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Appendix A
Questions and Answers About Greenhouse Warming

The Greenhouse Effect: What is Known, What Can be Predicted

1. What is the "greenhouse effect"?

In simplest terms, "greenhouse gases" let sunlight through to the earth's surface while trapping "outbound" radiation. This alters the radiative balance of the earth (see Figure A.1) and results in a warming of the earth's surface. The major greenhouse gases are water vapor, carbon dioxide (CO2), methane (CH4), chlorofluorocarbons (CFCs) and hydrogenated chlorofluorocarbons (HCFCs), tropospheric ozone (O3), and nitrous oxide (N2O). Without the naturally occurring greenhouse gases (principally water vapor and CO2), the earth's average temperature would be nearly 35°C (63°F) colder, and the planet would be much less suitable for human life.

2. Why is it called the "greenhouse" effect?

The greenhouse gases in the atmosphere act in much the same way as the glass panels of a greenhouse, which allow sunlight through and trap heat inside.

3. Why have experts become worried about the greenhouse effect now?

Rising atmospheric concentrations of CO2, CH4, and CFCs suggest the possibility of additional warming of the global climate. The panel refers to warming due to increased atmospheric concentrations of greenhouse gases as "greenhouse warming." Measurements of atmospheric CO2 show that the 1990 concentration of 353 parts per million by volume (ppmv) is about one-quarter larger than the concentration before the Industrial Revolution (prior



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Page 663 Appendix A Questions and Answers About Greenhouse Warming The Greenhouse Effect: What is Known, What Can be Predicted 1. What is the "greenhouse effect"? In simplest terms, "greenhouse gases" let sunlight through to the earth's surface while trapping "outbound" radiation. This alters the radiative balance of the earth (see Figure A.1) and results in a warming of the earth's surface. The major greenhouse gases are water vapor, carbon dioxide (CO2), methane (CH4), chlorofluorocarbons (CFCs) and hydrogenated chlorofluorocarbons (HCFCs), tropospheric ozone (O3), and nitrous oxide (N2O). Without the naturally occurring greenhouse gases (principally water vapor and CO2), the earth's average temperature would be nearly 35°C (63°F) colder, and the planet would be much less suitable for human life. 2. Why is it called the "greenhouse" effect? The greenhouse gases in the atmosphere act in much the same way as the glass panels of a greenhouse, which allow sunlight through and trap heat inside. 3. Why have experts become worried about the greenhouse effect now? Rising atmospheric concentrations of CO2, CH4, and CFCs suggest the possibility of additional warming of the global climate. The panel refers to warming due to increased atmospheric concentrations of greenhouse gases as "greenhouse warming." Measurements of atmospheric CO2 show that the 1990 concentration of 353 parts per million by volume (ppmv) is about one-quarter larger than the concentration before the Industrial Revolution (prior

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Page 664 image FIGURE A.1 Earth's radiation balance. The solar radiation is set at 100 percent; all other values are in relation to it. About 25 percent of incident solar radiation is reflected back into space by the atmosphere, about 25 percent is absorbed by gases in the atmosphere, and about 5 percent is reflected into space from the earth's surface, leaving 45 percent to be absorbed by the oceans, land, and biotic material (white arrows). Evaporation and mechanical heat transfer inject energy into the atmosphere equal to about 29 percent of incident radiation (grey arrow). Radiative energy emissions from the earth's surface and from the atmosphere (straight black arrows) are determined by the temperatures of the earth's surface and the atmosphere, respectively. Upward energy radiation from the earth's surface is about 104 percent of incident solar radiation. Atmospheric gases absorb part (25 percent) of the solar radiation penetrating the top of the atmosphere and all of the mechanical heat transferred from the earth's surface and the outbound radiation from the earth's surface. The downward radiation from the atmosphere is about 88 percent and outgoing radiation about 70 percent of incident solar radiation. Note that the amounts of outgoing and incoming radiation balance at the top of the atmosphere, at 100 percent of incoming solar radiation (which is balanced by 5 percent reflected from the surface, 25 percent reflected from the top of the atmosphere, and 70 percent outgoing radiation), and at the earth's surface, at 133 percent (45 percent absorbed solar radiation plus 88 percent downward radiation from the atmosphere balanced by 29 percent evaporation and mechanical heat transfer and 104 percent upward radiation). Energy transfers into and away from the atmosphere also balance, at the atmosphere line, at 208 percent of incident solar radiation (75 percent transmitted solar radiation plus 29 percent mechanical transfer from the surface plus 104 percent upward radiation balanced by 50 percent of incoming solar continuing to the earth's surface, 70 percent outgoing radiation, and 88 percent downward radiation). These different energy transfers are due to the heat-trapping effects of the greenhouse gases in the atmosphere, the reemission of energy absorbed by these gases, and the cycling of energy through the various components in the diagram. The accuracy of the numbers in the diagram is typically ±5. This diagram pertains to a period during which the climate is steady (or unchanging); that is, there is no net change in heat transfers into earth's surface, no net change in heat transfers into the atmosphere, and no net radiation change into the atmosphere-earth system from beyond the atmosphere.

