Ari A. Patrinos
Associate Director of Energy Research
Office of Health and Environmental
Research Department of Energy
The next group of papers discusses the environmental side of the Biological and Environmental Research (BER) program, and a few comments here will set the context.
Over the last 50 yr, the treatment of the environment in the BER program, like the treatment of the environment in most federal research programs, experienced a paradigm shift. The shift was away from the perception that the environment was simply the medium by which various insults, whether chemical or radiation, reached humans to the view that the environment is the ultimate recipient of those insults. We have seen many examples of that shift.
The BER program has done much important work and made major contributions to the understanding of many environmental problems—the invention of radioecology by Eugene Odum at the Savannah River Laboratory in the early 1960s, the first attempts to develop climate models by Chuck Leith at the Lawrence Livermore National Laboratory, the seminal work that Warren Washington did at the National Center for Atmospheric Research, the studies of the fallout of radioactivity from various weapons tests, the theoretical and practical studies dealing with turbulent diffusion and transport by Frank Gifford at the Oak Ridge Atmospheric Turbulence and Diffusion Laboratory, the studies of long-range transport of plumes from power plants and the early work dealing with the acid-precipitation problem, and the launching of the first federal research program 20 yr ago to look at the greenhouse effect, which at the time was known only to very few scientists and science managers. It also added its unique twist and its unique culture to the milieu of the federal agencies and the scientists who attacked many of those problems. The program sometimes stood alone in the midst of various interagency battles, but more often it was an essential and serious team player in interagency efforts. The BER program has contributed uniquely in promoting environmental science and in bringing environmental sciences to bear in the solution of national problems.
One unique characteristic of the program is its position in the Office of Energy Research and the Department of Energy (DOE). Where you stand on an issue depends on where you sit. The BER program is one example of that saying, and because of its position in the high-powered, big-time physics environment in which it exists in DOE, the program has managed to aim many of the physics capabilities, such as remote-sensing technologies and high-performance computing, at environmental problems that have eluded other agencies. Also, it has brought a culture of multidisciplinary science to the solution of environmental problems through its ability to launch teams of investigators, bringing together laboratory and university scientists to address problems. We are proud of the culture that we have applied to many environmental problems.
Atmospheric Carbon Dioxide: Contemporary Budget, Historical Context, and Implications for the Future
Chair, Department of Earth and Planetary Sciences
Carbon dioxide (CO2) produced by the burning of fossil fuel—coal, oil, and natural gas—is the largest waste product of modem industrial society. Emissions to the atmosphere globally in 1997 exceeded 6 billion tons, more than 1 ton for every person on the planet. The United States, with about 5% of the world's population, was responsible for 22% of global emissions. The People's Republic of China, which ranked number 10 in 1950, is now number 2, having just surpassed the former Soviet Union. Given current trends, there is little doubt that China will soon be number 1 and will set the pace globally for CO2 in the future.
We have an excellent account of variations in the concentration of atmospheric CO2 over the last 160,000 yr. The modem instrumental record, based on sampling of air at remote sites around the world, dates to 1958. The continuity and quality of the modem data for Mauna Loa, Hawaii, and the South Pole, displayed as weekly averages of daily measurements in figures 1 and 2, are tribute to the ingenuity and skill of C.D. Keeling at the Scripps Institute for Oceanography, who, long before it was popular, saw the need for a precise record of variations in atmospheric CO2.1 Studies of gases trapped in polar ice, pioneered by Swiss and French teams of scientists led by Hans Oeschger and Claude Lorius, extend the record back in time and indicate that the modem rise in CO2 began in the early part of the 18th century.
A summary of the ice-core data is presented in figures 3 and 4.2 The abundance of CO2 was relatively constant, at about 280 parts per million by volume (ppmv) of dry air, for more than 10,000 yr after the end of the last ice age. It rose from 280 ppmv in the 18th century to about 315 ppmv in 1958 and has since continued to climb to a contemporary value close to 360 ppmv. Concentrations of CO2 were much lower, about 200 ppmv, during both the last and the penultimate ice ages, which ended about 15,000 and 125,000 years ago, respectively.
The modem rise, as we shall see, may be attributed to complex contributions from industrial and agricultural society: Emission of CO2 associated with combustion of fossil fuel played a major role, but that was not the only influence. At least a portion of the modem increase must be attributed to transfer of carbon to the atmosphere from soils and the terrestrial biosphere. A particular challenge, if we are to forecast the future course of CO2, is to differentiate between contemporary contributions from fossil fuel and the soil-biosphere system.
Thanks mainly to work sponsored by the Department of Energy, we have a reasonably accurate estimate of the quantity of CO2 added to the atmosphere in recent years by the burning of fossil fuel. With somewhat lower precision based on historical data, trends in emission can be reconstructed back to the middle of the 19th
century. Results of this analysis, adapted from C.D. Keeling1 and Marland and co-workers,3 are presented in figure 5. As the data indicate, until recently, combustion of coal was the major industrial source of CO2. Oil supplanted coal as the largest source of CO2 in the late 1960s and the contribution from natural gas has been increasing in recent years.
Per unit of energy delivered, coal is the largest source of CO2, followed by oil and gas. In 1991, oil accounted for 37.1% of global consumption of commercial energy, coal 29.2%, and gas 23.7%, with the balance attributable to nuclear (7%) and hydroelectric (3%). The relative contributions of oil, coal, and gas to industrial emissions of CO2 in the same year were 42%, 38%, and 17%, respectively; the balance was associated with gas flaring and the manufacture of cement.
