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Energy and Climate: Studies in Geophysics (1977)

Chapter: Overview and Recommendations

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Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
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Overview and Recommendations

INTRODUCTION

Worldwide industrial civilization may face a major decision over the next few decades—whether to continue reliance on fossil fuels as principal sources of energy or to invest the research and engineering effort, and the capital, that will make it possible to substitute other energy sources for fossil fuels within the next 50 years. The second alternative presents many difficulties, but the possible climatic consequences of reliance on fossil fuels for another one or two centuries may be so severe as to leave no other choice.

A decision that must be made 50 years from now ordinarily would not be of much social or political concern today, but the development of the scientific and technical bases for this decision will require several decades of lead time and an unprecedented effort. No energy sources alternative to fossil fuels are currently satisfactory for universal use, and, in any case, conversion to other sources would require many decades. Similarly, finding ways to make reliable estimates of the climatic changes that may result from continued use of fossil fuels could very well require decades.

The climatic questions center around the increase in atmospheric carbon dioxide that might result from continuing and increasing use of fossil fuels. Four questions are crucial:

  1. What concentrations of carbon dioxide can be expected in the atmosphere at different times in the future, for given rates of combustion of fossil fuels?

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
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  1. What climatic changes might result from the increased atmospheric carbon dioxide?

  2. What would be the consequences of such climatic changes for human societies and for the natural environment?

  3. What, if any, countervailing human actions could diminish the climatic changes or mitigate their consequences?

NATURE OF THE PROBLEM

Three by-products of energy production and consumption—heat, particulate matter, and gases—were recognized at the start of the work of the Panel as having the potential for inadvertent modification of global climate. It has been known for some time that cities create their own microclimate (see Chapters 5 and 6). At first, the Panel speculated that increasing urbanization, large power-generating compounds (power parks), and similar developments might, by their output of heat and particles, disturb rainfall or influence other meteorological phenomena on a global scale. However, our study showed that the simple combustion product carbon dioxide has the greatest apparent potential for disturbing global climate over the next few centuries (see Chapters 4 and 10).

Carbon dioxide, although virtually transparent to shortwave solar radiation (visible light), strongly absorbs long-wave radiation (heat) at certain wavelengths where other atmospheric gases are transparent. In the atmosphere, it impedes radiation of heat from the earth’s surface into space. An increase in carbon dioxide concentration in the atmosphere could disturb the balance between incoming solar radiation and the radiation of heat from the earth into space with a resulting increase in the temperature of the lower atmosphere. Because glass in a greenhouse traps the sun’s heat, although mainly by preventing convection, this phenomenon has come to be known as the greenhouse effect.

In emphasizing questions related to increased atmospheric carbon dioxide we do not imply that serious consequences might not arise also from an increase in the load of particulates in the atmosphere or the growth of large “hot spots” resulting from the uneven distribution of human energy use. It is clearly possible, although expensive, to control the level of atmospheric particulates produced by human activity, and there are other reasons for doing so than the possible effects of high particulate concentrations on climate (see Chapter 3). Present climatic models are not adequate to predict reliably possible large-scale climatic changes resulting from the uneven geographic distribution of heat released by human energy use. But the greater understanding of climate required to answer questions about the effect of carbon dioxide could make it possible to give useful estimates of the effects of uneven heat releases. Even a future world population of ten billion people, with a per capita energy use several times greater than at present, would release an amount of heat equivalent to only one thousandth of the global net radiation received from the sun. The short residence times of tropospheric aerosols limit the threat that they pose because the atmosphere can be cleansed of them in a matter of weeks.

The average global temperature is only one of a constellation of dynamically related variables that, taken together, describe climate. Others include statistical properties of temperature, cloudiness, precipitation, and wind. The possibility that a moderate change in one of these variables could lead to a major shift in global climate cannot be ruled out. Historical records and indirect indices of past climates do indeed show marked shifts in temperature, precipitation, and ice volume. About 60 million years ago, the warm Mesozoic era ended and a gradual cooling began, leading to the present glacial age. The last 2 million years have been characterized by ice ages relieved by warm interglacial periods. The most recent ice age,

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
×

during which average midlatitude temperatures were 5 to 10°C below those we presently enjoy, ended about 10,000 years ago.

Some of the basic processes governing climatic change are poorly understood. It is not known whether climate changes occur in steplike transitions from one dynamically stable steady state to another or whether they represent a more gradual passage through a continuum of climate states. Either kind of change could be induced by variations in external parameters, such as the amount of radiation coming from the sun, or by spontaneous internal redistributions of energy within the physical components of the climate system (see Chapter 2). If changes in climate are steplike, then a disturbance of climate such as would be produced by a large increase in carbon dioxide production would be especially worrisome; a slow change might be the forerunner of a comparatively abrupt transition to a new climatic regime. If, on the other hand, changes in climate are gradual, then the effects of increasing carbon dioxide in the atmosphere would build steadily to produce a more gradual global shift in climate. In either case, agricultural belts would be shifted by changing seasonal precipitation and temperature patterns. For some countries with marginal agriculture, the impact on food production could be severe. For this and other reasons, the prospect of a man-made modification of global climate must be taken seriously.

If the potential for climatic change discussed in this report is further substantiated, then it may be necessary to reverse the trend in consumption of fossil fuels. Alternatively, carbon dioxide emissions will somehow have to be controlled or compensated for (no practical means of doing so seem to be readily at hand). In the face of so much uncertainty regarding climatic change, it might be argued that the wisest attitude would be laissez-faire. Unfortunately, it will take a millennium for the effects of a century of use of fossil fuels to dissipate. If the decision is postponed until the impact of man-made climate changes has been felt, then, for all practical purposes, the die will already have been cast.

MAGNITUDE OF THE PROBLEM

Man’s use of energy has increased manyfold since the industrial revolution, yet large-scale effects on climate are not readily apparent. One should, therefore, look ahead for significant relationships. Thus estimates of future world population, future energy uses, and future sources of energy are central to an assessment of future climatic impact. Harry Perry and Hans H. Landsberg have undertaken to produce such estimates,* which, as they note, are intended simply as plausible springboards for analysis and discussion—not as forecasts. Nevertheless, the model they present sets forth vividly the implications of growing population and a continuing demand for energy.

They envision a world population of some 10 billion by the latter part of the next century and total energy consumption over 5 times current levels. Perhaps surprisingly, all this energy could be provided by fossil fuels, mainly coal. On this basis, annual heat and carbon dioxide production would also be over 5 times current levels, while annual production of particles (because of the need to use dirtier fuels) could be perhaps 20 times current values. Thus considerable man-made heat would be released into the environment, but it would still be only a small fraction of the natural energy flows on a global or regional scale; local concentrations, however, could be much greater. While particulate production might be very high, there is no reason to expect that release of particulates to the environment would be correspondingly high. On the contrary, there is every reason to suppose that present

*

These estimates are discussed in Appendix A at the end of this chapter and in Chapter 1.

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
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means of controlling particle emissions will be improved to a high degree. Certainly a twentyfold increase in the emission of particles would be intolerable because of hazards to human health.

Perry and Landsberg have calculated that in 1973, energy use w as 7.6 billion metric tons of coal equivalent or 5.8 × 1016 kilocalories. This is about 0.01 percent of the incoming solar radiation. Their figures for 2075 indicate that the total energy used by human beings may correspond by then to about 0.1 percent of the incoming solar energy. On a worldwide basis, the climatic effect of the added heat would be small. On a local scale, however, it might be significant. Over Japan, for example, the heat released by human energy use could be nearly 2.6 percent of the solar radiation absorbed near the earth’s surface, and over Western Europe it could be about 0.6 percent. Even with a population of 20 billion people and a per capita energy demand 10 times the present world average (twice that of the United States in 1975) the total energy release would be 400 Gt of coal equivalent, or about 0.3 percent of the absorbed solar radiation. Current general circulation models indicate that, if the heat release were uniformly distributed on the earth’s surface, the corresponding increase in average surface temperature would be about 0.6°C, and at latitudes above 50° perhaps 2–3°C.

According to the data and estimates in Chapters 4 and 10, somewhat less than half of the carbon dioxide released by man since the industrial revolution has remained in the atmosphere. During that time, about a 13 percent rise in atmospheric concentration of carbon dioxide has taken place. Most of the remainder is inferred to have been taken up by the oceans and by the terrestrial biosphere. One can estimate the amount of carbon dioxide that may be released through the middle of the next century and, by the use of models of the carbon cycle, the amount that may be expected to remain in the atmosphere, It is not implausible that the peak atmospheric concentration occurring in A.D. 2150 to A.D. 2200 might be four to eight times the preindustrial level. Moreover, concentrations much higher than today’s may persist for many centuries thereafter.

Manabe and Wetherald (1975) have constructed a three-dimensional climatic model of the general circulation of the atmosphere that simulates the effects of a doubling in atmospheric carbon dioxide. This model, although recognized to be imperfect in a number of significant ways, is the most complete one yet devised. For a doubling of carbon dioxide in the atmosphere, the model predicts about a 2–3°C rise in the average temperature of the lower atmosphere at middle latitudes and a 7 percent increase in average precipitation. The temperature rise is greater by a factor of 3 or 4 in polar regions (see Chapter 9). For each further doubling of carbon dioxide, an additional 2–3°C increase in air temperature is inferred. The increase in carbon dioxide anticipated for A.D. 2150 to A.D. 2200 might lead to an increase in global mean air temperature of more than 6°C—comparable with the difference in temperature between the present and the warm Mesozoic climate of 70 million to 100 million years ago.

CONCLUSIONS

  • It does not now appear that the direct generation of heat from the production and consumption of energy over the next few centuries will cause a rise of more than 0.5°C in global average air temperature, although it may have substantial effects on local climates. If the correspondingly increased particulate emissions are properly controlled, there should be little global effect on climate from an increased atmospheric burden of aerosols.

  • The climatic effects of carbon dioxide release may be the primary limiting

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
×

factor on energy production from fossil fuels over the next few centuries. The prospect of damaging climatic changes may thus be the stimulus for greater efforts at conservation and a more rapid transition to alternate energy sources than is justified by economic considerations alone. The potential effect of carbon dioxide on climate could be exacerbated by fluorocarbons, nitrous oxide, and other industrial gases. The natural variability of climate could increase or reduce the impact of such man-made effects.

  • There are profound uncertainties regarding the carbon cycle, climate, and their interdependence. These uncertainties can be resolved only by a well-coordinated effort of extraordinarily interdisciplinary character. The focus for such an effort is not provided by any existing institutional mechanisms.