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Page 665 to 1750). Atmospheric CO2 is increasing at about 0.5 percent per year. The concentration of CH4 is about 1.72 ppmv, or slightly more than twice that before 1750. It is rising at a rate of 0.9 percent per year. CFCs do not occur naturally, and so they were not found in the atmosphere until production began a few decades ago. Continued increases in atmospheric concentrations of greenhouse gases would affect the earth's radiative balance and could cause a large amount of additional greenhouse warming. Increasing the capture of energy in this fashion is also called "radiative forcing." Other factors, such as variation in incoming solar radiation, could be involved. 4. Has there been greenhouse warming in the recent past? Best estimates are that the average global temperature rose between 0.3° and 0.6°C over about the last 100 years. However, it is not possible to say with a high degree of confidence whether this is due to increased atmospheric concentrations of greenhouse gases or to other natural or human causes. The temperature record much before 1900 is not reliable for estimates of changes smaller than 1°C–1.8°F). 5. What about CO2 and temperature in the prehistoric past? According to best estimates based on analysis of air bubbles trapped in ice sheets, ocean and lake sediments, and other records from the geologic past, there have been three especially "warm" periods in the last 4 million years. The Holocene optimum occurred from 6,000 to 5,000 years ago. During that period, atmospheric concentrations of CO2 were about 270 to 280 ppmv, and average air temperatures about 1°C (1.8°F) warmer than modern times. The Eemian interglacial period happened with its midpoint about 125,000 years ago. Atmospheric concentrations of CO2 were 280 to 300 ppmv, and temperatures up to 2°C (3.6°F) warmer than now. The Pliocene climate optimum occurred between 4.3 and 3.3 million years ago. Atmospheric concentrations of CO2 have been estimated for that period to be about 450 ppmv, with temperatures 3° to 4°C (5.4° to 7.2°F) warmer than modern times. The prehistoric temperature estimates are from evidence dependent

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Page 666 on conditions during growing seasons and probably are better proxies for summer than winter temperatures. The estimate for the Pliocene period is especially controversial. 6. What natural things affect climate in the long run? On the geologic time scale, many things affect climate: • Changes in solar output • Changes in the earth's orbital path • Changes in land and ocean distribution (tectonic plate movements and the associated changes in mountain geography, ocean circulation, and sea level) • Changes in the reflectivity of the earth's surface • Changes in atmospheric concentrations of trace gases (especially CO2 and CH4) • Changes of a catastrophic nature (such as meteor impacts or extended volcanic eruptions) 7. What is meant by ''atmospheric lifetime" and "sinks"? These concepts can be illustrated by referring to what is called the "carbon cycle." When CO2 is emitted into the atmosphere, it moves among four main sinks, or pools, of stored carbon: the atmosphere, the oceans, the soil, and the earth's biomass (plants and animals). The movement of CO2 among these sinks is not well understood. About 45 percent of the total emissions of CO2 from human activity since preindustrial times is missing in the current accounting of CO2 in the atmosphere, oceans, soil, and biomass. Three possible sinks for this missing CO2 have been suggested. First, more CO2 may have been absorbed into the oceans than was thought. Second, the storage of CO2 in terrestrial plant life may be greater than estimated. Third, more CO2 may have been absorbed directly into soil than is thought. However, there is no direct evidence for any of these explanations accounting for all the missing CO2. CO2 in the atmosphere is relatively "long-lived" in that it does not easily break down into its constituent parts. CH4, by contrast, decomposes in the atmosphere in about 10 years. The greenhouse gas with the longest atmospheric lifetime (except for CO2), CFC-115, has an average atmospheric lifetime of about 400 years. The overall contribution of greenhouse gases to global warming depends on their atmospheric lifetime as well as their ability to trap radiation. Table A.1 shows the relevant characteristics of the principal greenhouse gases. 8. Do all greenhouse gases have the same effect? Each gas has different radiative properties, atmospheric chemistry, typical atmospheric lifetime, and atmospheric concentration. For example, CFC-12 is roughly 15,800 times more efficient molecule for molecule at trapping heat than CO2. Because CFC-12 is a large, heavy molecule with many atoms and a

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Page 667 TABLE A.1 Key Greenhouse Gases Influenced by Human Activity   CO2 CH4 CPC-11 CFC-12 N2O Preindustrial atmospheric concentration 280 ppmv 0.8 ppmv 0 0 288 ppbv Current atmospheric concentration (1990)a 353 ppmv 1.72 ppmv 280 pptv 484pptv 310 ppbv Current rate of annual atmospheric accumulationb 1.8 ppmv (0.5%) 0.015 ppmv (0.9%) 9.5 pptv (4%) 17 pptv (4%) 0.8 ppbv (0.25%) Atmospheric lifetime (years)c (50–200) 10 65 130 150 NOTES: Ozone has not been included in the table because of lack of precise data. Here ppmv = parts per million by volume, ppbv = parts per billion by volume, and pptv = parts per trillion by volume. aThe 1990 concentrations have been estimated on the basis of an extrapolation of measurements reported for earlier years, assuming that the recent trends remained approximately constant. bNet annual emissions of CO2 from the biosphere not affected by human activity, such as volcanic emissions, are assumed to be small. Estimates of human-induced emissions from the biosphere are controversial. cFor each gas in the table, except CO2, the "lifetime" is defined as the ratio of the atmospheric concentration to the total rate of removal. This time scale also characterizes the rate of adjustment of the atmospheric concentrations if the emission rates are changed abruptly. CO2 is a special case because it is merely circulated among various reservoirs (atmosphere, ocean, biota). The "lifetime" of CO2 given in the table is a rough indication of the time it would take for the CO2 concentration to adjust to changes in the emissions. SOURCE: Intergovernment Panel on Climate Change. 1990. Climate Change: The IPCC Scientific Assessment, J. T. Houghton, G. J. Jenkins, and J. J. Ephraums, eds. New York: Cambridge University Press. Reprinted by permission of Cambridge University Press. CO2 molecule is small and light in comparison, there are fewer molecules of CFC-12 in each ton of CFC-12 emissions than CO2 molecules in each ton of CO2 emissions. Each ton of CFC-12 emissions is about 5,750 times more efficient at trapping heat than each ton of CO2. The comparatively greater amount of CO2 in the atmosphere, however, means that it accounts for roughly half of the radiative forcing associated with the greenhouse effect. 9. Do greenhouse gases have different effects over time? Yes. Figure A.2 shows projected changes in radiative forcing for different greenhouse gases between now and 2030. The potential increase for each