This paper seeks to develop a consistent model for the contemporary budget of atmospheric CO2. Emission of CO2 from industrial sources from July 1991 to July 1994 would have been sufficient to increase the concentration of atmospheric CO2 by 8.85 ppmv.4 The observational data indicate that the rise in CO2 over the interval was only about one-third of that value, 2.89 ppmv. One would expect that a portion of the CO2 added to the atmosphere was transferred to the ocean. Emission, or uptake, of CO2 associated with a change in the global content of carbon in soils and the terrestrial biosphere (defined here as the composite of all living, land-based plant material, including both roots and above-ground components) has an additional influence, complicating attempts to develop a balanced budget for CO2 in the atmosphere. I outline an approach using measurements of changes in the abundance of atmospheric O2 and the isotopic composition of CO2, allowing in principle for a separation of the relative contributions of industry, the soil-biosphere system, and the ocean to the budget of atmospheric CO2. I conclude with summary remarks discussing implications for policy and opportunities for further work.
THE CONTEMPORARY BUDGET OF ATMOSPHERIC CO2
We know that at least part of the modem increase in CO2 can be attributed to emissions associated with combustion of fossil fuel. We know that a portion of the carbon emitted by burning fossil fuel will remain in the atmosphere and that some fraction will be taken up by the ocean. A major uncertainty concerns the importance of exchange between the atmosphere and the combination of the terrestrial biosphere and soils. Net exchange of
carbon between the biosphere-soil system and the atmosphere reflects the composite influence of human activity on the global biosphere. Experience in the United States and Europe indicates that this influence can be extremely complex and difficult to quantify. In principle, the biosphere-soil system could represent either a source or a sink for atmospheric CO2 on a global basis. Compounding the problem, it could provide a source in 1 geographic region, a sink in another.
Deforestation in the tropics constitutes a source of CO2. But regrowth of forests in regions previously deforested should contribute a sink. A warmer climate could promote growth of vegetation Coy prolonging the growing season, for example) and thus produce a sink for CO2. Offsetting, higher temperatures could enhance decomposition of organic matter in soils, resulting in a net source of CO2. Higher concentrations of CO2 and industrial sources of fixed nitrogen and sulfur could contribute to increased growth of vegetation, representing a sink for CO2. But the carbon-to-nitrogen ratio in plants could change in response to a change in the availability of these essential elements, and feedbacks in terms of exchange of carbon with the atmosphere could be altered accordingly. These influences are complex and difficult to forecast. We need to develop a sense of the factors that regulate exchange of carbon between the biosphere-soil system and the atmosphere today if we are to project how they might vary in the future.
It is clear that the biosphere-soil system was an important net source of CO2 in the past. The beginning of the modem increase in CO2 in the 18th century predates important input from combustion of fossil fuel, and this situation persisted until the beginning of the 20th century. The carbon content of the atmosphere increased by about 15 billion tons between 1860 and 1890 (corresponding to a rise in the mixing ratio of CO2 by 7 ppmv), as indicated by the ice-core data. Combustion of fossil fuel over the same period contributed 5.8 billion tons of carbon as CO2, too small to account for the observed buildup of CO2 (the discrepancy is even larger if we allow for the fact that less than 60% of the fossil source would be expected to persist in the atmosphere).
It should come as no great surprise that 100 yr ago transfer of carbon from the biosphere and soil to the atmosphere was a larger source of atmospheric CO2 than was combustion of fossil fuel. Where it was available, wood was a convenient and relatively inexpensive fuel for use both in domestic situations (for heating and cooking) and in industry, where it was used extensively not only as a fuel but also as a source of essential industrial feedstock, supplying, for example, the large quantities of charcoal consumed in smelting ore. As forests were depleted in the early part of the 18th century, coal replaced wood as the primary fuel used in England and in much of western Europe. The transition from wood to coal occurred almost 200 yr later in the United States, in the early part of the 20th century. It is an even more recent phenomenon in China and India. As fossil fuel substituted for wood in developed societies, forests were often allowed to regrow. In the eastern United States, for example, forests were destroyed in the 18th century to provide wood for industry and land for agriculture. Construction of the railroads in the 19th century provided access to much-richer agricultural land in the Midwest. Plowing the relatively pristine prairies of the Midwest would have stimulated release of CO2, as organic matter that had been deposited over millennia was exposed to atmospheric oxygen. Farming in the East declined as a consequence of the more-efficient supply of agricultural products from the Midwest. Land was abandoned and allowed to revert slowly to forest. As we will see, it is likely that forests and soils in the eastern United States are now probably a net sink for atmospheric CO2.
Modem agricultural practices provide an important opportunity to enhance retention of carbon in soils—to capture at least a portion of the carbon released from soils in the early days of agricultural exploitation. Results from 27 studies primarily in the United States and 17 similar investigations in Canada indicate that soils subjected to nontill practices were more effective in retaining carbon than soils cultivated with conventional practices. Additional storage ranged from –4 to +10 Mg/ha with a mean of +3 Mg/ha. If nontill practices were instituted on a large scale in the United States and Canada, it is estimated that over a 20-yr period 765 teragrams of additional carbon could be sequestered in soils, significant in absolute terms but small compared with the fossil source.15
It would be helpful to know what is happening today, if only on a global scale. If we could provide spatial information, even on a coarse scale, this would be a bonus and an incentive for further, more-directed research. This paper describes an approach that promises at least to address these objectives. The architect and first practitioner of the approach is another Keeling: R.F. Keeling, son of the C.D. Keeling cited earlier for his contributions to modem studies of CO2. It is based on a recognition that careful measurements of the abundance
of O2 in the atmosphere, coupled with measurements of CO2, can provide a means to distinguish between the roles of the ocean and biosphere-soil system as a sink for fossil-fuel-derived CO2. While a graduate student at Harvard University, R.F. Keeling developed an instrument with the capability to measure the abundance of O2 to the precision required to implement this strategy and demonstrated its potential in an important series of papers analyzing implications of the changes in O2 that have occurred over the last several years.