RECOMMENDATIONS

  • The possibility of modification of the world’s climate by carbon dioxide released in the production of energy from fossil fuels should be given serious prompt consideration by concerned national and international organizations and agencies. Two kinds of action are needed: organization of a comprehensive worldwide research program and new institutional arrangements.

  • A worldwide comprehensive research program should be undertaken. It should include studies of the carbon cycle, climate, future population changes, and energy demands and ways to mitigate the effects of climatic change on world food production.

  1. Research on climate. The development and verification of climatic models should continue. Adequate means to monitor climate should be provided both to verify models and to give warning of climatic change. Further studies of the sensitivity of climate to disturbances in the radiation balance should be carried out using advanced climate models. Paleoclimatology should be pursued in the interest of understanding past climate changes as well as to provide data for verification of models, both of climate and of biogeochemical cycles.

  2. Studies of world population and energy demands. These studies should include estimates of the contribution to our energy requirements that can be made by renewable resources. Investigation of energy sources that do not release waste gases and particles and minimize the release of heat should be intensified. The problems associated with energy supply and demand call for the further study of conservation measures and their prompt implementation.

  3. Food. In view of the prospective impact of man’s activities on climate as well as the natural variability of climate, greater attention must be given to agriculture and water resources from the point of view of mitigating the effects of climatic change.

  4. Carbon dioxide and the atmosphere-ocean-biosphere system. A better understanding of the partitioning of carbon among the biosphere, oceans, and atmosphere is essential and might be obtained by the following measures:

  1. Measurements of variations with time in the ratio of the two stable isotopes of carbon (13C and 12C) in the atmosphere are needed to determine the net flux of carbon between the atmosphere and the biosphere. Past variations in this ratio can be obtained from measurements of these two isotopes in tree-ring sequences from trees in isolated locations—as distant as possible from major biological and industrial sources of carbon dioxide. Changes in the 13C–12C ratio are likely to be small relative to random measurement errors, and consequently many measurements are needed over a wide range of geographic locations.

  2. Better estimates should be made of the area of land annually cleared for

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
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agricultural and other purposes. From 1972 onward, these estimates can be obtained from earth-resources satellite data. To obtain estimates for the period before 1972, a historical study of the growth of cultivated areas in all continents from the early part of the nineteenth century should be made.

  1. Attempts should be made to estimate changes in forest biomass throughout the world, particularly in tropical and subtropical areas. The principal component of this biomass consists of wood in living trees; measurements of differences in the thickness of successive tree rings might indicate changes in the rate of net primary production of trees, at least in temperate latitudes. Many tree-ring sequences (of the order of several thousand) would be necessary for an adequate sample. Efforts should also be made to estimate time variations in the quantity of foliage and other organs of trees that participate actively in photosynthesis and in the rate of fall of “litter” (dead leaves and branches) from trees.

  2. Better estimates are needed of the fraction of soil humus from which carbon dioxide is released to the atmosphere. Changes in this quantity should be determined in agricultural and other cleared areas. Surveys of the present distribution of humus in soils throughout the world are needed to serve as a baseline for comparison with future measurements.

  3. Intercomparable mean monthly values of the partial pressure of atmospheric carbon dioxide should be obtained from continuous measurements at a number of carefully selected stations in different latitudes in both the northern and southern hemispheres. One of the principal objectives of such a network of stations would be to study year-to-year variations in the airborne fraction of carbon dioxide released to the atmosphere by fossil-fuel combustion and land clearing. These variations appear to be related to fluctuations in carbon dioxide uptake and release by near-surface ocean waters, and their elucidation should give us greater insight into oceanic processes affecting the partitioning of carbon dioxide between the ocean and the atmosphere.

  4. Further insight into these processes might be gained by time sequences of measurements of the total carbon dioxide content and partial pressure of free carbon dioxide at a global network of observing stations in surface and subsurface ocean waters. These quantities vary widely with local biological and other ocean processes, and hence it may not be possible to make useful interpretations of the measurements from the standpoint of the global carbon dioxide problem. Further analysis of the desirability of this type of measurement should be undertaken.

  5. Better estimates are needed of the quantity of carbon dioxide released by fossil-fuel combustion. International statistics on the consumption of fossil fuels should be supplemented by estimates of the carbon content of the fuels consumed each year. The present uncertainty in the amount of carbon dioxide released is of the order of 10 to 15 percent because the estimates of fuel consumption are given in terms of fuel energy rather than carbon content.

  6. A series of measurements of the dispersal of tritium from atmospheric nuclear-weapons tests in subsurface ocean waters should be made at approximately five-year intervals following the lines of the GEOSECS expeditions of the early 1970’s. Such measurements of time variations in the oceanic distribution of tritium appear to be the most satisfactory experimental means of studying “stirring” processes (advection, convection, and turbulent mixing) in the upper thousand or so meters of the ocean waters. These processes are critically important in estimating partitioning of fossil-fuel carbon dioxide between the ocean and the atmosphere.

  7. In principle, an independent check on oceanic stirring processes could be obtained if the Suess effect (the decrease in the radiocarbon content of the atmosphere from the beginning of the nineteenth century to 1950 resulting from injection of 14C-free fossil-fuel carbon dioxide into the atmosphere) were more accurately

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
×

known. The present uncertainty in the Suess effect is about ±25 percent. Many more measurements of 14C in tree rings covering the time span from 1800 to 1950 in trees from carefully selected locations are needed.

  1. The following kinds of observations should be given consideration for future studies but with a lower priority than the above recommendations relating to carbon dioxide.

    1. Knowledge of rates of interchange between the interstitial waters of calcareous sediments and the bottom waters overlying the sediments would allow better estimates of probable rates of solution of calcium carbonate and the corresponding increase in the capacity of the ocean to absorb carbon dioxide.

    2. More data on the distribution of aragonite (the more soluble of the two crystalline forms of calcium carbonate) in shallow and deep calcareous sediments are needed for better estimates of the possible solution of calcium carbonate.

    3. Whether solution of calcium carbonate has actually occurred and if so to what extent can be determined directly by measurement of changes in the alkalinity of seawater. New methods of measuring the alkalinity have a precision of about 1 part in 10,000, corresponding to a change in atmospheric carbon dioxide of about 0.1 percent,

    4. Carbon dioxide could be extracted from the atmosphere-subsurface ocean system if the rate of fallout of particulate organic matter from the subsurface layer into the deep ocean waters could be accelerated. This might be possible if the photosynthetic production of organic matter in the near-surface ocean waters could be increased. Photosynthetic production in these waters appears to be determined by the supply of dissolved phosphorus and nitrogen compounds. In the future, it might be possible to disperse large quantities of industrially produced phosphorus and nitrogen over extended areas in the oceans, at a cost that would be relatively small compared with the total cost of carbon dioxide-producing fossil fuels. The effectiveness of this tactic can probably be determined by comparative measurements of the rate of fallout of organic matter in both presently highly productive and unproductive ocean areas. In principle, fertilization of ocean waters with 10 million tons of phosphorus would produce fallout of about 300 million tons of organic carbon.

  • All the foregoing recommendations for research relate to global concerns and therefore the cooperation of such international agencies as the World Meteorological Organization, the Intergovernmental Oceanographic Commission, and the International Council of Scientific Unions should be sought in responding to them. The necessary research will be expensive in terms of scientific and technical manpower and research facilities and must by continued over many years. Consequently, it must be fostered and supported by governments. A high degree of international governmental cooperation is called for because of the need for a worldwide set of measurements and network of observing stations.

Consideration should be given to the establishment at the national level of a mechanism to weave together the interests and capabilities of the scientific community and the various agencies of the federal government in dealing with climate-related problems. Solutions to those problems will involve coordination of research in many scientific disciplines and are likely to require adjustments in national policy or the formulation of new legislation. Such a mechanism might be embodied in a Climatic Council with the following functions:

  1. To serve as the focal point within the United States for the development of a global research and action program.

  2. To coordinate activities that cross disciplinary, institutional, and organizational boundaries.

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
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  1. To serve as a link with international mechanisms that guide international research and action programs concerned with carrying out aspects of the above recommendations.

POTENTIAL CONSEQUENCES OF CLIMATIC CHANGES

WARMING OF THE OCEAN WATERS

A warmer atmosphere could not persist without a comparable warming of the upper layers of the oceans. The effects would be several: a reduction in the amount of sea ice, a rise in sea level, a flux of carbon dioxide from the ocean to the atmosphere, a reduction in the vertical stirring of the oceans, and a poleward shift in marine ecosystems including the fish population. Reduction of sea ice should decrease albedo and lead to further warming, higher precipitation, and possibly a new buildup of snow and ice poleward.

An increase by 5°C in the average temperature of the top 1000 m of ocean water would raise sea level by about 1 m, because of the expansion of water volume. Such an increase in ocean temperature would raise the partial pressure of carbon dioxide in the water by about 30 percent. After equilibration with the atmosphere, which would probably take place within a few years, the carbon dioxide content of the air would be increased by about 17 percent.

The expected polar warming would affect the rate of ventilation of all subsurface waters. A “lid” of relatively warm water would be formed over the colder deep waters, thereby increasing the vertical density stratification of the oceans. This would inhibit vertical mixing and stirring processes, which would in turn reduce the rate of nutrient supply to the near surface ocean waters and hence the productivity of marine plants. The quantity of dead organic carbon falling from the surface layers to the deep water would be lower, and consequently the rate of uptake of carbon dioxide by the deep sea would also be lower. According to our models, atmospheric and presumably oceanic near-surface temperatures would increase much more at high than at low latitudes. Circulation of the deep waters and vertical exchange between the deep and near-surface waters might be profoundly altered by reduction or even cessation of present vertical convection and formation of deep water in the north Atlantic.

Experience in previous periods of ocean warming indicates that the area of sea ice would be substantially reduced, probably so much that both the Northwest and Northeast Passages would be open for shipping throughout most of the year.

During earlier decades of this century, the slight rise in average air temperature over the northern hemisphere, and the concomitant warming of surface-water layers of the ocean, brought about a marked shift in the locations of certain commercially important fish populations, notably the North Atlantic cod. Much of the cod fishery shifted to the waters off Greenland and north of Iceland. Hence an abnormally large warming of the atmosphere would surely have significant effects in the geographic extent and location of important commercial fisheries. Because different marine organisms respond differently to temperature changes, marine ecosystems might be seriously disrupted.