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Page 668 image FIGURE A.2 Additional radiative forcing of principal greenhouse gases from 1990 to 2030 for different emission rates. The horizontal axis shows changes in greenhouse gas emissions ranging from completely eliminating emissions (-100 percent) to doubling current emissions (+100 percent). Emission changes are assumed to be linear from 1990 levels to the 2030 level selected. The vertical axis shows the change in radiative forcing in watts per square meter at the earth's surface in 2030. Each asterisk indicates the projected emissions of that gas assuming no additional regulatory policies, based on the Intergovernmental Panel on Climate Change estimates and the original restrictions agreed to under the Montreal Protocol, which limits emissions of CFCs. Chemical interactions among greenhouse gas species are not included. For CO2 emissions remaining at 1990 levels through 2030, the resulting change in radiative forcing can be determined in two steps: (1) Find the point on thecurvelabeled "CO2" that is vertically above 0 percent change on the bottom scale. (2) The radiative forcing on the surface-troposphere system can be read in watts per square meter by moving horizontally to the left-hand scale, or about 1 W/m2. These steps must be repeated for each gas. For example, the radiative forcing for continued 1990-level emissions of CH4 through 2030 would be about 0.2W/m2. SOURCE: Courtesy of Michael C. MacCracken.

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Page 669 gas is plotted for different emissions of each gas compared to 1990 emission levels. The figure shows the impact of different percentage changes in emissions (compared to 1990 emission rates) on the radiative forcing. Figure A.3 extends this to show the impact on equilibrium temperature for different sensitivities of the climatic system (in degrees Celsius). 10. What is meant by a "feedback" mechanism? One example of a greenhouse warming feedback mechanism involves water vapor. As air warms, each cubic meter of air can hold more water vapor. Since water vapor is a greenhouse gas, this increased concentration of water vapor further enhances greenhouse warming. In turn, the warmer air can hold more water, and so on. This is an example of a positive feedback, providing a physical mechanism for "multiplying" the original impetus for change beyond its initial force. Some mechanisms provide a negative feedback, which decreases the initial impetus. For example, increasing the amount of water vapor in the air may lead to forming more clouds. Low-level, white clouds reflect sunlight, thereby preventing sunlight from reaching the earth and warming the surface. Increasing the geographical coverage of low-level clouds would reduce greenhouse warming, whereas increasing the amount of high, convective clouds could enhance greenhouse warming. This is because high, convective clouds absorb energy from below at higher temperatures than they radiate energy into space from their tops, thereby effectively trapping energy. Satellite measurements indicate that clouds currently have a slightly negative effect on current planetary temperature. It is not known whether increased temperatures would lead to more low-level clouds or more high, convective clouds. 11. Can the temperature record be used to show whether or not greenhouse warming is occurring? The estimated warming of between 0.3° and 0.6°C (0.5° and 1.1°F) over the last 100 years is roughly consistent with increased concentrations of greenhouse gases, but it is also within the bounds of "natural" variability for weather and climate. It cannot be proven to a high degree of confidence that this warming is the result of the increased atmospheric concentrations of greenhouse gases. There may be an underlying increase or decrease in average temperature from other, as yet undetected, causes. 12. What is the basis for predictions of global warming? General circulation models (GCMs) are the principal tools for projecting climatic changes. GCMs project equilibrium temperature increases between 1.9° and 5.2°C (3.4° and 9.4°F) for greenhouse gas concentrations equivalent to a doubling of the preindustrial level of atmospheric CO2. The midpoint of this range corresponds to an average global climate warmer than