Combustion of fossil fuel is responsible both for an increase in production of CO2 and simultaneously for a (predictable) decrease in O2. The relative decrease in O2 is largest if the fuel consumed is natural gas (CH4), less for oil, and least for coal, reflecting the carbon-to-hydrogen ratios of the different fuels. On the average, with the current mix of fossil fuels, accounting for production of cement, it is estimated that 1.4 moles of O2 is removed from the atmosphere for each mole of CO2 released by the burning of fossil fuel. Independent analysis suggests that uptake of CO2 by the biosphere (photosynthesis) is associated with release of O2 in relative proportions of 1 mole of CO2 for each 1.1 moles of O2, proportions are assumed to be similiar for release of CO2 and consumption of O2 by the biosphere and soils (the combined effects of respiration and decay). The net global source of CO2 (the composite release due to transfer from the biosphere-soil system and burning of fossil fuel) will be distributed between the atmosphere and ocean in proportions that depend on the capacity of the ocean to absorb excess CO2, measured in terms of a quantity known as the airborne fraction or Keeling fraction.5 The change in O2, however, is essentially confined to the atmosphere, reflecting the relatively low solubility of O2 in water. Given a measurement of the change in O2 and knowing the change expected because of combustion of fossil fuel, we can estimate the contribution of oxidation (or reduction) of biosphere-soil organic carbon to the observed change. Given a measurement of the change in the abundance of atmospheric CO2 and information on the contribution of burning of fossil fuel and oxidation of organic carbon in soils and the biosphere to this change, we can obtain empirical estimates of the quantity of carbon transferred from the atmosphere to the ocean and of the net exchange of atmospheric carbon with the biosphere and soil.
Changes in O2 are quoted conventionally in terms of the fractional change in the ratio of the concentration of O2 to N2 with respect to the ratio in a standard (typically a sample of air taken in 1988 when R.F. Keeling began his program of sustained modem measurements of O2). The change in O2 relative to N2 is expressed in terms of the delta notation.4 Assuming that the concentration of N2 stays fixed, a change in the mixing ratio of O2 relative to N2 by 1 ppmv corresponds to a change in delta of 1/0.2095 = 4.77 per meg (0.2095 is the mixing ratio assumed for O2 by volume in dry air).
Emission of CO2 from combustion of fossil fuel from July 1991 to July 1994 would have been sufficient to cause an increase in the mixing ratio of CO2 by 8.65 ppmv. The equivalent decrease in delta for O2 would be given by (8.65)(1.39)(4.77) = 57.4 per meg. The observed decrease is 42.2 per meg. It follows that exchange of O2 between the biosphere-soil system and the atmosphere must be responsible for a net increase in delta of 15.2 per meg. That implies that the biosphere-soil system accounted for a net source of O2 over this period, and thus for a net sink of CO2. The equivalent sink for CO2 (converting the change in delta to an equivalent change in the mixing ratio of O2 and accounting for the fact that 1.1 mole of O2 is released for every mole of CO2 taken up by the biosphere-soil) would be given by [(15.2)(0.2095)]/(1.1) = 2.89 ppmv. The observed increase in CO2 is 3.36 ppmv. The ocean must have accounted for removal of CO2 equivalent to (8.65-2.89-3.36) = 2.4 ppmv.
A summary of this analysis is presented in figure 6. Point A denotes the (CO2, O2) combination measured at the beginning of the record, in July 1991. Point D summarizes observations for the end of the record, in July 1994. The influence of fossil-fuel combustion (production of CO2 and consumption of O2) is represented by the segment A-B (slope,-0.15 ppmv meg). The role of the ocean, providing a sink for CO2 with minimal impact on O2, is indicated by the horizontal line B-C. Finally, the path from A to D is completed by exchange between the atmosphere and the biosphere-soil system as described by segment C-D (slope, = –0.19 ppmv meg). Point C lies at the intersection of a horizontal line passing through B and a line of slope –0.19 ppmv meg passing through the final state D.
In summary, from the middle of 1991 to the middle of 1994, according to R.F. Keeling and co-workers,4 we added enough CO2 to the atmosphere from combustion of fossil fuel to increase the atmospheric abundance by 8.65 ppmv—about 18.3 × 109 tons of carbon. Of that, a quantity of CO2 equivalent to 2.89 ppmv (6.13 × 109 tons of carbon) was incorporated either in soils or in the terrestrial biosphere, 2.39 ppmv (5.07 × 109 tons of carbon)
was transferred to the ocean, and 3.36 ppmv (7.12 × 109 tons of carbon) remained in the atmosphere. Of the carbon added to the atmosphere by the burning of fossil fuel, 39% remained in the atmosphere, 33% was incorporated in a combination of soils and the biosphere, and the balance, 28%, was absorbed by the ocean. The analysis implies that 58% of net carbon added to the atmosphere (fossil-fuel source minus biosphere-soil sink) over the period 1991-1994 remained in the atmosphere and the balance was transferred to the ocean. On a global basis, it appears that in the early years of the 1990s the carbon content of soils and the terrestrial biosphere increased at an annual rate of close to 2 × 109 tons per year.4
From an analysis of gradients in CO2 and O2 observed between the Northern and Southern hemispheres, R.F. Keeling and associates4 concluded that the growth of the biosphere-soil reservoir in the 1990s occurred primarily in the north. Transfer of air between the Northern and Southern hemispheres is relatively slow. The associated time constant—as indicated, for example, by analysis of spatially distributed data for the industrial halocarbons—is about 1 yr. Fossil fuel is consumed mainly in the industrial Northern Hemisphere. If net input of CO2 to the Northern Hemisphere was dominated by fossil fuel, we would expect the abundance of CO2 in the atmosphere of the Northern Hemisphere to exceed that in the Southern Hemisphere by about a year's worth of production. The abundance of O2 would be correspondingly lower in the north. The observations indicate that the gradient of concentrations between the hemispheres is less than one would expect according to that scenario. The dilemma is resolved if we suppose that the net input of CO2 (and net consumption of O2) in the Northern Hemisphere was less than would have been expected as a consequence of fossil-fuel consumption. The apparent reduction in the net northern CO2 source (and the smaller decrease in O2) can be explained if the global biosphere-soil sink for CO2 (source of O2) inferred from the CO2-O2 analysis is associated primarily with growth of the biosphere-soil system in the north.