EFFECTS ON THE POLAR ICE CAPS

In the present state of understanding, it is impossible to forecast what might happen to the Greenland and Antarctic ice caps as a result of a rise of several degrees in global average air temperature. It is likely that the temperature in the Antarctic

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
×

would still remain below the freezing point, so that melting at or near the surface of the ice probably would not occur. On the other hand, a substantial change in climate might greatly increase the annual snowfall in Antarctica and Greenland, resulting in a substantial increase in the thickness of the ice. This, in turn, would increase horizontal stresses at the base of the ice caps, which might result in surges or slides of ice masses into the sea. If these surges resulted in destruction of the West Antarctic ice cap, there might be a corresponding rise in sea level of about 5 m within 300 years (Hughes, 1974, 1977).

EFFECTS ON WORLDWIDE AGRICULTURE

Far-reaching consequences of a large increase in atmospheric carbon dioxide would be felt in agriculture—mankind’s basic industry. With our present level of understanding, we cannot specify these consequences completely, let alone quantitatively. We can only suggest some of the possible effects. A few of these would be beneficial; others would be disruptive. Five factors must be considered: the effect of high levels of carbon dioxide on plant metabolism; higher annual average temperatures; spatial shifts in agroclimatic regions, especially in the precipitation patterns in different regions; the possibilities of greater or less variability from year to year in different regions; and the effects of possible increased cloudiness on the growth of crops.

Effects on Photosynthesis

Both theory and experiment show that raising the carbon dioxide content of the air in contact with plants increases the photosynthetic production of organic matter, provided other requirements for plant growth—nutrients, water, and sunshine—are present in abundance and the plants are not under stresses caused by too low or too high temperatures, soil acidity or alkalinity, lack of oxygen in the root zone, diseases, or other factors. With modern farming technology it is possible to provide adequate supplies of water and major and minor nutrients and to eliminate most causes of stress. Atmospheric carbon dioxide, incoming solar radiation, and the genetic potential of crop plants can then become the limiting factors of agricultural production.

Under normal farming conditions, the net photosynthetic product, that is, the organic matter remaining after the plant has used some of its own product in respiration, will not increase so rapidly as the atmospheric carbon dioxide (Waggoner, 1969). For the terrestrial biosphere as a whole, we have estimated that the factor of proportionality (designated as β) is about 30 percent, but it can be much higher for agricultural crop plants. Future agronomic and genetic research might bring this factor close to 1 (Waggoner, 1969; Hardman and Brun, 1971).

Other changes that might be brought about by higher atmospheric carbon dioxide would work in an opposite direction. If average air temperatures increase significantly, plant respiration is also likely to increase (Waggoner, 1969; Botkin et al., 1973), The net photosynthetic product may be reduced even though gross photosynthesis is raised.

If average cloudiness (the proportion of the land area covered by clouds) rises, the quantity of incoming solar radiation will be lowered, and the energy available to crop plants for photosynthesis will diminish. Excessive cloudiness during the monsoon season in India and Bangladesh already limits crop yields, compared with those obtained on the same farms in the sunnier months of October through March.

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
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Poleward Shift of Agroclimate Zones

A rise in average annual global air temperature, increasing toward higher latitudes as predicted in the model of Manabe and Wetherald (1975) would result in a general poleward movement of agroclimatic zones. At higher latitudes, there would be a longer frost-free growing season than at present, and the boundaries of cultivation could be extended northward in the northern hemisphere. At the same time, summer temperatures might become too high for optimum productivity of the crops presently grown at middle latitudes, such as corn and soy beans in Iowa, Illinois, Indiana, and Missouri, and it might be necessary to shift the Corn Belt toward the north. But the acid podzol soils over large areas in these higher latitudes are badly leached, and extensive and expensive soil amendments would be required even to approach the yields now obtained in the remarkable soils of the Corn Belt.

The model predicts a global rise in average annual precipitation, which at first glance would seem to benefit agriculture. But the accompanying higher temperatures would also increase evapotranspiration from cultivated lands, and some part, perhaps all, of the benefits from the additional water supply would be lost. In some regions, higher evapotranspiration might exceed the increase in precipitation; in others, the reverse might be true (Manabe and Wetherald, 1975).

In general, the most serious effects on agriculture would arise not from changes in global average conditions but from shifts in the location of climatic regions and changes in the relationships of temperature, evapotranspiration, water supply, cloudiness, and radiation balance within regions. Present cropping patterns, crop varieties, and farming technology in different climatic regions are based on cumulative experience over many years in the selection of appropriate crop species and varieties for each region and in adapting both the plants and their physical environment to each other in as nearly an optimal fashion as possible. These adaptations have remained fairly satisfactory over the relatively narrow range of climatic changes that have occurred in the historic past. But large changes in climatic relationships within regions such as might be brought about by a doubling or quadrupling of atmospheric carbon dioxide would almost certainly exceed the adaptive capacity of presently grown crop varieties.

The regional changes in temperature-precipitation relationships that can accompany even comparatively small excursions in global average temperatures are illustrated by paleoclimatic studies. For example, during the so-called climatic optimum of several thousand years ago, when the average temperature was perhaps 1.5°C higher than at present, precipitation probably increased over southern Europe, northern Africa, southern India, and eastern China, while over large parts of the United States, Canada, and Scandinavia, the climate was drier (Kellogg, 1977).

It cannot be expected that regional climatic changes resulting from a large rise in atmospheric carbon dioxide would be simply an exaggerated replica of past changes. Both the seasonal and latitudinal effects of added carbon dioxide should be different from, for example, the effects of a global change in incoming solar radiation. Since both water vapor and carbon dioxide absorb and reradiate infrared energy, the effect of added carbon dioxide will be relatively more important in the dry air of high latitudes, in the upper troposphere, and in the stratosphere than in the moist air of the tropics. Similarly, because absolute humidity in winter is less than in summer, the effects of added carbon dioxide will be relatively more significant in the winter months. These latitudinal and altitudinal differences in the role of carbon dioxide are taken into account in the model of Manabe and Wetherald (1975); and they must form an integral part of future three-dimensional dynamic models that attempt to specify regional climatic changes in temperature, precipitation, evapotranspiration, and cloudiness.

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
×

Studies of oxygen and carbon isotope ratios in deep-sea cores suggest that the higher temperatures of the climatic optimum may have been due to a temporary increase in atmospheric carbon dioxide resulting from the changes in ocean circulation that followed the melting of the ice cap. If this suggestion can be substantiated, further paleoclimatic studies of seasonal differences in temperature-precipitation relationships during the climatic optimum should provide valuable insights into the future effects of increased atmospheric carbon dioxide (Berger and Garner, 1975; Berger and Killingley, 1977).

Serious Effects in Arid and Semiarid Regions

The most serious effects of possible future climatic changes could be felt along the boundaries of the arid and semiarid regions in both hemispheres. These are the zones of atmospheric subsidence where precipitation is scanty and highly variable: the southwestern United States and northern Mexico; the belt of relatively dry lands extending from southern Europe and northern Africa (including the Sahara), eastward across Arabia and south Asia to Pakistan and northwestern India; northeastern Brazil, northern Chile and southern Peru, western Argentina, southwest Africa, and Australia. We need to be able to estimate whether these belts of aridity and semiaridity will move toward or away from the poles and whether they will expand or contract in area.

Short-term variations in climate, especially in rainfall, often persisting for several years, are characteristic of the semiarid regions. The human disasters caused by drought in these regions are familiar and dramatic. But the variations are also closely related to destruction of the resource base through processes of desertification—wind and water erosion and sedimentation—which make large areas unfit for agriculture or pasturage, deterioration in soil and groundwater quality through salinization, and destruction of natural vegetation and its replacement by plants unfit for grazing.

The effects of relatively short-term climatic variations in the semiarid zone are worsened by the seemingly inevitable behavior patterns of farmers and pastoralists, who, during relatively wet periods, expand their cultivated areas and drive their livestock into marginal lands beyond the region’s carrying capacity. During dry periods, these marginal lands, with their natural protective cover destroyed by cultivation and grazing, are eroded at rates exceeding by orders of magnitude those that had prevailed before.

From this discussion of the human consequences of a marked climatic change that might be brought about by the addition of large quantities of carbon dioxide to the atmosphere, it may be concluded that world society could probably adjust itself, given sufficient time and a sufficient degree of international cooperation. But over shorter times, the effects might be adverse, perhaps even catastrophic.

COUNTERVAILING MEASURES

Two kinds of countervailing measure against the possible climatic effects of added carbon dioxide can be visualized: measures to reduce possible climatic changes themselves and measures to reduce the impact on human affairs. In the first category, it is possible to conceive of ways of reversing the changes in the radiation balance of the earth that might result from added carbon dioxide or ways of removing the added carbon dioxide from the air. In the second category, we are concerned primarily with ways to increase the robustness and flexibility of the world’s food-supply systems. We shall consider these first because they are less problematic and largely within the range of present technology.

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
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Improving the Food Supply System

From an agricultural point of view, arid and semiarid lands are defined as regions where water is not available in adequate quantities for crop production. Irrigation is the classical and still the most practical remedy. In this process, water is transported from mountainous and hilly regions or humid lowland areas, where the amount exceeds that which can be used for agriculture, to the arid or semiarid regions. Because the water supply is usually widely variable from season to season and from year to year, the water is commonly stored during wet periods, in surface reservoirs or underground, for use during the dry times when it is needed.

The stabilization of water supply provided by storage of irrigation water, and the increased quantities made available where they are needed by transporting the water, are the essential basis of modern, high-yielding agriculture, particularly in semiarid zones and in subtropical latitudes. To be of permanent benefit, irrigation development must nearly always be accompanied by parallel development of drainage facilities. Combined development of irrigation and drainage requires large capital investments of the order of $500 to $1000 per hectare (1 hectare = 0.01 km2). For example, the cost of full development of irrigation of the Ganges Plain in India, which contains over 50 million hectares of irrigable land, has been estimated to be close to $50 billion. Such development could lead to an annual increase in crop production of several hundred million tons of food grain, worth $20 billion to $40 billion (Revelle, 1976).

It will be of particular importance to counter effects of possible increases in short-term climatic variation. For this purpose, large reservoirs are needed, either on the surface behind dams or in underground aquifers. Wherever feasible, underground storage is preferable, because it is cheaper per unit volume of stored water; water losses by evaporation are small; and the volume stored can be large enough to provide stable supplies even in the face of the persistent drought periods with time constants of a decade. But in some situations, no practical reservoir could provide long-term protection against climate change.