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Page 670 image FIGURE A.3 Commitment to future warming. An incremental change in radiative forcing between 1990 and 2030 due to emissions of greenhouse gases implies a change in global average equilibrium temperature (see text). The scales on the right-hand side show two ranges of global average temperature responses. The first corresponds to a climate whose temperature response to an equivalent of doubling of the preindustrial level of CO2 is 1°C; the second corresponds to a rise of 5°C for an equivalent doubling of CO2. These scales indicate the equilibrium commitment to future warming caused by emissions from 1990 through 2030. Assumptions are as in Figure A.2. To determine equilibrium warming in 2030 due to continued emissions of CO2 at the 1990 level, find the point on the curve labeled "CO2" that is vertically above 0 percent change on the bottom scale. The equilibrium warming on the right-hand scales is about 0.23°C (0.4°F) for a climate system with 1° sensitivity and about 1.2°C (2.2°F) for a system with 5° sensitivity. For CH4 emissions continuing at 1990 levels through 2030, the equilibrium warming would be about 0.04°C (0.07°F) at 1°sensitivity and about 0.25°C (0.5°F) at 5° sensitivity. These steps must be repeated for each gas. Total warming associated with 1990-level emissions of the gases shown until 2030 would be about 0.41°C (0.7°F) at 1°sensitivity and about 2.2°C (4°F) at 5° sensitivity. Scenarios of changes in committed future warming accompanying different greenhouse gas emission rates can be constructed by repeating this process for given emission rates and adding up the results. SOURCE: Courtesy of Michael C. MacCracken.

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Page 671 any in the last 1 million years. The consequences of this amount of warming are unknown and may include extremely unpleasant surprises. 13. What is "equilibrium temperature"? The oceans, covering roughly 70 percent of the earth's surface, absorb heat from the sun and redistribute it to the deep oceans slowly. It will be decades, perhaps centuries, before the oceans and the atmosphere fully redistribute the absorbed energy and the currently "committed" temperature rise is actually "realized." The temperature at which the system would ultimately come to rest given a particular level of greenhouse gas concentrations is called the "equilibrium temperature.'' Since atmospheric concentrations of greenhouse gases are constantly changing, the temperature measured at any time is the "transient" temperature, which lags behind the committed equilibrium warming. The lag depends in part on the sensitivity of the climate system and is believed to be between 10 and 100 years. This phenomenon makes it difficult to use temperature alone to "prove" that greenhouse warming is occurring. 14. How can we know when greenhouse warming is occurring? The only tools we have for trying to produce credible scientific results are observations combined with theoretical calculation. Detecting additional greenhouse warming will require careful monitoring of temperature and other variables over years or even decades. Further development of numerical models will help characterize the climatic system, including the atmosphere, oceans, and land-based elements like forests and ice fields. However, only careful interpretation of actual measurements can reveal what has occurred and when. 15. How can credible estimates of future global warming be made? Several approaches can be used. Scientific "first principles" can be used to estimate physical bounds on future trends. GCMs can be used to conduct "what if" experiments under differing conditions. Comparisons can be made with paleoclimatic data of previous interglacial periods. None of these methods is absolutely conclusive, but it is generally agreed that GCMs are the best available tools for predicting climatic changes. Substantial improvements in GCM capabilities are needed, however, for GCM forecasts to increase their credibility. 16. What influences future warming? The amount of climatic warming depends on several things: • The amount of sunlight reaching the earth • Emission rates of greenhouse gases • Chemical interactions of greenhouse gases in the atmosphere

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Page 672 • Atmospheric lifetimes of greenhouse gases until they decompose or transfer into sinks • Effectiveness of positive or negative feedback mechanisms that enhance or reduce warming • Human actions, which effect radiative forcing in both positive and negative directions 17. What are the major "unknowns" in predictions? Major uncertainties include: • Future emissions of greenhouse gases • Role of the oceans and biosphere in uptake of heat and CO2 • Amount of CO2 and carbon in the atmosphere, oceans, biota, and soils • Effectiveness of sinks for CO2 and other greenhouse gases, especially CH4 • Interactions between temperature change and cloud formation and the resulting feedbacks • Effects of global warming on biological sources of greenhouse gases • Interactions between changing climate and ice cover and the resulting feedbacks • Amount and regional distribution of precipitation • Other factors, like variation in solar radiation 18. How can the uncertainties best be handled? Data can be arrayed to validate components of the models. Increasing the number of data sets can also help. In addition, the variation in GCM results can be compared to provide a sense of their "robustness." A major "intercomparison" of GCMs is being conducted, and has shown large differences in regional precipitation and reduction of snow and ice fields at high latitudes. 19. Are these changes associated with an equivalent doubling of the preindustrial level of atmospheric CO2 that can be stated with confidence? Because of the uncertainty in our understanding of various factors, projections reflect different levels of confidence. Highly plausible: Global average surface warming   Global average precipitation increase   Reduction in sea ice   High-latitude surface winter warming Plausible: Global sea level rise   Intensification of summer mid-latitude, mid-continental drying   High-latitude precipitation increase