Direct (in situ) measurements of CO2 and O2 sufficient to carry out the analysis of the relative importance of the ocean and the biosphere-soil system in the global budget of atmospheric CO2 are available only for the 1990s.
In a remarkable tour de force, a group of scientists from the University of Rhode Island, Pennsylvania State University, the National Oceanographic and Atmospheric Administration, and the University of Colorado6 extended the study by R.F. Keeling and co-workers4 back to 1977 by measuring the composition of air in the upper unconsolidated (firn) layer of snow at the South Pole. Analysis of the firn data indicates that the biosphere-soil system played a much smaller role in the global budget of atmospheric CO2 over the period 1977-1985 than it did in the 1990s; trends in CO2 and O2 observed in the earlier data can be attributed straightforwardly to the influence of fossil-fuel combustion without invoking a role for either the biosphere or soil. It appears, therefore, that the
biosphere-soil system played a minor role in the global CO2 budget in 1977-1985 even though it was clearly important in the 1990s.
An interesting perspective on the factors influencing changes in the abundance of CO2 since 1958 was presented in a paper published by C.D. Keeling and associates7 in 1995. As indicated in figure 7, they found that the trend in CO2 from 1958 to 1980 could be explained if fossil-fuel combustion was the dominant source of CO2 and the airborne fraction was taken as 56%. The airborne fraction that they used is almost identical to the value (58%) inferred from the CO2-O2 observations for 1991-1994 when we allow in the latter case for uptake of carbon by the biosphere-soil system. Assuming that the capacity of the ocean to take up excess CO2 is relatively constant, the obvious conclusion to be drawn is that the role of the global biosphere-soil system, although important in the 1990s, must have been small throughout the period 1958-1985; the conclusions of C.D. Keeling and co-workers7 are consistent with those of Battle and co-workers. 6
There is a simple resolution to the dilemma. The gradient in CO2 between the Northern and Southern hemispheres throughout the record is consistently less than would be expected if input from combustion of fossil fuel were the only factor influencing this gradient. Moreover, as indicated in figure 8, the gradient appears to increase in proportion to the source of CO2 contributed by fossil fuel. We can account for all the existing constraints if we assume a persistent sink for CO2 associated with uptake of carbon by the biosphere-soil system at the middle latitudes of the Northern Hemisphere. The analysis of R.F. Keeling and co-workers4 suggests that this sink contributed to a net annual removal of about 2 × 109 tons of carbon from the atmosphere in the early 90s—roughly one-third of the input from fossil fuel. For most of the time, the middle-latitude Northern Hemisphere sink is offset by a source of CO2 of comparable magnitude in the tropics, probably attributable to deforestation in some countries, including Brazil and Indonesia. The tropical source appears to have been much reduced in the early 1990s in the period covered by the observations of R.F. Keeling and co-workers.4 Indeed, there are data to support that conjecture. Burning of forests in Brazil appears to have diminished in the early 1990s largely because of changes in economic incentives, helped perhaps by sensitivities heightened by Brazil's role in hosting the Earth
Summit in Rio de Janeiro in 1992. Anecdotal evidence suggests that the pace of tropical deforestation has increased more recently.
C.D. Keeling and co-workers7 used a combination of CO2 data and measurements of the isotopic composition of carbon in CO2 to isolate influences of the ocean and the biosphere-soil system on the trends in CO2 observed since 1978. Between 1980 and 1989, they found that the concentration of CO2 rose more rapidly than expected, given the earlier trend (that is, if fossil fuel had continued as the major source, we would have needed to assume a higher airborne fraction to account for the data in the 1980s). The anomalous rise ended in 1989; by 1994, concentrations of CO2 had reverted to the long-term behavior observed before 1980. As illustrated in figure 9, adapted from C.D. Keeling and co-workers, 7 globally averaged surface temperatures rose steadily over the decade of the 1980s, peaking in 1990 and declining by several tenths of a degree after the 1991 eruption of the volcano on Mount Pinatubo. They concluded that, with reference to the long-term trend, there was a net cumulative increase of about 2.3 × 109 tons of carbon released from soils and the biosphere from 1980 to 1989 with a compensating increase of 3.5 × 109 tons in carbon taken up by soils and the biosphere from 1989 to 1991. Those numbers, when expressed in terms of mean annual-exchange rates, are small relative to the mean annual uptake of 2 × 109 tons of carbon per year inferred for the 1990s by R.F. Keeling and co-workers4: Averaged over the period 1980-1991, the anomalous biosphere-soil uptake inferred from the isotopic analysis amounts to less than 108 tons of carbon per year. The pattern reported by C.D. Keeling and co-workers7 is consistent generally with results based on the CO2-O2 analyses discussed above. They argued that higher ocean temperatures contributed to a cumulative reduction of 2.9 × 109 tons in carbon uptake by the ocean over the period 1980-1991; again, this is small relative to the annual mean uptake of 1.7 × 109 tons of carbon per year inferred from the CO2-O2 analysis. There are additional indications in the isotopic data of an influence of El Niño. Release of carbon from the tropical ocean is reduced as a consequence of suppressed upwelling during El Niño. At the same time, it appears that tropical terrestrial environments are responsible for increased release of CO2 associated most probably with regional changes in tropical climate.