Equally careful research, planning, and investment are needed to develop methods for conserving water. With present methods of farm-water management in less-developed countries, only about a third of irrigation water supplies are beneficially used. Large savings in water could be obtained by improved water management and in many cases by adoption of new irrigation practices. Even more useful in most areas will be the introduction of water-saving crops; for example, crops grown during the season of minimum evapotranspiration; crop varieties with the shortest possible growing season; and row crops such as peanuts and sugar-beets where evapotranspiration is reduced by the presence of clean, bare ground between the planted rows. The best way to save water, however, is through the use of high-yielding crop varieties. Hardly any more water is required to irrigate a variety of wheat or corn yielding three or four tons per acre than is required for a variety yielding less than one ton per acre.

The impact on world and regional food supplies of short-term variations in climate can be greatly lessened by maintaining food reserves. On a worldwide basis, under the present climatic regime, such reserves should represent about 5 percent of average annual production; that is, the excess or deficit over periods of several years of food grain production with respect to demand is about 5 percent of the average annual production. The primary purpose of food reserves of this magnitude would be to stabilize prices of food staples for both farmers and consumers. Because food is an essential requirement for human life, the demand for food is extremely inelastic in relation to price; food supplies cannot be rapidly increased in response to rising prices. Experience shows that food prices may rise or fall by several

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
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hundred percent with a deficit or surplus of only a few percent in supply in relation to demand.

Counteracting a Changed Radiation Balance

One way to counteract the climatic effects of added carbon dioxide in the air would be to increase the albedo, or reflectivity, of the earth and thereby to reduce the incoming solar radiation. No practical, plausible, and reliable means to accomplish such an increase seem to be at hand.

One means to accomplish this that has been discussed might be to spread small reflecting particles over large areas of the ocean surface. To reduce the cost and increase the effectiveness of this measure, the particles should have a density very close to that of seawater and should be chemically stable over periods of many months. One material that has been suggested is very thin platelets of latex. If these platelets had a thickness of 0.01 mm, about 10 tons would be required per square kilometer or 50 million tons per year to cover 5 million square kilometers—1 percent of the earth’s surface. At $100 per ton, the cost would be $5 billion per year—about 0.2 percent of the world’s annual expenditure for fossil-fuel energy anticipated during the next century (PSAC, 1965).

The disadvantages of such a scheme are obvious and may be insuperable. The material might eventually pile up on the coastlines of the world, with unacceptable environmental consequences, and the effect on fisheries might be disastrous.

A completely different kind of countervailing measure might be to store the added carbon dioxide in the biosphere. The present biospheric organic carbon is about four times the carbon in the atmosphere. Perhaps a fourth of this mass is in the roots, trunks, branches, and leaves of living trees. Most of the remainder is in the soil humus or in dead organic matter in lakes, marshes, and wetlands.

Forests now cover about 50 million square kilometers, a third of the land surface of the earth. Doubling this area or doubling the mass of living trees in the existing forests would permit storage of 700 Gt (1 gigaton = 109 metric tons) of carbon—about a seventh of the carbon in fossil fuels but between a third and a sixth of the carbon that might otherwise be added to the atmosphere by fossil-fuel combustion. Such an increase in forest mass would have a significant mitigating effect, but it would be extremely difficult to accomplish, even on the hundred-year time scale we are considering. Major changes in land use on a worldwide basis, and consequently in world political and social organization, would be required. With the continuing growth in human populations and economic production, and the resulting needs for expanded food, fuel, and timber production, present trends are in exactly the opposite direction, since forests are being cut down for fuel and lumber and land is being cleared for agriculture.

Without human intervention, increased photosynthesis due to rising atmospheric carbon dioxide will probably cause forests and soil detritus to grow, perhaps by over 1000 Gt in the next 200 years (see Chapter 10).

It has been suggested that if wood grown in fertilized and irrigated forest plantations were cut and preserved against decay, significant amounts of carbon dioxide could be removed from the atmosphere, counteracting to some extent the increase of atmospheric carbon dioxide from fossil-fuel combustion. But it seems obvious that if large quantities of organic material were to be grown and collected, it would make more sense to use the material as an alternative source of energy to fossil fuels. If this were done, carbon dioxide would simply be recycled between the atmosphere and the biosphere, and the net addition of carbon dioxide to the atmosphere from fossil-fuel combustion would be reduced by the amount of biomass energy substituted for coal, oil, and natural gas.

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
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It appears that any attempt to reduce the climatic impact of additional carbon dioxide in the atmosphere would be formidably difficult, especially since the effort would need to be continued over the next millennium and might even have unacceptable consequences. On the other hand, mitigation of the climatic effects on human affairs might be possible and even desirable from points of view other than that of climatic change; but it would require planning, research, and investment of international scope on an unprecedented scale.

It may well be the case that increasing reliance on renewable resources, with the concomitant reduction in the carbon dioxide burden in the atmosphere, will emerge as a more practical alternative to these countervailing measures.

NATURE AND LIMITATIONS OF MODELS

THE CARBON CYCLE

During the last 110 years, the carbon dioxide content of the air (expressed as weight of carbon) has increased by 72 to 83 Gt, or 11.5 to 13.5 percent. During the same period, 127 Gt of carbon in fossil fuels and in limestone have been converted to carbon dioxide and released to the atmosphere (2 percent of this amount came from cement manufacturing and the remaining 98 percent from combustion of fossil fuels). Volcanoes may have added around 4 Gt, less than 3 percent of the “anthropogenic” carbon. Rock weathering has probably subtracted an amount equal to the volcanic flux. The clearing of forests, woodland, savannas, and grasslands for farming and other human modification of land biota and soils throughout the world may have brought about a further net release of roughly 70 Gt of carbon, as carbon dioxide, into the atmosphere.

The excess of carbon dioxide released over that remaining in the atomosphere has undoubtedly been absorbed in the subsurface water layers of the ocean and in the pool of organic matter on the land (about 70 percent of this organic pool, around 2000 Gt, consists of dead organic material—mostly soil humus—and roughly 30 percent is in the trunks, stems, roots, branches, and leaves of living plants). Our calculations indicate that about 40 percent of the carbon dioxide released to the air has been absorbed in the land organic pool, about 20 percent has been absorbed by the oceans, and about 40 percent remains in the air. General discussion and review of these questions are contained in Woodwell and Pecan (1973), Wilson and Matthews (1970), Matthews et al. (1971), Dryssen and Jagner (1972), Proceedings of the Dahlem Workshop (1977), Proceedings of the ERDA Workshop (1977), Proceedings of the SCOPE Workshop (1977), and Baes et al. (1976).

If fossil fuels remain the principal energy source for world society during the next hundred years, our estimates indicate that around 2500 Gt of carbon, as carbon dioxide, could be released to the air by 2090, about 20 times as much as the amount produced to date by fossil-fuel combustion and over 4 times the preindustrial content of the atmosphere. More than half of this amount would probably remain in the air. At first thought, this statement appears paradoxical, because the ocean contains nearly 60 times as much carbon as the air, and the terrestrial organic pool nearly 4 times as much, and it might be supposed that the division of added carbon dioxide between the atmosphere, the oceans, and the land organic pool would be in proportion to the amounts already present. But the amount of carbon dioxide that the ocean is able to take up is limited by the small amounts of carbonate ion in sea-water and by the low solubility of free carbon dioxide. Similarly, uptake of carbon in the biosphere is limited by the balance between photosynthetic production and oxidation of organic matter.

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
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Because of the highly stratified nature of the oceans, the exchange between surface waters and deep waters is very slow. Hence, as long as the rate of addition of fossil-fuel carbon dioxide continues to increase exponentially, only a small fraction of the total volume of the oceans can serve as an absorber of an important fraction of the added carbon dioxide. Keeling and Bacastow (see Chapter 4) have calculated that this slow turnover of the oceans, combined with the relatively low carbonate-ion concentration of surface seawater, could result in about 80 percent of the carbon dioxide added during the next century being retained in the air. If so, the concentration of carbon dioxide in the atmosphere in the twenty-second century might be 7 times the preindustrial value. As they point out and as we discuss below, considerable uncertainty attends this figure.

DEEP CIRCULATION OF OCEANS

The real nature of the intermediate and deep circulation of the ocean (see Chapters 4 and 7) and the location and size of the regions where the deep waters come in contact with the surface are poorly understood at present. The variations from year to year in the proportion of fossil-fuel carbon dioxide removed from the air, shown in measurements made at both Mauna Loa and the South Pole, suggest that the interchanges between the intermediate water and the surface occur in irregular pulses. No very reliable estimate can be made of how the rates of interchange might be modified by changes in air and surface ocean temperatures or in circulation patterns accompanying possible climatic changes.

Because of the relatively large carbon dioxide content of the oceans and the large variability in the partial pressure of carbon dioxide in the waters near the ocean surface, the fraction of the added carbon that has been absorbed in the sea cannot be measured directly. But it can be estimated by comparison with the distribution in the upper ocean layers of tritium and other trace substances produced in nuclear-weapons tests during the two decades after World War II, principally in 1962–1963. Measurements of tritium in the upper 500 to 700 m of the ocean were begun in 1964–1965 by Östlund and co-workers in the Atlantic (Östlund et al., 1969) and by Suess and co-workers in the Pacific (Michel and Suess, 1975). These have been extended by Östlund on nearly 1000 samples collected on the GEOSECS expeditions (Östlund et al., 1974; Östlund, Institute of Marine Science, Miami, personal communication, 1977).

These measurements show that tritium has been mixed down to an average depth of about 360 m over the last 12 years. Apparently the tritium is carried downward partly by advection along surfaces of equal density, partly by sinking of cold surface waters in high latitudes of the Atlantic, and partly by vertical mixing. The observed downward diffusion of tritium can be used to estimate the time constant for diffusion of material added to the well-mixed surface layer of the sea into the main mass of the ocean (see Chapter 10). Since the concentration of tritium in the 100-m-thick mixed surface layer has diminished by a factor of 1/e in about 12 years, the time constant for diffusion into the deeper waters, with a volume 40 times larger than that of the mixed layer, should be at least 500 years.

Because of the buffer mechanism of seawater, a 10 percent increase in atmospheric carbon dioxide corresponds at equilibrium to a 1.1 percent increase in the carbon dioxide content of surface ocean waters (see Chapter 4). The mixed surface layer contains approximately two moles of carbon dioxide per cubic meter, or 864 Gt of carbon in a 100-m-thick layer. Hence, for a 13 percent increase in atmospheric carbon dioxide from 1860 to 1973, the carbon dioxide content of the mixed layer should have increased by 12.5 Gt. If the carbon dioxide has diffused downward to an average depth of 360 m, the total added to the ocean would be 45 Gt of carbon—

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
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about 22 percent of the carbon added to the atmosphere by fuel combustion and land clearing.