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Page 673 Highly uncertain: Local details of climate change   Regional distribution of precipitation   Regional vegetation changes   Increase in tropical storm intensity or frequency 20. What about storms and other extreme weather events? The factors governing tropical storms are different from those governing mid-latitude storms and need to be considered separately. One of the conditions for formation of typhoons or hurricanes today is a sea surface temperature of 26°C (79°F) or greater. With higher global average surface temperature, the area of sea with this temperature should be larger. Thus the number of hurricanes could increase. However, air pressure, humidity, and a number of other conditions also govern the creation and propagation of tropical cyclones. The critical temperature for their creation may increase as climate changes these other factors. There is no consistent indication whether tropical storms will increase in number or intensity as climate changes. Nor is there any evidence of change over the past several decades. Mid-latitude storms are driven by equator-to-pole temperature contrast. In a warmer world, this contrast will probably weaken since surface temperatures in high latitudes are projected to increase more than at the equator (at least in the northern hemisphere). Higher in the atmosphere, however, the temperature contrast strengthens. Increased atmospheric water vapor could also supply extra energy to storm development. We do not currently know which of these factors would be more important and how mid-latitude storms would change in frequency, intensity, or location. 21. Can projections be improved? Better computers alone will not solve the problems associated with positive and negative feedbacks. Better understanding of atmospheric physics and chemistry and better mathematical descriptions of relevant mechanisms in the models are also needed, as are data to validate models and their subcomponents. Significant improvements may require decades. 22. Is it possible to avoid the projected warming? It is possible only at great expense or by incurring risks not now understood, unless the earth is itself self-correcting. Continued increases in atmospheric concentrations of greenhouse gases would probably result in additional global warming. Avoiding all future warming either would be very costly (if we significantly reduce atmospheric concentrations of greenhouse gases) or potentially very risk (if we use climate engineering). However, a comprehensive action program could slow or reduce the onset of greenhouse warming.

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Page 681 to assess how much to spend on emission reductions or offsets. However, all estimates are approximations with very little precision. The amount to allocate to prevent additional greenhouse warming depends significantly on the preferred degree of risk aversion. Preventing or Reducing Additional Greenhouse Warming 32. What are the sources of greenhouse gas emissions? All of the major greenhouse gases except CFCs are produced by both natural processes and human activity. Table A.4 summarizes the principal sources of greenhouse gases associated with human activity. 33. What interventions could reduce greenhouse warming? It is useful to examine two different aspects of reducing emissions or offsetting emissions: • ''Direct" reduction or offsetting of emissions through altering equipment, products, physical processes, or behaviors • "Indirect" reduction or offsetting of emissions through altering the behavior of people in their economic or private lives and thus affecting the overall level of activity leading to emissions It is much easier to estimate potential effectiveness and costs of direct reductions than of indirect incentives on human behavior. This is mostly because of the many factors that affect behavior in addition to the incentives in any particular program. 34. How can specific mitigation options be compared? Mitigation options can be compared quantitatively in terms of their cost-effectiveness and qualitatively in terms of the obstacles to their implementation and in terms of other benefits and costs. The standard quantitative unit used to compare mitigation options is the cost per metric ton of carbon emissions reduced or per metric ton of carbon removed from the atmosphere. The amount of carbon can be converted to the amount of CO2 in the atmosphere by multiplying by 3.67, which is the ratio of the molecular weights of carbon and CO2. Other greenhouse gases can be "translated" to CO2 equivalency by using two calculations. First, the amount of radiative forcing caused by a specific concentration of the gas is estimated in terms of the change in energy reaching the surface (in watts per square meter). This estimate accounts for atmospheric chemistry, atmospheric lifetime of the gas, and other relevant factors affecting the total contribution of that gas to greenhouse warming. Second, the amount of

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Page 682 TABLE A.4 Estimated 1985 Global Greenhouse Gas Emissions from Human Activities       Greenhouse Gas Emissions (Mt/yr) CO2-equivalent Emissionsa (Mt/yr) CO2 Emissions           Commercial energy 18,800 18,800 (57)     Tropical deforestation 2,600 2,600 (8)     Other 400 400 (1)     TOTAL 21,800 21,800 (66) CH4 Emissions           Fuel production 60 1,300 (4)     Enteric fermentation 70 1,500 (5)     Rice cultivation 110 2,300 (7)     Landfills 30 600 (2)     Tropical deforestation 20 400 (1)     Other 30 600 (2)     TOTAL 320 6,700 (20)b CFC-11 and CFC-12 Emissions           TOTAL 0.6 3,200 (10) N2O Emissions           Coal combustion 1 290 (>1>     Fertilizer use 1.5 440 (1)     Gain of cultivated land 0.5 150 (>1)     Tropical deforestation 0.5 150 (>1)     Fuel wood and industrial biomass 0.2 60 (>1)     Agricultural wastes 0.4 120 (>1)     TOTAL 4 1,180 (4)   TOTAL   32,880 (100) NOTE: Mt/yr = million (106) metric tons (t) per year. All entries are rounded because the exact values are controversial. aCO2-equivalent emissions are calculated from the Greenhouse Gas Emissions column by using the following multipliers:     CO2 1         CH4 21         CFC-11 and -12 5,400         N2 290     Numbers in parentheses are percentages of total. bTotal does not sum due to rounding errors. SOURCE: Adapted from U.S. Department of Energy. 1990. The Economics of Long-Term Global Climate Change: A Preliminary Assessment—Report of an Interagency Task Force. Springfield, Va.: National Technical Information Service.