Observations of turbulent exchange of CO2 between the atmosphere and a deciduous forest in New England (Harvard Forest in central Massachusetts) provide independent support for the existence of a middle-latitude Northern Hemisphere sink for CO2.8 Net ecosystem exchange of carbon varied from a low of –1.4 tons per hectare
per year in 1992-1993 to a high of –2.8 tons per hectare per year in 1990-1991 (negative values indicate that the forest represented a net sink for atmospheric CO2). Interannual variability was associated with changes in climatic conditions and was sensitive particularly to the length of the growing season (regulated primarily by temperatures in spring and early fall), cloud cover in summer, drought in summer, snow depth, and other factors that affect temperatures of soils in the dormant season. If we were to assume that Harvard Forest was representative of forests and woodlands in North America (obviously a leap of faith) and adopt an average of the range of values observed for carbon uptake at Harvard Forest over the period 1990-1995, we would conclude that forests and woodlands in North America could be responsible for net regional uptake as large as 1.3 × 109 tons of carbon per year, with comparable regional emissions of 1.5 × 109 tons of carbon per year in 1991 contributed by combustion of fossil fuel.
Additional evidence supporting a Northern Hemisphere sink for CO2 comes from an analysis by Fan and co-workers9 of measurements of CO2 reported from a distributed network of 62 sampling stations.10 Using 2 distinct general-circulation models for the atmosphere and a model to describe ecosystem exchange of carbon, they concluded that the combination of biosphere-soil systems in Eurasia and North America accounted for a net CO2 sink of about 2 × 109 tons of carbon per year for the period 1981-1987, with somewhat larger removal in 1988-1992. The sink inferred in their analysis is remarkably similar in magnitude to the result derived above. Their study suggested that the sink is primarily in temperate regions of North America and in the boreal forests of northern Eurasia. Consistently with the speculation above, they raised the possibility that release of CO2 associated with combustion of fossil fuel and cement manufacture in North America can be offset significantly at present by uptake in temperate forests.
I have shown that geographically distributed measurements of O2 and CO2, in combination with data on the isotopic composition of CO2, can be used to place important constraints on the budget of atmospheric CO2. A convincing body of evidence suggests the presence of an important and persistent sink for CO2 associated with the biosphere-soil system in the Northern Hemisphere. The analysis indicates that the sink can accommodate as much as one-third of the carbon added to the atmosphere today by the burning of fossil fuel. The sink is most likely in regions of North America and Europe where forests were once abundant but where overuse resulted in their depletion (early in Europe, more recently in North America). As coal replaced wood as a primary fuel, and as industrial practices evolved, forests were allowed to regrow. The sink for CO2 at northern middle latitudes could be temporary and likely to diminish in importance as the biosphere-soil system approaches a new steady state reflecting mixed current patterns of land use.
IMPLICATIONS FOR THE FUTURE
Soils at high latitudes provide an important reservoir for organic carbon—between 200 billion and 500 billion tons of carbon.11-13 The eddy-correlation method used to study exchange of carbon between the atmosphere and Harvard Forest has been applied also to the carbon balance of a mature black spruce forest in central Canada.14 It was found that decomposition of organic carbon in soils of this system resulted in a small but important net source of CO2 emission to the atmosphere—0.4 ton of carbon per hectare per year—in 1994-1996. Emission rates increased by a factor of 10, however, as temperatures rose from –2º C to 5º C, raising the possibility that high-latitude soils could be a much more important source of CO2 in the future if the climate warms significantly at high latitudes.
In forecasting upcoming trends in CO2, it will be important to allow for feedbacks between the climate system and the complex suite of processes that regulate the distribution of carbon over its dominant reservoirs—the atmosphere, soils, biosphere, and ocean. As indicated by the analysis of C.D. Keeling and co-workers,7 a warmer ocean might constitute a less-efficient sink for excess concentrations of atmospheric CO2. It appears that the global biosphere-soil system played a relatively minor role in the budget of CO2 over the last 40 yr, with release from tropical ecosystems offset by regrowth of forests at northern middle latitudes. But the signature of the biosphere-soil system could change over the next few decades as middle-latitude forests approach a new steady state, completing their recovery from earlier disturbance. The global biosphere-soil system could switch from its current neutral role to become an important net source of CO2 if higher temperatures develop, stimulating an increase in emissions from soils at high latitudes.
The analysis indicates that for most of the last 40 yr, tropical ecosystems might have been a net source of CO2 for the atmosphere, equal to as much as one-third of emissions associated with global combustion of fossil fuel; that is, the excess of CO2 released by deforestation in recent years over CO2 absorbed by plants in regions deforested earlier but abandoned might amount to close to 2 billion tons of carbon per year. This suggests that the climate-change issue is linked inextricably to the challenge of preserving the rain forests. Such countries as Brazil and Indonesia could bear responsibility as much as the United States and China for the contemporary buildup of atmospheric CO2.
The suggestion that regrowth of forests in North America can provide a sink for CO2 comparable with that associated with local consumption of fossil fuel, although plausible, is clearly speculative, on the basis of current evidence. It merits further attention, however. If validated, it would complicate the task of those charged with developing a strategy to minimize the future growth of greenhouse gases in the atmosphere.