Carbon dioxide from fossil fuels contains no 14C and hence the proportion of 14C in the atmosphere prior to the nuclear tests of the post-World War II period should have been less than that of 100 years ago. The magnitude of this phenomenon—the Suess effect—is about 2 ± 0.5 percent for A.D. 1950, indicating that the atmospheric carbon dioxide has mixed with biosphere and ocean reservoirs, which are some 5 times larger than the atmospheric reservoir. If the part of the biosphere that exchanges with the atmosphere during 29 years (the average age of carbon added from fossil-fuel combustion) is 1.8 times the atmospheric reservoir, the exchanging part of the ocean must be 3.2 times the size of the atmospheric reservoir, corresponding to a water layer about 230 m thick. However, consideration of the exchange rates of carbon dioxide between the oceans and the atmosphere suggests that the mean depth of complete isotopic mixing calculated from the Suess effect may be less than the mean depth of diffusion of fossil-fuel carbon dioxide. According to Broecker (1977), the chemical equilibration time between the ocean and the atmosphere is smaller than the isotopic equilibration time. Thus the apparent isotopic equilibration depth of 230 m calculated from the Suess effect implies an equivalent chemical depth of perhaps 400 m. This would allow absorption of about 25 percent of the carbon dioxide added to the atmosphere from land clearing and fossil-fuel combustion.

From either method of estimating the absorption of carbon dioxide in the oceans we are left with the conclusion that a large fraction must have been taken up by the biosphere. Bacastow and Keeling (1973) also reached this conclusion using a time-dependent model. This requires that the rate of net primary production by photosynthesis should have increased relative to the rate of oxidation by animals, microorganisms, and fires.

PROCESSES IN THE BIOSPHERE

The portion of the terrestrial biosphere that exchanges carbon dioxide with the atmosphere consists of two components: the biomass of living plants and animals, mostly the trunks, branches, roots, and leaves of trees, and litter and soil organic matter (humus). Whittaker and Likens (1975) have compiled and evaluated data on the biomass (see Table 10.1). They conclude that 90 percent of the total of around 830 Gt is in the world’s forests, which cover nearly 50 million km2. Tropical forests, with an area of 24.5 km2, contain more than half of the total. Woodland and shrub-land, savannas, grasslands, desert and semidesert scrub, swamps and marshes, and cultivated land together contain only 84 Gt or 10 percent of the total biomass, although they cover nearly 75 million km2. Net primary production of organic matter (photosynthesis minus plant respiration) is more evenly divided: forests produce 33 Gt of carbon per year, and all other vegetation produces nearly 20 Gt. These estimates correspond to an average efficiency of photosynthetic conversion of solar energy on the earth’s land surface of about 0.1 percent. In a steady state, net primary production must be balanced by the oxidative activities of animals and microorganisms (heterotrophic respiration) and fires. Thus about 8 percent of the carbon dioxide content of the atmosphere is turned over each year by terrestrial biological activities.

Bohn (1976) has recently estimated from the World Soil Map of FAO-UNESCO (1971) and other sources that the content of organic carbon in the world’s soils in somewhat less than 3000 Gt, about three times the previously accepted value (Baes et al., 1976). Of this total, about 860 Gt is in peaty materials (dystric and gelic histosols in the FAO nomenclature), covering 4.3 million km2, which presumably ex-

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
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change carbon very slowly, if at all, with the atmosphere. The remaining approximately 2000 Gt of carbon in soil organic matter can be assumed to lose carbon to the atmosphere by oxidation at about the same rate as carbon is added by the accumulation of dead plant material.

The world’s soils contain on the average 18 kg/m2 of exchangeable soil organic matter, or 180 tons per hectare. There is a wide variation in different soil types from 40 to 600 tons per hectare.

The average residence time of carbon in the terrestrial biosphere is simply the total mass of organic matter, about 2800 Gt, divided by 53 Gt per year (the rate of heterotrophic respiration plus fires), or 53 years. Variations in global temperature and precipitation should bring about short-term variations in the rates of photosynthesis and oxidation of organic matter in the terrestrial biosphere, but over periods of ten years or more these changes should be small and tend to balance out. They are likely to be considerably less than the changes brought about by the secular increase in atmospheric carbon dioxide or by human activities such as clearing of forests, reforestation, and destruction of soil humus.

The quantity of organic carbon in the oceanic biosphere is estimated to be about half as large as that of the terrestrial biosphere, 1650 ± 500 Gt, but with the important difference that almost all of it consists of dead organic matter in small suspended particles and dissolved substances distributed fairly evenly throughout the ocean waters. Carbon in the living marine biomass, almost all in the top 100 m of the ocean, is approximately 0.1 percent of the total marine organic carbon (Whittaker and Likens, 1975). The rate of primary production (photosynthesis minus plant respiration) in the ocean is estimated at 20–25 Gt per year (Whittaker and Likens, 1975), probably about half that of the terrestrial biosphere. This rate is believed to be determined by the rate at which the essential plant nutrients, phosphorus, and nitrogen compounds are brought to the sunlit surface waters by vertical motions within the oceans.

Broecker (1974) estimates that about 1.7 Gt per year, 8 percent of the organic matter produced by photosynthesis in the subsurface ocean waters, sinks into the depths, where most of it is oxidized to carbon dioxide and water. This process is balanced by a slow upwelling of deep waters with higher carbon dioxide content into the surface layers. The deep ocean waters contain about 10 percent more carbon dioxide than they would if they were at equilibrium with the partial pressure of carbon dioxide in the present atmosphere. The combination of biological and gravitational processes can be thought of as a pump that maintains a high carbon dioxide content in the deep water and a low content in the surface waters and in the atmosphere. If the pump ceased to act, the atmospheric carbon dioxide would eventually increase severalfold. Variations in the effectiveness of the pump could have occurred without detection during the past 100 years and could have caused significant changes in the atmospheric carbon dioxide content.

Several effects of human activities during the past 100 years may have changed the quantity of organic material in the terrestrial biosphere and, correspondingly, the content of carbon dioxide in the atmosphere. Perhaps the most important of these has been the clearing of lands for agriculture, which is taking place with the rapid increases in world population and food demand. Some workers have estimated that about 2 million hectares of forest lands are being cleared each year (or destroyed by continued shortening of the cycle of slash and burn agriculture) in Africa (Persson, 1976); about 6 million hectares in Latin America (FAO, 1976); and perhaps 600 thousand hectares in Southeast Asia (Swedish Royal College of Forestry, 1974). The total area of land cleared for agriculture between 1950 and 1970 has been estimated by Revelle and Munk (see Chapter 10) from data published by the U.S. Department of Agriculture and the Food and Agriculture Organization of the United

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
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Nations, which show annual increases in cultivated areas in 35 countries. During this period, the areas sown to crops in the Soviet Union may have risen by 88 million hectares, and in Africa south of the Sahara, Latin America, and Asia (outside the Soviet Union and China) by 74, 50, and 84 million hectares, respectively. Thus, the average annual increase in crop area for these regions combined was 15 million hectares. Assuming that somewhat less than 30 percent of this area was originally forest, and that approximately 30 tons per hectare of soil humus is oxidized after cultivation has commenced, the total loss of carbon from the biosphere to the atmosphere over 20 years was close to 26 Gt, an average of 1.3 Gt annually.

The loss of carbon dioxide from agricultural clearing during the past 100 years can likewise be estimated from the probable worldwide increase in cultivated land. From 1860 to 1970, the world population rose from somewhat more than 1200 to over 3400 million people, a nearly threefold increase. Agricultural yields per hectare increased much more slowly than food demand arising from the increase in population and rise in income. The increased food demand must have been met mainly by enlarging the area of cultivated land, perhaps from 500 million hectares in 1860 to 1370 million hectares in 1970. Assuming that about 30 percent of the new farm land was originally forest and that 30 tons of carbon dioxide per hectare in the soil humus were oxidized, addition of carbon dioxide to the atmosphere could have been 72 Gt in 110 years. This is less than 3 percent of the mass of organic carbon in the biosphere but nearly two thirds the addition to the atmosphere from fossil-fuel combustion. The total mass of carbon dioxide released by fossil-fuel combustion and land clearing combined could have been over 200 Gt, a third of the initial atmospheric content.

Populations in the less-developed countries have increased roughly exponentially, and agricultural clearing may well have followed the same course (see Chapter 10), with an annual growth rate of about 3 percent from 1860 to the present. The total potential arable land on earth is limited, however, to 2600 million hectares (Revelle, 1976) compared with a presently cultivated area of about 1400 million hectares. Even assuming that all presently uncultivated but potentially arable land is now in forests, the maximum future addition of carbon dioxide from land clearing is only about 240 Gt, less than 5 percent of the total carbon reserve in fossil fuels. Land clearing in the future is likely to follow a logistic curve, similar to that used by Keeling and Bacastow (see Chapter 4) for fossil-fuel carbon dioxide production but with a maximum rate of clearing around 2010, compared with the maximum carbon dioxide production from fossil fuels near the end of the twenty-first century.

Reforestation, the planting of trees in previously cleared areas, will increase the size of the terrestrial biosphere at the expense of atmospheric carbon dioxide. According to Bolin (1977) reforestation has occurred over 30 million to 60 million hectares in China and about 10 million hectares in other less-developed countries, while in the developed countries it is not certain whether there has been an increase or a decrease in the forested area by as much as 12 million hectares. The increase in net primary production, less oxidation and fires, resulting from reforestation may be as much as 2.5 tons per hectare per year; hence, the annual subtraction of carbon from the atmosphere by reforestation may lie between 0.07 and 0.2 Gt.

Bolin (1977) suggests that wood cut for lumber, paper pulp, and especially firewood may exceed net primary production in existing forests. Wood is “the poor man’s oil,” and in Africa, Latin America, and parts of Asia the average per capita use of firewood (primarily for cooking) is about 0.7 ton per year (see Chapter 10), corresponding to 350 kg of carbon per capita, or 0.875 Gt for the 2500 million people of the poor countries. This is more than the FAO estimate of the harvest of roundwood

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
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from the world’s forests. But, even accepting this figure, the total wood harvest is only 5 percent of the estimated net primary production in tropical and subtropical forests (see Table 10.1). Thus, it seems unlikely that wood cutting has resulted in a net addition of carbon to the atmosphere.

As G. C. Delwiche of the University of California at Davis (personal communication) has pointed out, several other human activities may result in adding or subtracting carbon from the biosphere or the inorganic components of the soil. Cultivation of calcareous soils in arid and semiarid regions, and expansion of areas of irrigated land, may result in an uptake of carbon by the soil as calcium carbonate or organic matter. Drainage and cultivation of swamps and marshes will liberate organic carbon to the atmosphere as will some improved forest practices. At present, we are unable to estimate the magnitude of these various effects.

M. Stuiver of the University of Washington (personal communication) has estimated that the total amount of carbon dioxide released from the terrestrial biosphere by clearing of forests and other human activities corresponded to 120 Gt of carbon between 1850 and 1950 and possibly to an additional 25 Gt between 1950 and 1970, a total of 145 Gt. This is approximately twice our estimated value. He bases his estimates on an observed decrease with time in the ratio of the two stable carbon isotopes, 13C/12C, in the atmosphere over the past 120 years, as measured in tree rings. Biosphere carbon, like fossil-fuel carbon, is deficient in 13C, and hence a transfer of carbon from the biosphere and the fossil-fuel reservoir to the atmosphere should result in a lowering of the atmospheric ratio of 13C to 12C. Tree-ring measurements by Stuiver and others suggest a lowering of this ratio by 1.5 per thousand since 1850. This is a small effect and could be largely accounted for by changes in the heights of the sample trees relative to the average height of the leaf canopy in the surrounding forest. (Trees at different relative heights in the forest are bathed in carbon dioxide with a different 13C–12C ratio.) The measurements do not show an exponential decease in 13C/12C with time, which should be expected from the accelerating use of fossil fuels and the probable acceleration of land clearing in less-developed countries. Moreover, the changes in the 13C–12C ratio would not occur unless the carbon released from the biosphere had remained in the atmosphere. This requires the carbon dioxide content of the atmosphere in the middle of the nineteenth century to have been only about 268 parts per million, much lower than the most probable value. It is more likely that a large fraction of the released carbon has been reabsorbed in the biosphere.

Many more measurements of possible changes with time in the 13C–12C ratio, using tree-ring sequences from trees in isolated locations, are needed to resolve this question. The 13C–12C ratio should also be measured directly on air samples collected over a wide range of geographic locations at the present time as a baseline for the detection of future changes.

Besides clearing of land for agriculture, the two principal effects of human activities on the biosphere should be an increase in net primary production over oxidation, resulting from the fertilization of the biosphere by carbon dioxide added to the air, and a similar increase resulting from the excess of nitrogen fixation over denitrification.

According to G. C. Delwiche and G. K. Likens of the University of California at Davis (personal communication), the present annual fixation of atmospheric nitrogen from human activities amounts to 68 Mt (40 Mt of nitrogen fertilizer, 10 Mt of other chemical products, and 18 Mt in combustion of fossil fuels). This is 35 percent of world total nitrogen fixation. The remaining 65 percent (127 Mt) is fixed by terrestrial and marine bacteria—including blue-green algae—and in lightning discharges. Denitrification is estimated roughly at 160 Mt annually. Thus, the excess of nitrogen fixation over denitrification is perhaps 30 Mt per year. If this

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
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increment of fixed nitrogen were stored in the terrestrial biomass, principally in the wood of forest trees that have a high carbon-to-nitrogen ratio, the carbon added annually to the biosphere would be 2300 Mt. If it were stored in soil humus, the additional carbon would be only 300 Mt. It seems unlikely that the excess fixed nitrogen has had much effect on the marine biosphere, which, as we have indicated, is limited by the flux of phosphorus as well as nitrogen to ocean surface waters.

Any uptake of carbon dioxide by the biosphere requires that the rate of net primary production by photosynthesis should be greater than the rate of oxidation by microorganisms, animals, and fires. It has long been known that photosynthetic production increases when crop plants are bathed in a carbon dioxide-rich atmosphere, provided other factors of production are not limiting. In the natural world, however, plant production is limited by the availability of nitrogen, phosphorus, and minor nutrients; the intensity of solar radiation; the adequacy of the supply of water; and environmental stresses, such as low and high temperatures; in addition to the concentration of atmospheric carbon dioxide. Experimental data indicate that in the absence of other limiting factors, net primary production increases with the logarithm of the increase in carbon dioxide (see Chapter 10 and Bacastow and Keeling, 1973). In the heterogeneous conditions of the natural world, we may introduce a factor β, which takes account of the presence of other limiting factors. This leads to simple mathematical relationships among net primary production, atmospheric carbon dioxide, photosynthesizing biomass, rate of oxidation, and the total weight of carbon in the biosphere (see Chapter 10).

Except for the changes associated with agricultural land clearing, the portion of the terrestrial biomass that carries out photosynthesis may not vary significantly in the future, if, as seems likely, plants now optimally occupy the land surfaces available to them. The size of the terrestrial biosphere is then uniquely determined by the atmospheric carbon dioxide content. Computations by Revelle and Munk (see Chapter 10) based on this assumption indicate that the mass of the biosphere would ultimately increase by 700 Gt or 25 percent, if all fossil fuels are burned over the next one or two centuries, and the mass of carbon dioxide in the ocean and the atmosphere would be correspondingly reduced. The peak concentration of carbon dioxide in the atmosphere near the end of the twenty-second century would be 2900 Gt or about four times the present value. Somewhat similar outcomes would result from any other model in which the size of the photosynthesizing biomass does not vary linearly with the size of the biosphere. Present empirical data on past changes in the terrestrial biosphere and the ecological relationships involved do not provide evidence sufficient to determine a choice among possible models of the terrestrial biosphere. Consequently, a considerable range of uncertainty must be associated with predictions of atmospheric carbon dioxide levels.

We conclude that a principal uncertainty in estimating the likely increase in atmospheric carbon dioxide from future fossil-fuel combustion lies in our present inability to estimate the uptake of carbon by the terrestrial biosphere. Strenuous research efforts and extensive surveys should be undertaken to reduce this uncertainty.

GEOLOGIC SOURCES AND SINKS

Beside absorption of carbon in the oceans and terrestrial biosphere, five geological processes can appreciably affect the carbon dioxide content of the atmosphere over periods of centuries: (1) the flux of carbon dioxide from the earth’s interior through volcanos and in other ways, (2) weathering of silicate rocks on land, (3) weathering of carbonate rocks on land and the subsequent redeposition of carbonates on the

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
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sea floor, (4) burial of organic material in marine and lacustrine sediments, and (5) solution of calcium carbonate in the sediments of the sea floor.

Bowen (1966) estimates that the flux of carbon dioxide from the earth’s interior adds about 40 Mt of carbon to the atmosphere each year. This is less than 1 percent of the addition from fossil fuels. It is approximately balanced by the removal of carbon dioxide in the weathering of silicate rocks, which is estimated by Garrels et al. (1975) from the average content of dissolved silica in river waters to be 42 Mt of carbon per year.

Weathering of limestones and dolomites removes carbon dioxide from the air in the ratio of one mole per mole of calcium or magnesium. The amount of bicarbonate in river waters resulting from limestone and dolomite weathering is about twice the amount of carbon dioxide removed from the air; half of this quantity is contributed by the rocks themselves.

The total quantity of bicarbonate ions carried to the sea by rivers each year is estimated by Garrels et al. (1975) as equivalent to 368 Mt of carbon. Of this amount, 205 Mt is removed from the atmosphere—163 Mt from the weathering of carbonates and 42 Mt from the weathering of silicates. The annual loss of atmospheric CO2 to river waters is thus about 3 percent of the total added annually at present to the atmosphere by combustion of fossil fuels and land clearing. In a steady state, the carbon from the weathering of limestone and dolomites is ultimately restored to the atmosphere when calcium carbonate is precipitated in the upper layers of the sea and deposited in ocean sediments, releasing an amount of carbon dioxide equal to the amount of carbonate deposited. Broecker (1974) estimates the rate of deposition of carbon as calcium carbonate as 69 to 216 Mt per year, in fair agreement with the calculated quantity removed from the atmosphere by weathering of limestones and dolomites.

Future increases in the amount of carbon dioxide in the atmosphere should increase the rate of weathering of both silicate and carbonate rocks, while calcium carbonate precipitation in the sea should be reduced because of the increased acidity resulting from addition of carbon dioxide to the seawater directly from the air. But as the following considerations indicate, the effects are likely to be small.

For silicate rocks, the rate of weathering could conceivably grow as rapidly as the rate of increase in atmospheric carbon dioxide. A quadrupling of atmospheric CO2 would then increase the rate of extraction of carbon dioxide from the atmosphere to 170 Mt of carbon per year. This is less than 0.01 percent of the postulated increase in atmospheric CO2.

With respect to carbonate rock weathering it might be supposed that if present river waters are saturated with calcium carbonate, a quadrupling of atmospheric carbon dioxide would result in a 60 percent increase in the rate of extraction of carbon dioxide from the air, since at equilibrium

To a fair approximation,

and therefore

In fact, present river waters are substantially undersaturated with calcium car-

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
×

bonate. Their content of “free” carbon dioxide is on the average ten times higher (and their CO32− concentration correspondingly lower) than that at equilibrium with present atmospheric carbon dioxide, while the average calcium content is somewhat lower than the saturation value for water with a CO2 content at equilibrium with the present pressure of CO2 in the atmosphere (Garrels et al., 1975). The marked undersaturation of calcium carbonate in present rivers is demonstrated by the close correlation between the contents of calcium and bicarbonate in different rivers and the total dissolved solids. If the rivers were saturated with calcium carbonate, their calcium and bicarbonate concentrations would be relatively constant regardless of the total quantities of dissolved solids.

Garrels et al. (1975) examined the content of “free” carbon dioxide in 17 different rivers covering a wide range of latitudes, rock types in the drainage basins, river sizes and stages, and total dissolved solids. The CO2 pressures were 3–30 times higher than that in the present atmosphere and were nearly independent of all the above river characteristics. The average CO2 pressure was about the same as that of the interstitial waters of soils. Apparently the CO2 pressure in the world’s rivers depends on microbiological oxidation processes that are relatively independent of the carbon dioxide pressure in the atmosphere.

It seems reasonable to conclude that even with a quadrupling of atmospheric carbon dioxide, the annual loss of carbon in river waters would not exceed 500 Mt, less than 0.03 percent of the additional carbon in the atmosphere.

Organic carbon is deposited in marine sediments at a rate, according to Broecker (1974), about one fourth of that of carbon as calcium carbonate, and this represents a net loss of atmospheric carbon that must, on a geologic time scale, be counterbalanced by other processes. The rate of deposition of organic matter is apparently determined by the amount of phosphorus carried by rivers from the land to the sea, which may be related to the atmospheric carbon dioxide concentration through weathering processes. Photosynthetic reduction of carbon dioxide by plants and the subsequent deposition of a small fraction of the plant remains in marine sediments, leaving the photosynthetically produced oxygen in the air, is probably the chief process that, over geologiceons, has resulted in our planet having an atmosphere with free oxygen. But the present annual rate of deposition is probably not more than 20 to 50 Mt. This is negligible compared with the rate of addition of carbon dioxide to the air.

A process of greater potential significance could be the dissolution of calcium carbonate in marine sediments. The upper water layers of the sea in the tropics and subtropics appear to be markedly oversaturated with respect to both calcite and aragonite, the two crystalline forms of calcium carbonate. Carbonate-rich sediments, including coral reefs and atolls and deposits that may be formed inorganically, are widespread at shallow depths. In the deeper waters of low latitudes and midlatitudes, pelagic sediments containing a high percentage of calcareous skeletons of marine animals and plants occur above a “compensation depth,” at which, because of the combined effects of pressure and temperature, the water appears to be about saturated with calcium carbonate. Above the compensation depth, the waters are supersaturated; below it, they are undersaturated, and the fraction of calcium carbonate in pelagic sediments sharply diminishes. The compensation depth is different in different oceans, because of their differing carbonate ion concentrations, being about 4500 and 1000 m for calcite and aragonite, respectively, in the Atlantic (and probably the Indian Ocean), and perhaps as shallow as 500 and 300 m for these two mineral species in the North Pacific north of 10° N. South of 10° N, the saturation depth for calcite in the Pacific is probably between 3500 and 4000 m, judging by the distribution of calcium carbonate in the pelagic sediments (Sverdrup et al., 1942).

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
×

When some of the carbon dioxide produced by fossil-fuel combustion and forest clearing reaches the deep sea, the carbonate ion concentration in the deep water will diminish and consequently the compensation depth will rise. Calcium carbonate on the surface of the sediments should dissolve at depths between the old and new compensation depths. At a depth of the order of 10 cm below the sediment surface (beneath the zone of burrowing organisms), calcium carbonate will be effectively insulated from the overlying waters and will dissolve very slowly, if at all. Between 1 and 10 cm below the sediment surface, the rate of solution of sedimentary calcium carbonate is limited by the rates of exchange between the interstitial water in the sediment and the overlying seawater. Experimental data summarized by Guinasso and Schink (1975) indicate that this rate is several orders of magnitude lower than the rate of molecular diffusion in water. Calculations based on the data suggest that the rate of solution of calcium carbonate, even if the superjacent waters become more acid, is likely to correspond to less than 1 Gt of carbon a year, a relatively small proportion of the anticipated increase in atmospheric carbon. The alkalinity of the seawater, and hence its ability to absorb carbon dioxide, will increase in proportion to the quantity of calcium carbonate dissolved.

If the compensation depth rose to the sea surface, the ultimate result could be, as Keeling and Bacastow show (see Chapter 4), that the carbon dioxide content of the atmosphere in the early twenty-second century would, ceteris paribus, be only five times the preindustrial value, instead of eight times if carbonate solution did not occur, and it would drop off to twice the preindustrial value a thousand years from now. However, this would require solution of a layer of shallow-water carbonate sediments several meters thick, which probably could not occur without massive human intervention. Observed mixing rates resulting from the activities of burrowing organisms in shallow water sediments (Guinasso and Schink, 1975; Guinasso, 1976; Sundquist et al., 1977; Goreau, 1977; Berger, 1977) indicate that the diffusion of interstitial waters between these sediments and the superjacent waters is slow, although much more rapid than in deep-sea sediments, and hence the rate of solution of calcium carbonate in undisturbed sediments will result in only a small rate of increase in the capacity of the ocean to absorb carbon dioxide.

Up to the present, this effect is probably negligible. The carbon dioxide added from the air has not penetrated to sufficient depths to raise the compensation depth, except in the North Pacific, where the compensation depth may have become a few meters shallower. But the total quantity of carbon dioxide in the top meter of sediments in the uppermost thousand meters of the entire Pacific Ocean corresponds only to 230 Gt of carbon (Sverdrup et al., 1942), and perhaps 90 percent of this lies south of 10° N where the compensation depth has almost certainly not been affected. An upper limit to the quantity of sedimentary calcium carbonate dissolved up to the present is probably about 1 Gt of carbon, less than one fifth of the addition from fossil fuels in the single year 1973.

We conclude that none of the geologic processes described above would significantly affect the increase in atmospheric carbon dioxide from fossil-fuel combustion that is likely to occur during the next few centuries.

CLIMATE

Any useful prediction of the effect of carbon dioxide added to the air must depend on an understanding of the physical processes that determine climate. In general, we know that both carbon dioxide and water vapor in the air absorb and reradiate infrared radiation. Hence the temperature of the lower air must be at the level where the fraction of the infrared radiation passing outward to space is equal to the total incoming radiation from the sun, less the amount reflected from clouds

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
×

and from the earth’s surface. Because of this greenhouse effect produced by carbon dioxide and water vapor, the average temperature at the earth’s surface is about 30°C higher than it would be in the absence of these two gases.

As mentioned above, Manabe and Wetherald (1975) have calculated the magnitudes of the effects of a doubling of the atmospheric carbon dioxide—it would result in about a 2– 3°C rise in the earth’s average air temperature, about a 2 percent increase in relative humidity, and a 7 percent increase in average precipitation. The temperature rise would be greater at high latitudes, about 10°C at latitude 80° N. Temperatures in the stratosphere would decrease. At a height of 30 km, this decrease would be about 5°C. The increase in average air temperatures near the ground would be about 4–6° for a fourfold increase in CO2 and 6–9° for an eightfold increase (see Chapter 9). A global average rise of 6°C would be comparable with the extreme changes of global temperature believed to have occurred in geologic history of the last billion years.

Although the model used by Manabe and Wetherald is the most sophisticated that can be constructed at present, it does not take account of four climatic feedback mechanisms that could change the calculated results: atmosphere-ocean carbon dioxide interaction, cloud reactions, ocean temperature and heat transport changes, and aerosol reactions.

Atmospheric-Ocean Carbon Dioxide Interactions

If an increase in atmospheric carbon dioxide raises the air temperature over the sea it will also warm the waters near the surface and thereby reduce the solubility of carbon dioxide. Some carbon dioxide will be driven from the sea into the air, or conversely less carbon dioxide will be absorbed. A more important effect will be the increase in the vertical density gradients in the ocean, which may significantly change interactions between the air and deep-ocean circulation.

Cloud Reactions

An increase in the water vapor content of the air could result in an increase in low-level clouds. This might be expressed as a larger area of the earth’s surface being covered by clouds and also by changes in the heights of cloud tops, which determine their temperature and hence their radiative properties. An increase in the surface area covered by clouds would result in a proportional increase in albedo—that is, in the reflectivity of the earth. A higher albedo in turn would reduce the percentage of solar radiation reaching the earth’s surface and thus would cause a lowering of air temperatures. This could partly compensate for the rise in temperature that would otherwise be produced by the rise in atmospheric carbon dioxide, although Manabe and Wetherald find in some of their numerical experiments that increases in low cloudiness are offset from the standpoint of radiation balance by changes in middle and high cloudiness.

Ocean Temperature and Heat Transport Changes

The Manable-Wetherald model assumes a swamplike ocean with no heat capacity but with the ability to provide water vapor to the atmosphere. Their result would have been completely different if a constant sea-surface temperature had been assumed, implying an ocean of infinite heat capacity. This would greatly diminish the response of the ocean-atmosphere system to changes in the carbon dioxide content or even to changes in solar radiation. In fact, the ocean has a large but finite heat capacity, and it surrounds continental land masses that have very low

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
×

heat capacity. This fact is particularly important in considering seasonal cycles of heating and cooling, because of the long lag time and persistence of changes in ocean temperatures near the surface.

The ocean also transports heat from low latitudes to high latitudes. The quantity of heat transported poleward by the oceans is about equal to that transported by the air. Any model that attempts to predict atmospheric temperature changes, let alone the diffusive mixing in the oceans that determines the amount of carbon dioxide absorbed, must take into account possible changes of the ocean circulation. That is, it must be a coupled ocean-atmosphere model. The construction of such a model is difficult at present, because the main motions of the sea appear to be in eddies of the order of 200 km in diameter (much smaller than the main components of the atmospheric circulation), and satisfactory methods for arriving at even a statistical description of these oceanic motions have not yet been developed. Ways of expressing the effects of these eddies in terms of larger-scale oceanic parameters are needed.

Aerosol Interactions

Aerosol particles absorb and scatter sunlight, and they also absorb and reradiate infrared radiation. The backscattering of solar radiation tends to increase the net albedo, while the absorption and reradiation of sunlight in effect reduce the net albedo. Thus, “dark” particles, which have a high ratio of absorption to backscatter, tend to decrease the albedo over a light surface such as a snow field, a low cloud deck, or desert areas. Aerosols over the ocean, which has a low albedo, tend to increase the net albedo. Modeling the interaction of aerosols such as dust, sea salts, or sulfates cannot be done very well at present (see Chapter 3) because their introduction into the atmosphere is either not coupled to the dynamics of climate (fires, volcanoes) or is coupled in a very complex way (wind-raised dust and sea spray).

OUTLOOK

We now summarize what will be needed to improve our understanding of the phenomena involved in the carbon dioxide problem and indicate some of the elements that should be included in planning to close gaps in our knowledge, so that future decisions regarding the exploitation of energy resources can be made on as sound a basis as possible.

CARBON CYCLE

Further work is needed to understand the carbon cycle that takes into account buffering by the biosphere, rock weathering, and calcium carbonate precipitation and solution in lakes and the oceans. A program of measurement of tracers that indicate the mixing rates of the upper oceans (e.g., tritium), and also of oceanic carbon dioxide content both near the surface and at a series of depths below the surface, should be undertaken. Present efforts to develop better methods of studying the ocean circulation in three dimensions should be broadened and intensified. Research on the biological and chemical processes in ocean banks and reefs, and in the calcareous oozes of the deep seafloor, should be encouraged. Baseline measurements of soil humus and of the other living and dead components of the terrestrial organic pool should be made on a worldwide basis, and these measurements should be repeated at suitable intervals to allow direct estimates of the net uptake or net release of carbon from this reservoir.