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Page 683 CO2 that would produce the same amount of forcing at the surface is calculated. This is the CO2 equivalent for that specific concentration of the other greenhouse gas. The respective costs per ton for different options can then be compared directly. It is important to recognize, however, that these calculations allow comparison only of initial contributions. They do not account for changes in energy-trapping effectiveness over the various lifetimes of these gases in the atmosphere. 35. What mitigation options are most cost-effective? The panel ranks options for reducing greenhouse gas emissions or removing greenhouse gases from the atmosphere according to their cost-effectiveness. Some of these options have net savings or very low net implementation costs compared to other investments. The options range from net savings to more than $100 per metric ton of CO2-equivalent emissions avoided or removed from the atmosphere. The most cost-effective mitigation options are presented in Table A.5. 36. What are examples of options with large potential to reduce or offset emissions? The so-called geoengineering options have the potential of substantially affecting atmospheric concentrations of greenhouse gases. They have the ability to screen incoming sunlight, stimulate uptake of CO2 by plants and animals in the oceans, or remove CO2 from the atmosphere. Although they appear feasible, they require additional investigation because of their potential environmental impacts. 37. How much would it cost to significantly reduce current U.S. greenhouse gas emissions? It depends on the level of emission reduction desired and how it is done. The most cost-effective options are those that enhance efficient use of energy: efficiency improvements in lighting and appliances, white roofs and paving to enhance reflectivity, and improvement in building and construction practices. Figure A.4 compares mitigation options, and Table A.5 gives the panel's estimates of net cost and emission reductions for several options. It must be emphasized that the table presents the panel's estimates of the maximum technical potential for each option. The calculation of cost-effectiveness of lighting efficiency, for example, does not consider whether the supply of light bulbs could meet the demand with current production capacities. Nor does it consider the trade-off between expenditures on light bulbs and on health care, education, or basic shelter for low-income families. In addition, there is a danger of some "double counting." For example, in the area of energy supply both nuclear and natural gas energy options assume replacement

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Page 684 TABLE A.5 Comparison of Selected Mitigation Options in the United States Mitigation Option Net Implementation Costa Potential Emission Reductionb (t CO2 equivalent per year) Building energy efficiency Net benefit 900 millionc Vehicle efficiency (no fleet change) Net benefit 300 million Industrial energy management Net benefit to low cost 500 million Transportation system management Net benefit to low cost 50 million Power plant heat rate improvements Net benefit to low cost 50 million Landfill gas collection Low cost 200 million Halocarbon-CFC usage reduction Low cost 1400 million Agriculture Low cost 200 million Reforestation Low to moderate costd 200 million Electricity supply Low to moderate costd 1000 millione NOTE: Here and throughout this report, tons are metric. aNet benefit = cost less than or equal to zero Low cost = cost between $1 and $9 per ton of CO2 equivalent Moderate cost = cost between $10 and $99 per ton of CO2 equivalent High cost = cost of $100 or more per ton of CO2 equivalent bThis "maximum feasible" potential emission reduction assumes 100 percent implementation of each option in reasonable applications and is an optimistic "upper bound" on emission reductions. cThis depends on the actual implementation level and is controversial. This represents a middle value of possible rates. dSome portions do fall in low cost, but it is not possible to determine the amount of reductions obtainable at that cost. eThe potential emission reduction for electricity supply options is actually 1700 Mt CO2 equivalent per year, but 1000 Mt is shown here to remove the double-counting effect. of the same coal-fired power plants. Table A.5, however, presents only options that avoid double counting. Finally, although there is evidence that efficiency programs can pay, there is no field evidence showing success with programs on the massive scale suggested here. Thus there may be very good reasons why "negative cost options" on the figure are not implemented today. The United States could reduce its greenhouse gas emissions by between 10 and 40 percent of the 1990 levels at low cost, or perhaps some net savings, if proper policies are implemented.

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Page 685 image FIGURE A.4 Comparison of mitigation options. Total potential reduction of CO2 equivalent emissions is compared to the cost in dollars per ton of CO2 reduction. Options are ranked from left to right in CO2 emissions according to cost. Some options show the possibility of reductions of CO2 emissions at a net savings. Adapting to Additional Greenhouse Warming 38. Will human and natural systems adapt without assistance? Farmers adjust their crops and cultivation practices in response to weather patterns over time. Natural ecosystems also adapt to changing conditions. The real issue is the rate at which human and natural systems will be able to adjust. 39. At what rates can human and natural systems adapt? Many human systems have decision and investment cycles that are shorter than the time in which impacts of climate change would become manifest. These systems in the United States should be able to adjust to climate change without governmental intervention, as long as it is gradual and information about the rates of change is widely available. This applies to agriculture, commercial forestry, and most of industry. Industrial sectors with extremely long investment cycles (e.g., transport systems, urban infrastructure, and major structures and facilities) or requiring high volumes of water may require special attention. Coastal urban settlements would be

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Page 686 able to react quickly (within 3 to 5 years) if sea level rises. Response would be much more difficult, however, where financial and other resources are limited, such as in many developing countries. Some natural systems adjust at rates an order of magnitude or more slower than those anticipated for global-scale temperature changes. For example, the observed and theoretical migration of large trees with heavy seeds is an order of magnitude slower than the anticipated change in climate zones. Furthermore, natural ecosystems cannot anticipate climate change but must wait until after conditions have changed to respond. 40. What is the value of the vulnerable natural ecosystems? Natural ecosystems contribute commercial products, but their value is generally considered to exceed this contribution to the economy. For example, genetic resources are generally undervalued because people cannot capture the benefits of investments they might make in preserving biodiversity. Many species are unlikely to ever have commercial value, and it is virtually impossible to predict which ones will become marketable. In addition, some people value natural systems regardless of their economic value. Loss of species, in their view, is undesirable whether or not those species have any commercial value. They generally hold that preservation of the potential for evolutionary change is a desirable goal in and of itself. Humanity, they claim, should not do things that alter the course of natural evolution. This view is sometimes also applied to humanity's cultural heritage—to buildings, music, art, and other cultural artifacts. 41. How much would it cost to adapt to the anticipated climatic changes? The panel's analysis suggests that some human and natural systems are not very sensitive to the anticipated climatic changes. These include most sectors of industry. Other systems are sensitive to climatic changes but can be adapted at a cost whose present value is small in comparison to the overall level of economic activity. These include agriculture, commercial forestry, urban coastal infrastructure, and tourism. Some systems are sensitive, and their adaptation is questionable. The unmanaged systems of plants and animals that occupy much of our lands and oceans adapt at a pace slower than the anticipated rate of climatic change. Their future under climate change would be problematic. Poor nations may also adapt painfully. Finally, some possible climatic changes like shifts in ocean currents have consequences that could be extremely severe, and thus the costs of adaptation might be very large. However, it is not currently possible to assess the likelihood of such cataclysmic changes. No attempt has been made to comprehensively assess the costs of anticipated climatic changes on a global basis.

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Page 687 42. How much should be spent in response to greenhouse warming? The answer depends on the estimated costs of prevention and the estimated damages from greenhouse warming. In addition, the likelihood and severity of extreme events, the discount rate, and the degree of risk aversion will modify this first-order approximation. The appropriate level of expenditure depends on the value attached to the adverse outcomes compared to other allocations of available funds, human resources, and so on. In essence, the answer depends on the degree of risk aversion attached to adverse outcomes of climate change. The fact that less is known about the more adverse outcomes makes this a classic example of dealing with high-consequence, low-probability events. Programs that truly increase our knowledge and monitor relevant changes are especially needed. Implementing Response Programs 43. What policy instruments could be used to implement response options? A wide array of policy instruments of two different types are available: regulation and incentives. Regulatory instruments mandate action, and include controls on consumption (bans, quotas, required product attributes), production (quotas on products or substances), factors in design or production (efficiency, durability, processes), and provision of services (mass transit, land use). Incentive instruments are designed to influence decisions by individuals and organizations and include taxes and subsidies on production factors (carbon tax, fuel tax), on products and other outputs (emission taxes, product taxes), financial inducements (tax credits, subsidies), and transferable emission rights (tradable emission reductions, tradable credits). The choice of policy instrument depends on the objective to be served. 44. At what level of society should actions be taken? Interventions at all levels of human aggregation could effectively reduce greenhouse warming. For example, individuals could reduce energy consumption, recycle goods, and reduce consumption of deleterious materials. Local governments could control emissions from buildings, transport fleets, waste processing plants, and landfill dumps. State governments could restructure electric utility pricing structures and stimulate a variety of efficiency incentives. National governments could pursue action in most of the policy areas of relevance. International organizations could coordinate programs in various parts of the world, manage transfers of resources and technologies, and facilitate exchange of monitoring and other relevant data.

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Page 688 45. Is international action necessary? The greenhouse phenomenon is global. Unilateral actions can contribute significantly, but national efforts alone would not be sufficient to eliminate the problem. The United States is the largest contributor of CO2 emissions (with estimates ranging from 17 to 21 percent of the global total). But even if this country were to totally eliminate or offset its emissions, the effect on overall greenhouse warming might be lost if no other countries acted in concert with that aim. 46. What about differences between rich and poor countries? Poor and developing countries are likely to be the most vulnerable to climate change. In addition, many developing countries today are sorely pressed in a variety of other ways. They may conclude that other issues have more immediate consequences for their citizens. Incentives in all parts of the world for intervention in the area of greenhouse warming may thus draw heavily on the industrialized nations. They may be called upon to help poor countries stimulate economic development and thus become better able to cope with climate change. They may also be asked to provide expertise and technologies to help poor countries adapt to the conditions they face. Actions to be Taken 47. Do scientific assessments of greenhouse warming tell us what to do? Current scientific understanding of greenhouse warming is both incomplete and uncertain. Response depends in part on the degree of risk aversion attached to poorly understood, low-probability events with extremely adverse outcomes. Lack of scientific understanding should not be used as a justification for avoiding reasoned decisions about responses to possible additional greenhouse warming. 48. Is it better to prevent greenhouse warming now or wait and adapt to the consequences? This complicated question has several parts. • First, will it be possible to live with the consequences if nothing is done now? The panel's analysis suggests that advanced, industrialized countries will be able to adapt to most of the anticipated consequences of additional greenhouse warming without great economic hardship. In some regions, climate and related conditions may be noticeably worse, but in other regions better. Countries that currently face difficulty coping with extreme climatic events, or whose traditional coping mechanisms are breaking down, may be sorely pressed by the climatic changes accompanying an equivalent doubling of atmospheric CO2 concentrations. It is important to recognize that there may be dramatic improvement or disastrous deterioration in specific locales. In addition, this analysis applies to the next 30 to 50 years. The situation may be different beyond that time horizon.

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Page 689 Natural communities of plants and animals, however, face much greater difficulties. Greenhouse warming would likely stress such ecosystems sufficiently to break them apart, resulting in a restructuring of the community in any given locale. New species would be likely to gain dominance, with a different overall mix of species. Some individual species would migrate to new, more livable locations. Greenhouse warming would most likely change the face of the natural landscape. Similar changes would occur in lakes and oceans. In addition, there are possible extremely adverse consequences, such as changing ocean currents, that are poorly understood today. The response to such possibilities depends on the degree of risk aversion concerning those outcomes. The greater the degree of risk aversion, the greater the impetus for preventive action. • Second, does it matter when interventions are made? Yes, for three different kinds of reasons. Because greenhouse gases have relatively long lifetimes in the atmosphere, and because of lags in the response of the system, their effect builds up over time. These time-dependent phenomena lead to the long-term "equilibrium" warming being greater than the "realized" warming at any given point in time. These dynamic aspects of the climate system show the importance of acting now to change traditional patterns of behavior that we have recently recognized to be detrimental, such as heavy reliance on fossil fuels. In addition, the implications of intervention programs for the overall economy vary with time. Gradual imposition of restraints is much less disruptive to the overall economy than their sudden application. Finally, the length of investment cycles can be crucial in determining the costs of intervention. In addition, some investments can be thought of as insurance, or payments now to avoid undesirable outcomes in the future. The choice is made more complicated by the fact that the outcomes are highly uncertain. • Third, what discount rate should be used? The selection of a discount rate is very controversial. Macroeconomic calculations for the United States show a return on capital investment of 12 percent. The choice of discount rate reflects time preference. The panel has used discount rates of 3, 6, and 10 percent in its analysis. Finally, consumers often behave as if they have used a discount rate closer to 30 percent. The panel has also included this rate for comparison when options involve individual action.

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Page 690 49. Are there special attributes of programs appropriate for response to greenhouse warming? Yes. The uncertainties present in all aspects of climate change and our understanding of response to potential greenhouse warming place a high premium on information. Small-scale interventions that are both reversible and yield information about key aspects of the relevant phenomena are especially attractive for both mitigation and adaptation options. Monitoring of emission rates, climatic changes, and human and ecologic responses should yield considerable payoffs. Perhaps the most important attribute of preferred policies is that they be able to accommodate surprises. They should be constructed so that they are flexible and can change if the nature or speed of stress is different than anticipated. 50. What should be done now? The panel developed a set of recommended options in five areas: reducing or offsetting emissions, enhancing adaptation to greenhouse warming, improving knowledge for future decisions, evaluating geoengineering options, and exercising international leadership. The panel recommends moving decisively to undertake all of the actions described under questions 51 through 55 below. 51. What can be done to reduce or offset emissions of greenhouse gases? Three areas dominate the panel's analysis of reducing or offsetting current emissions: eliminating CFC emissions and developing substitutes that minimize or eliminate greenhouse gas emissions, changing energy policy, and utilizing forest offsets. Eliminating CFC emissions has the biggest single contribution. Recommendations concerning energy policy are to examine how to make the price of energy reflect all health, environmental, and other social costs with a goal of gradual introduction of such a system; to make conservation and efficiency the chief element in energy policy; and to consider the full range of supply, conversion, end use, and external effects in planning future energy supply. Global deforestation should be reduced, and a moderate domestic reforestation program should be explored. 52. What can be done now to help people and natural systems of plants and animals adapt to future greenhouse warming? Most of the actions that can be taken today improve the capability of the affected systems to deal with current climatic variability. Options include maintaining agricultural basic, applied, and experimental research; making water supplies more robust by coping with present variability; taking into consideration possible climate change in the margins of safety for long-lived structures; and reducing present rates of loss in biodiversity.

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Page 691 53. What can be done to improve knowledge for future decisions? Action is needed in several areas. Collection and dissemination of data that provide an uninterrupted record of the evolving climate and of data that are needed for the improvement and testing of climate models should be expanded. Weather forecasts should be improved, especially of extremes, for weeks and seasons to ease adaptation to climate change. The mechanisms that play a significant role in the responses of the climate to changing concentrations of greenhouse gases need further identification, and quantification at scales appropriate for climate models. Field research should be conducted on entire systems of species over many years to learn how CO2 enrichment and other facets of greenhouse warming alter the mix of species and changes in total production or quality of biomass. Research on social and economic aspects of global change and greenhouse warming should be strengthened. 54. Do geoengineering options really have potential? Preliminary assessments of these options suggest that they have large potential to mitigate greenhouse warming and are relatively cost-effective in comparison to other mitigation options. However, their feasibility and especially the side-effects associated with them need to be carefully examined. Because the geoengineering options have the potential to affect greenhouse warming on a substantial scale, because there is convincing evidence that some of these cause or alter a variety of chemical reactions in the atmosphere, and because the climate system is poorly understood, such options must be considered extremely carefully. If greenhouse warming occurs, and the climate system turns out to be highly sensitive to radiative forcing, they may be needed. 55. What should the United States do at the international level? The United States should resume full participation in international programs to slow population growth and contribute its share to their financial and other support. In addition, the United States should participate fully in international agreements and programs to address greenhouse warming, including representation by officials at an appropriate level.