I am indebted to Andrew Abban for invaluable support in the preparation of this paper.
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W. Lawrence Gates
Lawrence Livermore National Laboratory
I will comment on some of the remarks of Michael McElroy from the viewpoint of a climate modeler. Paleoclimate data are limited in space and limited in the variables that they can reveal, but they pose an important challenge to models because they represent climates different from today's, against which our models are necessarily calibrated. Paleoclimate data give us a unique opportunity to see the excursions that climate can undergo. It is a major challenge to models to be able to recreate selected paleoclimates and someday to simulate the changes from 1 paleoclimate to another.
Nature follows only 1 path, but models can be run under many conditions; this is a major advantage of modeling. We can change the internal structure of a model, we can change the external forcing, and we can run large numbers of climate simulations. Thus, we have been able to simulate with reasonable confidence the climate of an ice age, for example, but the transition between an ice age and the intervening interglacial periods has not yet been stimulated. As valuable as paleoclimate data are, they represent only a slice-of-time "validation of opportunity" as a supplement to model validation with the 4-dimensional instrumental data on the current climate.
Michael McElroy also referred to the ocean-atmosphere system. We have had climate models of one sort or another for some 40 yr. These have been developed by the atmospheric and oceanic research communities more or less independently. The workhorse of modem climate modeling is the coupled model in which atmosphere, ocean, cryosphere, and the land surface are joined to form an interacting dynamic system.
Some 20 groups now have coupled models. Many have performed simulations covering a century or more in which solar radiation and atmospheric composition are given modem values. These simulations show irregular fluctuations on seasonal, yearly, and decadal time scales when there are no changes in external conditions. We refer to these fluctuations as natural climate variability—fluctuations that occur without a change in the forcing and that are essentially unpredictable. In long-term integrations, these fluctuations tend to average out, but they are partly responsible for the systematic errors found in every model. On the whole, however, current climate models do a reasonably good job of simulating the average large-scale seasonal distribution of climate.
One important application of coupled models in recent years has been in simulating the climatic consequences of increased CO2 concentration in the atmosphere. In general, such experiments yield a warming of several degrees Celsius, with larger warming over the continents than over the oceans. When we search the observed record of the last few decades, during which CO2 in the atmosphere has increased by perhaps 30%, we find almost no correspondence between the models' simulations of warming for this CO2 increase. This has led some to conclude that the models are wrong. But probably the models are simulating the nearly correct response to CO2 increases, and other forces are operating in nature.
More recent simulations with coupled models consider the transient increase in both CO2 sulfate aerosols, which have a cooling effect that might offset some of the CO2-induced warming. When such runs are made for 100 yr or so (it takes about 70 yr to double the CO2 concentration), we find that the large-scale pattern of surface-air-temperature changes simulated by the models bears a marked resemblance to that observed over comparable periods.
That important result was highlighted in the 1995 report of the United Nations Intergovernmental Panel on Climate Change, which contains the now-famous statement that "the balance of evidence suggests a discernible human influence on global climate." This finding has now been verified by several models in the continuing search for the details of an anthropogenic climate-change "fingerprint."
We cannot, however, predict all aspects of a future climate. Whether a warmed climate, modified by aerosols, will increase the frequency of El Niño or change the intensity or onset of monsoons, for example, has not yet been established. Coupled models, like their atmospheric and oceanic components, contain systematic errors that need to be reduced. As we add interactive biology and chemistry to the models, they will become even more complex, but presumably they will also be more accurate.
The climate-modeling community spends a great deal of effort in building models and in deciding on the parameters for individual physical processes, such as cloud formation, radiation, and ocean mixing. We also use a great deal of computer time in integrating the models over hundreds or even thousands of years. But we pay relatively little attention to the systematic extraction of information from the results. In some ways, climate modeling is in a situation analogous to that of biology, which (as we have heard here) is trying to move from the genome scale to the organism scale. We know something about the individual components of climate models, but we are not very clever in diagnosing the behavior of the coupled climate system—we cannot point to an error as having specific causes. Such model diagnosis and interpretation of model performance is an underdeveloped part of climate research.
National Center for Atmospheric Research
The Department of Energy (DOE) has been a pioneer in the use of computers. As we go into the next generation of climate models, we will be trying to address several major problems. One, of course, is to increase the spatial resolution of the models. This is very important for understanding the atmospheric circulation that is generated by the models and the ocean and sea-ice components of the models. We are finding, for example, in the ocean models that at very high resolution the models generate realist current systems that can be validated against observed satellite and in situ data. However, there are still some substantial biases, and the oceanographic research community is trying to address them. With the advent of highly parallel and cluster computer systems that we see becoming available in the next few years, we should be able to exploit such computers in going to the next generation of climate models.
One persistent problem in climate modeling has been to identify the physical constituents that are not working properly. Clouds have always been very high on the list. I am pleased to see that, under the ARM program, DOE has taken the leadership role in trying to identify the defects of the models with respect to radiation processes.
Another issue in climate modeling is how we handle atmospheric water in all forms—water vapor, liquid, and ice. We are making substantial inroads in this regard in our models.
The chemistry of the carbon cycle, the sulfur cycle, and other cycles in the atmosphere and in the oceans is important. Instead of just having models with specified concentrations of these gases and aerosols, we will have models that can predict changes in chemistry and in aerosol distributions.
Modeling of ecology is also important. We are moving to models with realistic ecologic systems that take into account the vegetation of the tropics, prairie, middle latitudes, and tundra in a much more accurate manner. The different systems will be validated with satellite data and other field data.