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
×

CLIMATE

Clearly, a fundamental understanding of the mechanisms and dynamics of climate is needed. The question of how to meet that need has been addressed in a number of recent reports (U.S. Domestic Council, 1974; Matthews et al., 1971; GPS No. 16, 1975; U.S. Committee for the Global Atmospheric Research Program, 1975). Broadly speaking, these reports agree that modeling is the most promising approach to understanding climate. They document the considerable gains achieved through modeling but point out that much remains to be done. Further progress will require new observations, both to point the way to new advances and to test models, and technological innovation in observational and data-gathering systems, more powerful computers, and perhaps new institutions through which to plan, organize, and execute research programs and to provide access to the results of these programs to the widest possible audience (see Chapter 8).

Problems that will have to be considered in improving our understanding of energy-related aspects of climate include the following.

Ocean-Atmosphere Interaction

The oceans play an important part in storing and transporting heat involved in climate dynamics. Coupled ocean-atmosphere models, including mechanisms of sea-ice formation and its effect on radiation balance and of the exchange of heat between the ocean and the atmosphere, are needed (for further discussion see Ocean Science Committee, 1975).

Radiation Balance

The physical and dynamical mechanisms of cloud formation are not well understood. Cloud formation in turn affects the radiation balance. This is one of the most challenging problems in climatic modeling.

Fluorocarbons and oxides of nitrogen from fertilizers can, like carbon dioxide, cause a greenhouse effect. Global warming due to the use of chlorofluoromethanes and the increase in atmospheric content of nitrous oxide has been predicted (Committee on Impacts of Stratospheric Change, 1976; Chapter 4). Thus, carbon dioxide and other atmospheric gases may act together to produce a global warming trend.

Monitoring

The principal requirement for improvement of general circulation climatic models is an integrated program of measurements of the distribution in space and time of key physical properties and processes in the ocean and the atmosphere—key properties indicated as essential by the models themselves. These include many kinds of observation that can be made from satellites, such as those of total solar flux, net radiation budget, cloud cover, albedo, extent of snow, sea ice and polar ice sheets, tropospheric and stratospheric aerosols, and atmospheric turbidity. Other properties and processes must be measured in situ, including river runoff, soil moisture (to some extent measurable from space), subsurface ocean temperatures, precipitation, sea level, carbon dioxide, and ozone. Many of the necessary measurements are included in the Global Atmospheric Research Program under the joint aegis of the World Meteorological Organization and the International Council of Scientific Unions. Almost as important as the actual measurements are methods of data compilation and processing that will compress the enormous amounts of data that will be obtained into usable information that can be handled in practical models of the ocean-atmosphere system (see Chapter 8).

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
×

The benefits to be gained from establishing a global monitoring system are not only scientific but also practical, for such a system can provide us with early warning of climatic change. Climate cannot be predicted in the sense that weather can (see Chapter 9). Weather is determined by the short-term behavior of the atmosphere and oceans, which is, in turn, determined primarily by initial conditions. The statistical properties that define climate are determined primarily by boundary conditions and the basic parameters of the system; initial conditions are not likely to be as relevant to these properties. The potential climate changes discussed in this report are largely responses to a change in one of those basic parameters: the total quantity of carbon dioxide in the air, waters, and organic matter of the earth. An effective global monitoring system, through the observation of regional and local climatic changes as well as other changes in physical parameters as outlined above under Recommendations, may be expected to provide evidence of such responses before their consequences become unpleasantly obvious.

APPENDIX A:
ENERGY CONSUMPTION IN 2075

Hans H. Landsberg and Harry Perry, Resources for the Future, Inc.

INTRODUCTION

The following calculations require a preliminary note. Anyone having engaged in projections for even a decade ahead is aware of the fragility of such estimates. Vulnerability tends to increase with time horizons as well as with widening geographic scope. Thus a 100-year projection must offhand seem foolhardy. It probably is. As the only excuse for having engaged in it the authors offer the insistent demand for it by those asked to speculate on the consequences of energy consumption far enough into the future to enable them to deal with significant cumulative effects.

Undertaking a methodical projection then has the advantage of being subject to inspection and scrutiny but does not bring one any closer to a “forecast.” In this context it is desirable to proceed with the least sophistication. Thus the reliance on population and per capita energy consumption in preference to any more complex formula. Both involve arbitrariness and provide fuel for disagreement. As providing orders of magnitude, however, they seem serviceable and keep the discussion on the level of simplicity that matches their merit. It is hoped that no reader or user will be tempted to elevate them to any higher status or divorce them from their sole intended function as tools for the calculations that utilize them.

ASSUMPTIONS CONCERNING POPULATION

World population is expected to stabilize at some time in the future, but the net reproduction rate (NRR) should reach 1.0 at different times for different regions. Table A.1 indicates the five-year range when the NRR is estimated to reach 1.0 for each region of the world and the population that would exist in each region in 2075 as a consequence. The year range selected for the NRR to reach 1.0 was a “most probable” estimate for each region.

Based on these estimates, world population would be about 10.7 billion in 2075.

ASSUMPTION CONCERNING ENERGY PER CAPITA CONSUMPTION

Annual per capita energy consumption now varies widely among regions. For the world as a whole, it is about 52.5 × 106 Btu. For the United States the annual per

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
×

TABLE A .1 World Population in 2075a

Region

Year Range NRR Equals 1.0

Population in 2075 (millions)

United States

1975–1980

285

North America (excluding United States)

1975–1980

36

Western Europe

1975–1980

579

Oceania

1985–1990

31

Latin America

2010–2015

989

Japan

1995–2000

139

Other Asia

2019–2024

4901

Africa

2020–2025

1430

Soviet Union

1975–1980

332

Communist East Europe

1990–1995

139

Communist Asia

2000–2005

1835

TOTAL WORLD

 

10696

aSource: T. Frojkn (1973).

capita consumption is 306 × 106 Btu, while for Africa it is only 9.2 × 106 Btu, or 3 percent of the level of the United States.

Estimates of future world per capita energy consumption were made by Weinberg and Hammond (1971). They projected that annual world per capita consumption would stabilize at about twice the current U.S. level, or about 600 × 106 Btu. In a more recent article, Puiseux (1975) estimated that an “advanced civilization” could be achieved with a world per capita energy consumption of 185 × 106 Btu to 264 × 106 Btu. He also estimated a world population of 10 billion people.

Table A.2 shows an estimate of the levels of energy consumption that might be reached in different regions by 2075. For the regions that now have large per capita energy consumptions, the values shown in Table A.2 are the maximum that is believed will be used in any future year. Values of per capita consumption below 265 × 106 Btu per year are for regions that would not yet have reached a maximum per capita consumption by 2075 since they currently have such a small base of energy use. North American values already exceed the maximum and have been allowed to increase at a moderate pace.

TABLE A.2 Per Capita Consumption and Total Energy Consumption by Regions, 2075

Region

Per Capita Consumption (Btu × 106)

Total Consumption (Btu × 1015)

United States

741

211

North America (excluding United States)

623

23

Western Europe

290

168

Oceania

577

18

Latin America

130

128

Japan

497

69

Other Asia

40

194

Africa

64

92

Soviet Union

760

252

Communist East Europe

580

81

Communist Asia

58

107

TOTAL WORLD

 

1343

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
×

LEVELS OF ENERGY CONSUMPTION AND HEAT RELEASE

By combining Table A.1 with column 1 of Table A.2, it is possible to determine the heat released annually in 2075 for each of the regions shown on the tables. These data are shown in column 2 of Table A.2. The heat released per km2 for each region in 2025 and 2075 can be calculated and is shown and compared with the heat that each region would receive from solar energy in those years. These data are shown in Table A.3.

Table A.4 shows the projected world consumption of fuels for 1973, 2025, and

TABLE A.3 Estimated Energy Densities by Region

 

Btu × 109/km2

Man-made Heat Release as Percentage of Solar Energy

Region

2025

2075

2025

2075

United States

20.93

22.56

0.289

0.311

North America (excluding United States)

1.56

1.87

0.022

0.026

Western Europe

33.60

44.03

0.464

0.608

Oceania

1.76

2.13

0.024

0.029

Latin America

5.54

6.24

0.077

0.087

Japan

182.80

186.29

2.523

2.571

Other Asia

10.65

12.50

0.147

0.172

Africa

2.57

3.03

0.036

0.042

Soviet Union

10.89

11.26

0.150

0.155

Communist Eastern Europe

60.39

63.65

0.834

0.879

Communist Asia

6.28

9.08

0.087

0.126

TOTAL WORLD

8.63

9.89

0.119

0.136

TABLE A.4 World Heat Releasea and Carbon Dioxide and Particulate Inputsb

 

1973

2025

2075

2090c

2170d

Ultimate Total

1. Heate

 

 

 

 

 

 

Gt of coal equivalent per year

7.6

42

48

 

 

7500

Quadrillion (1015) Btu per year

220

1170

1340

 

 

210000

Cumulative percent of total recoverable reserves of fossil fuels

2.5

17

42

 

 

100

2. Carbon Dioxidef

 

 

 

 

 

 

Annual production, Gt of carbon

4.8

16

 

37

13

5000

Annual increase in air (airborne portion), Gt of carbon

3.0

9

 

21

0

900

Airborne portion/production, percent

56g

53

 

57

0

18

Annual increase of atmospheric carbon relative to 1860, percent

0.5

1.3

 

3.4

0

Total atmospheric carbon, Gt

700

945

 

1950

2900

1500

3. Particulatese

 

 

 

 

 

 

Emissions, Mt

140

800

2800

 

 

 

aIncluding all nonrenewable energy forms.

bIncluding coal, oil, and natural gas for carbon dioxide and particulate calculations.

cYear of peak carbon dioxide production in Revelle-Munk model (see Chapter 10).

dYear of peak atmospheric carbon dioxide in Revelle-Munk model (see Chapter 10).

eCalculated with Perry-Landsberg model (see Chapter 1).

fCalculated with Rovelle-Munk model (see Chapter 10).

gAverage airborne fraction from 1959 to 1973 inclusive.

Suggested Citation:"Overview and Recommendations." National Research Council. 1977. Energy and Climate: Studies in Geophysics. Washington, DC: The National Academies Press. doi: 10.17226/12024.
×

2075. Comparing these values with those in Table A.3 using a total inhabited surface area of 136 × 106 km2 indicates that by 2075 all the energy needed could still be provided by fossil fuels, mostly from the production of coal.

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