To make full use of those models, we need to look at the interface between the climate-model simulations and societal and economic variables more closely. This field is called integrated assessment. DOE has been a leader in trying to explain how to use interaction of economic models with climate models to produce information useful to policy-makers and the public.
Now that we understand most aspects of the climate system, including the effects of burning fossil fuels and other climatic forcings, we should not just have a simple view. This is not the time to slack off in funding global-change research. I think, with the wide range of uncertainties and probably some surprises in our understanding, that this is not the time to decrease support for a very aggressive research program.
Brookhaven National Laboratory
Upton, New York
I would like to express my gratitude to the Department of Energy for its long-term support of multi-disciplinary research on the environment. It is the only federal agency that has made such commitment, which I hope will continue.
My comments pertain to the role of the oceanic biota and climate feedback. By feedback, I am referring to what climate does to affect the abundance and distribution of organisms and to what organisms do to affect biogeochemical cycles that, in turn, influence climate. Understanding such feedback is critical not only for evaluating how natural climatic changes that are recorded in the geologic history of the earth have been affected by and affect biologic processes, but also for using historical data and observations to represent such processes in general-circulation models that are used to predict climatic change and its potential effects. The researchers who have developed general-circulation models are trained primarily in physics. The language of biology differs from that of physics, so communication between them is often strained. I will briefly illustrate how critical it is to bridge the language barrier in climate-change research and thus how essential a multidisciplinary approach is for developing a comprehensive earth-system science program. The latter is a great challenge in the coming decade.
A century or so ago, scientific disciplines were not so segregated from each other, and individuals could be truly multidisciplinary researchers. In 1830, the British geologist Charles Lyell1 published the first volume of his classic text, Principles of Geology,* in which rational hypotheses were developed to explain geologic formations. Because geologists relied heavily on fossil biologic groups to identify epochs, Lyell was a superb biologic taxonomist and was able to identify numerous fossil marine invertebrates, many of which appeared to be extinct. While on a trip to the continent, he noted that marble columns of the Roman ruins of the Temple of Scerapi in Pozzuoli (just south of Naples) had obvious signs of marine boring organisms. The columns, which were (and still are) above water, must have been partially submerged in the ocean for some period after the construction of the temple but before Lyell's visit to it. How could the temple have been under water, and how did it come back to the surface?** Similarly, why were there fossils of marine origin high up in the Alps and Dolomites? Were the mountains under water at some point as well? Moreover, why were many of the fossil organisms extinct?
It is difficult to imagine today what explanations had been offered for those phenomena before Lyell. They included eruptions of submarine volcanoes that spewed marine organisms onto the land, evidence of the great flood and later subsidence of waters as described in Genesis, and clear evidence that God was testing one's belief (the "fossils" were not really extinct marine organisms, but rocks made to look like marine organisms to fool one into believing in false creations). Lyell elaborated on the absurdity of many of the hypotheses and went on to attribute the changes in the earth's surface to causes still in operation. Thus, earthquakes, floods, volcanism, and erosion (causes now in operation), shaped the earth. The presence of fossils of marine organisms in the Alps must mean that the mountains (or the rocks of which they are made) were at one time under water. Lyell's concept, which came to be known as uniformitarianism, forms a fundamental paradigm in geology and geochemistry to this day*** and is embraced (mostly unconsciously) by climate-change researchers. It is used, for example, to explain
glacial and interglacial (or paleoclimatic) changes on the basis of astronomically predictable variations in solar-energy fluxes (such as "Milankovich cycles"), meteorite impacts, volcanism, changes in ocean circulation and heat transport, and so on. It is also used to rationalize changes in the radiative forcing resulting from variations in greenhouse gases, such as CO2, which in the past were generally believed to have been a response to, rather than a cause of, climate change.
An ecologic analogue of uniformitarianism was succinctly stated by G. Evelyn Hutchinson, who wrote that "the ecological theatre of the present is only a scene on the ongoing evolutionary drama." This statement implies that ecologic processes must be considered within a continuum of change, not the least of which is a consequence of climatic variation. In the roulette wheel of evolution, some organisms have persisted beyond and despite large changes in climate, and others have become extinct or have evolved by natural selection to adapt to new conditions. Among the most-persistent and most-dynamic organisms (from an evolutionary perspective) are marine unicellular algae, the phytoplankton. (Plankton is from a Greek word meaning "wandering"; phytoplankton are photosynthetic organisms that drift with the currents.)
Let us now turn to the ecologic theater of the contemporary interglacial ocean. Satellite imaging has provided high-resolution pictures of the chlorophyll distributions in the oceans. High chlorophyll (that is, phytoplankton) concentrations are found in coastal regions and in the open ocean, where physical processes provide a source of nutrients, light, and simultaneous stability within the water column. Low-chlorophyll regions, characteristic of most of the central gyres (circular ocean currents), is also a consequence primarily of physics; in such regions, the vertical flux of nutrients from the ocean interior is relatively weak. The phytoplankton in the oceans are extraordinarily diverse from a genetic viewpoint, belonging to over 18 phyla (all flowering plants on land belong to a single phylum), and, although their composite biomass accounts for only about 1012 kg of carbon, they fix about 40 × 1012 kg of CO2 per year. For comparison, terrestrial plants, which account for about 500 × 1012 kg of carbon biomass, fix about 60 × 1012 kg of carbon per year. Hence, marine phytoplankton are more than 300 times as productive per unit of biomass as their terrestrial counterparts. The productivity of marine phytoplankton is limited primarily by the availability of essential nutrients, such as nitrate and phosphate, but not CO2.
In the ocean, about 15% of the phytoplankton organic carbon sinks out of the upper ocean into the interior, where it is oxidized back to inorganic carbon. In so doing, marine phytoplankton help to maintain a negative diffusive gradient between the ocean and atmosphere that drives a nonequilibrium flux of carbon between the 2 reservoirs. This process, called the "biologic pump," is analogous to the decomposition and burial of organic matter (such as leaf litter) in terrestrial ecosystems. The oceans differ from terrestrial systems, however, in that the carbon stored in the oceans resulting from biologic activity is primarily inorganic carbon. Only a very small fraction of the organic carbon is preserved in the sediments. Over geologic time, a small fraction of the organic carbon preserved in the sediments undergoes chemical transformations that lead to the production of oil. Simply put, oil is relic fossil phytoplankton. The burial of organic carbon results in the net release of oxygen. Hence, the oxygen in the earth's atmosphere is fossil oxygen and reflects, in part, the burial and sequestration of fossil phytoplankton carbon in the oceans.
For carbon to be buried and oxygen evolved, phytoplankton production must not have been in equilibrium with oxidation processes; that is, there must have been a change in the biologic pump on geologic time scales. How can such changes occur, and what is their effect on atmospheric CO2? Geochemically, the most-obvious mechanism for enhancing the biologic pump is to add nutrients to the ocean. But which nutrients, and how should they be added? Let me briefly consider a particular nutrient and its consequences.
In the eastern equatorial Pacific, there are high concentrations of the essential plant nutrients nitrate and phosphate in the upper ocean, but phytoplankton chlorophyll concentrations are remarkably low. In a series of experiments in the 1990s, infinitesimal concentrations of iron were added directly to a small region of the high-nutrient, low-chlorophyll waters. The iron additions led to dramatic and immediate increases in phytoplankton biomass and photosynthetic fixation of carbon. In fact, photosynthetic rates more than doubled within 24 hr, and high rates were sustained for several days, until the iron was depleted. It turns out that a major source of iron for the oceans is aerosol deposition, particularly wind-driven transport of dust from deserts. Desertification leads to enhanced production of wind-blown iron, and analyses of ice cores suggest that the flux of such iron was about 100
times as high during glacial periods as it is today. Perhaps this flux stimulated the biologic pump and contributed to the drawdown of atmospheric CO2 during glacial periods.
Iron is extraordinarily important in the global nitrogen cycle. With very few exceptions, phytoplankton must use a fixed-nitrogen source for growth. Nitrogen fixation is carried out by a very small number of species of cyanobacteria, whose abundance in the modem ocean appears to be extremely low. The distribution of nitrogen fixers appears to be highly correlated with the flux of wind-blown iron. A small change in the amount of nitrogen fixed could have a large effect on the net photosynthetic fixation of CO2.2
Thus, iron can both directly affect biologic fixation of CO2 in the high-nutrient, low-chlorophyll regions of the oceans and indirectly affect the same process in the other regions by limiting nitrogen fixation. Will the flux of iron to the oceans change in the coming decades? If so, how, and in what direction, and how will the change affect the biologic pump? My point here is not the specific process of wind transport of iron or its effects, but the lack of recognition of the process in coupled atmosphere-ocean general-circulation models. Virtually all modelers assume that oceanic biology is in a steady state and that the net uptake of atmospheric CO2 by the oceans is accomplished by physical-chemical processes without any biologic enhancement. That is certainly not true on geologic time scales, and it is unlikely in the coming decades. If such feedback processes as I have described are ignored, model prognostications with regard to atmospheric CO2 and its effects will be quantitatively, if not qualitatively, incorrect.
Many people doubt that human influences on CO2 have had any effect on the earth's climate. Without a control in the experiment, it is extremely difficult to prove beyond doubt that changes in climate since the Industrial Revolution are a direct consequence of human activities. As Michael McElroy stated, ice-core data clearly indicate that current atmospheric concentration of CO2 are about 30% higher than at any time in the last several hundred thousand years. We do not completely understand climate-change processes in the geologic past, but if the consensus interpretation of the climatic forcings and feedbacks during the Holocene is even roughly correct (I assume that the consensus interpretation of the glacial-interglacial forcing is based on Milankovich cycles, with a feedback on ocean chemistry and biology that led to changes in atmospheric CO2), such information cannot necessarily be a guide to the present situation, in which changes in atmospheric gas composition are assumed to be a cause, rather than a consequence, of climate change. Nonetheless, as I have briefly described, many complicating and interactive biogeochemical processes can attenuate or amplify the direct effect of anthropogenic emissions of CO2. Unless the ecologic feedbacks are included with the physical processes in mathematical representations of the earth's climate system, coupled atmosphere-ocean models will remain crude approximations of a complex system.
In conclusion, I would like to return to a quotation from Charles Lyell1: ''The system of scholastic disputations encouraged in the Universities of the middle ages had unfortunately trained men to habits of indefinite argumentation, and they often preferred absurd and extravagant propositions, because great skill was required to maintain them; the end and object of such intellectual combats being victory and not the truth.'' The debate on the causes and consequences of change in the earth's climate system should be vigorous and open and should include of biologic feedbacks. As scientists, we have an obligation to search for the truth. As human beings, with families and children, we should recognize that the potential consequences of our research on our lives and quality of life could be profound. The search for the truth will not be simple; it will require substantial vision and consistent commitment on the part of the Department of Energy and other federal agencies to long-term multidisciplinary research.
1. Lyell C. Principles of geology. Vol. 1. London: John Murry; 1830;511p.
2. Falkowski P. Evolution of the nitrogen cycle and its influence on the biological sequestuation of CO2 in the ocean. Nature 1997;387:272-6.
This paper has been authored under Contract No. DE-AC02-76CH00016 with the U.S. Department of Energy. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes.