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Recent data on the longevity of excess human-generated CO, in the atmosphere argue for only modest reductions in greenhouse-gas emissions

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carly 7 years ago in these sume pages, I gave my views on the state of research on global warming (Nierenberg. 1988). The purpose of this article is to update that carlier assessment. In the time since my 1988 article was published in The Bridge, the United States has spent some $6 billion on research related to climate change and global warming. The nation currently invests about $1.6 billion annually on such studies. Given this level of support, what have been the results? What is known now that was not known in the mid1980s? Accepting that increasing CO2 concentrations will induce climate changes, when and where will these changes occur, and what will be their extent? Most important, what are the implications of the data that have been gathered, both for science and public policy?

Atmospheric CO2

The one clear piece of information to emerge over the last decade is that CO, concentrations in the atmo sphere are increasing steadily. This supports the findings of C.D. Keeling and his colleagues (Keeling and Bacastow, 1977), who studied the impact of industrial gases on climate nearly 20 years ago. The rate of increase slowed between 1980 and 1990, but this sort of fluctuation is not unusual. Most believe the upward trend will continue to parallel global population growth. Reductions in CO2 output by industrial nations through their use of energy efficient technologies will probably be offset by increases in greenhousegas production by transitional and developing countries as their standards of living improve.

Scientists understand much less about the time dependence of the anthropogenic excess of CO2 in the atmosphere. Most observers agree that the pattern of rising global average surface temperatures over the last 100 years does not conclusively point to CO, as the culprit; "normal" climate fluctuation may be to blame. Still, the near irreversibility of the predicted phe

early efforts to calculate the longevity of excess atmospheric CO came up with estimates as high as 1,000 years (Keeling and Bacastow, 1977). That is, it would take a millennium for the COą to be absorbed into the total global system-essentially, the world's oceans.

The implications of this estimate are important. If true, there would be no clear path to take to reverse CO-induced climatic changes. And since it will take 200 to 300 years to exhaust the world's supply of fossil carbon fuels, modest reductions (on the order of 20 percent) in CO2 emissions would probably have little effect on climate. Reductions on the order of 80 percent would be necessary. That would be a daunting challenge.

More recently, with the development of reasonable ocean models coupled to atmospheric climate models, researchers have arrived at greatly reduced estimates of the decay time of atmospheric CO2. Research now suggests that the decay curve is really a sum of exponentials. Current estimates indicate that it takes between 50 and 150 years for anthropogenic CO2 to be absorbed into the oceans (O'Neill et al., 1994).'

The amount of CO2 in the atmosphere may be able to be "tailored" in some optimum fashion.

From a policy standpoint, these findings raise two important issues. First, given these decay times, there may be no urgent need to take action to reduce emissions until the severity of the climate effects are either firmly predicted or clearly evident. If the climatechange "signal" does emerge from the natural background fluctuations and is unfavorable, there is ample time at that point to take whatever remedial action, including emissions controls, may be needed. Second, in contrast to the case if CO were expected to stay in the atmosphere for 1,000 years, the new estimates mean that reducing emissions will result in a corresponding reduction of the peak CO2 extess. Therefore, not only is there considerable elasticity in atmo spheric response to CO2, but the amount of CO2 in the atmosphere may be able to be “tailored” in some opti

One aspect of the climate warming issue that has not changed is the uncertainty surrounding how much the global average surface temperature will rise in response to a doubling of the concentration of greenhouse gases.2 There are approximately 15 computer facilities around the world devoted to modeling the hydrodynamics and thermodynamics of the atmosphere and oceans. In 1983, a committee assembled by the National Academy of Sciences (NAS) predicted that a twofold jump in greenhouse gas amounts would boost the global average surface temperature by between 1.5 and 4.5 degrees Celsius (Carbon Dioxide Assessment Committec, 1983). This temperature range was similar to values reported in an earlier NAS report (Assembly of Mathematical and Physical Sciences, 1979).

Estimates of how much the seas will rise due to increasing concentrations of CO2 in the atmosphere have improved considerably.

Today, despite continuing efforts to refine these models, the imprecision in the temperature-change predictions remains. This poses a dilemma, since the observed temperature rise in the last 100 years, if attributable to increased CO2 concentrations, lies just at the bottom limit of the range proposed by the 1983 NAS panel. Furthermore, the shape of the curve is not consonant with the predicted model behavior.

It is unlikely that researchers will be able to better pin down CO2-associated temperature fluctuations in the foreseeable future. The models they use are mathematically complex, often embodying a large number of empirically determined physical and biological phenomena (such as the effect of soil moisture, the water retention of soils, ice formation and breakup, cloud formation in height, thickness, and density, as well the distribution of water droplets and aerosols). Some models also try to account for feedback phenomena, such as increased photosynthetic activity and oxidation of clathrates, while others attempt to incorporate changes in the rate of growth of other atmospheric

greenhouse gases like methane, whose sources are poorly understood. Differences in the computers, software, numerical analyses, and phenomenologies used by the various research groups add to the problem.

Another key weakness of the models, apparent even a decade ago, is their inability to capture adequately the interactions between water and solar radiation. Water vapor is the principal greenhouse gas, and its effects far exceed those of CO. Increasing CO2 concentrations actually enhance the thermodynamic behavior of atmospheric water. Although our understanding of these interactions has improved, we are still far from resolving their complexities. Thus, even as the individual elements of the models are better understood, the necessary introduction of new real-world climate variables will continue to hinder precise predictions of temperature change.

Efforts to study climate have placed great emphasis on global average surface temperature. In the 1988 Bridge article, I pointed out that this number is a surrogate measure of regional climate changes that can be of serious consequence, such as droughts, floods, and storms, as well as of long-term ecological changes. While the average global temperature may vary by only a small amount, alterations in certain aspects of regional climates may be appreciable. Unfortunately, current models of climate change rarely yield consistent results in this area. Over the past 10 years, researchers have also learned that the global increase of 0.6 degree Celsius recorded during the past 100 years is attributable largely to a shift in nighttime temperatures. The fluctuation of daytime temperatures has been minimal.

Estimates of how much the seas will rise due to increasing concentrations of CO2 in the atmosphere have improved considerably. The 1983 NAS report predicted that a doubling of CO2 would raise ocean heights about 2 feet. Now, however, scientists believe the correct amount is closer to 1 foot, a figure that agrees with satellite measurements of the current rate of change in sea level. This is supported by the very small change in the thickness of the principal ice caps.

Research on Climate, Temperature, and CO2

During the last 10 years, long-awaited data from several research projects have shed light on some important climate questions. Ice cores drilled in the Antarctic and Greenland ice caps, for example, provided a historical picture of the Earth's climate going back 100,000 years. One of the first bits of information to be extracted from trapped gases inside the cores was the aumospheric CO2 concentration in the period immediately preceding the Industrial Revolution. Despite

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some disagreement among researchers about the cise amount of CO, during this period, the ice-core data are useful benchmarks against which the Keeling findings on CO2 (Barnola et al., 1979; Jouzel et al., 1987) can be compared. By calculating the ratio of oxygen isotopes to CO2 in the ice, scientists have gained insights into whether changes in CO2 concentrations led or followed alterations in surface temperature. High-resolution analyses suggested that the temperature changes may have actually preceded changes in atmospheric CO2 (Barnola et al., 1991). The full significance of this is not completely clear. However, it is possible that a global climate shift led to a general warming of the oceans, which expelled CO2 into the aumosphere. The ice-core results also indicate that the fast 10,000 years have been exceptionally stable in terms of climate.

Climatologists have been able to glean important information about global surface temperatures from the microwave emissions of atmospheric oxygen (Christy and Spencer, 1993; Spencer and Christy, 1990; Spencer and Christy, 1992a; Spencer and Christy, 1992b). Satellite observations of the intensities of the microwave spectra have provided good temperature estimates over the last 15 years. The strength of the method is that it allows almost total coverage of the planet, including the oceans, without the need for spe cial corrections, such as for the effects of urbanization. Although meteorological records show a sharp rise in surface temperature during this period, the satellite measurements show essentially no temperature fluctuation, and they do not reveal the differences between daytime and nighttime temperatures reported by others. While a 15-year temperature record is too short to draw conclusions about long-term trends, the satellite measurements have raised questions about the representativeness of the surface observations.

Researchers have also taken advantage of test oil borings-holes drilled and then abandoned in prospecting for petroleum-on the Canadian shield to gather information about temperature (Lachenbruch and Marshall, 1986; Lewis, 1992). Because these holes were drilled many years ago, they are geologically stable and therefore useful for temperature studies. Temperature "logging" of bore holes is routine in the oil exploration industry. By inverting the thermal diffusion equations using temperature versus depth data, investigators determined local surface temperatures going back several thousand years. The bore-hole measurements show a larger increase in the global average temperature, a finding that agrees with most model predictions but is inconsistent with surface meteorological measurements.

During the past decade, what we have learned about the balance between the sources and sinks of CO has led to considerable confusion. According Keeling, each year the amount of carbon in the atmosphere increases by 3 gigatons (3x1015 gm). Human activity vents close to twice this amount into the atmosphere, and as recently as 10 years ago researchers believed that the oceans absorbed the difference. Our ability to measure the direct flux of CO2 into the oceans has since improved, and it now appears that these bodies of water take up much less of the gas than first thought (Tans et al., 1990). While the ocean measurements do not cover Earth as uniformly as might be wished, the results are enlightening. What accounts for this discrepancy? One answer is that the excess CO2 is being incorporated into forests and other biomass in the northern hemisphere. This view does not conform to generally accepted notions about deforestation, however.

It now appears that the oceans take up much less CO2 than first thought.

Concerns about global warming have been based on predictions that the growth in CO2 emissions during this century would exceed that of the last. Economists now see a generally more complex picture (Nordhaus, 1994). During the last 10 years, worldwide CO2 emissions have departed markedly from the average growth of the preceding 100 years, remaining reasonably flat. Emissions can be expected to rise again, but the most likely result is that the rate of their increase will, on the average, continue as it has during the past 100 years.

One of the most important contributors to the greenhouse effect besides CO2 methane. Up until about 10 years ago, methane amounts in the atmosphere were increasing exponentially. Many thought this trend would continue, but recent measurements suggest this growth has largely subsided. The same may be true for the chlorofluorocarbons (CFCs). Although a significant element of the greenhouse warming effect a decade ago, CFC concentrations in the atmosphere are expected to decline because of provisions in the Montreal Convention intended to protect stratospheric ozone.

During the past 10 years, we have begun to under

stand that variations in global temperatures correspond in a logarithmic rather than linear fashion to atmospheric CO2 amounts. (This is because normal concentrations of CO2 are quite high, approaching saturation.) Since atmospheric CO2 concentrations are expected to continue to grow exponentially, the temperature increase should be linear. If this is true, then the observed temperature rise due to CO2 increases will be the same (about 0.5 degree Celsius) over the next 100 years as it has been during the last century. This half-degree rise falls below the range currently projected, which was calculated assuming higher greenhouse gas emissions and which takes into account the effects on temperature of water in the atmosphere.

The overall effect of carbon taxes or other methods of constraining CO2 emissions is not at all clear.

Mitigation: Prospects and Problems

Researchers have learned a good deal about mitigating the effects of global warming. Most attention has focused on two approaches, one technical and the other socioeconomic. Mathematically, the two approaches are similar. Indeed, the climate and socioeconomic models both are based on highly nonlinear coupled equations containing many dependent variables.

Technical strategies are those that would use some physical or biological method to either absorb excess CO2 or alter the natural environment in order to reduce solar insulation, thus compensating for the effects of increased CO2 concentrations. An example of the latter approach would be to enhance artificially the amount of aerosols in the atmosphere. A socioeconomic strategy would be to lower CO2 emissions by reducing energy usage.

It is not my purpose here to present a complete review of the many climate-change mitigation propos als that have been outlined in the past. Assessing their viability would require estimates of regional climate change. Because current climate models are not capa

ble of accurately predicting regional perturbations caused by CO, increases, we are not now in a position to gauge the effects of deliberately altering the climate. Take the case of adding artificial aerosols to the atmosphere. However introduced, we cannot project their distribution by latitude, longitude, or height Without this information, it is impossible to assess how this approach might offset the many regional climate effects caused by increased CO2 concentrations.

Other proposed mitigation plans also have weaknesses. One popular plan suggests the planting of vast expanses of forests that would absorb CO2 and be recy cled for fuel, thus helping to maintain an equilibrium concentration of the gas. However, estimates indicate that warming caused by the reflectivity of the increased forest acreage would nearly cancel out the cooling effect of enhanced CO, absorption. In addition, the geographical differences in surface reflectivity probably would not match the regional effects of CO2, leading to variable and highly unpredictable regional climate changes.

With less clear examples, the same can be said of the socioeconomic models. There are strong links and considerable feedback between the climate and economic models. Therefore, it is not at all clear what the overall effect would be of carbon taxes or other methods of constraining CO2 emissions.

Conclusion

This article raises questions that need to be addressed as scientists and policymakers consider the future course of climate warming. How can we better separate the greenhouse “signal" from the variety of background fluctuations in the global climate system? If science cannot sharpen its predictions in this area, then how much are we willing to spend on mitigation efforts, such as reducing the use of carbon fuels, investing in "acceptable" nuclear sources of energy, or exploiting breakthroughs in solar and other nonfossil sources of energy?

This review began by noting the dramatic reductions in the longevity estimates for excess atmospheric CO. I believe that this new understanding of the CO2 lifecycle argues for relatively modest reductions in greenhouse gas emissions. It also makes plausible a strategy of waiting for the "signal" to emerge from the climate "noise" before taking serious remedial action. Finally, it also suggests future directions for climate research, which might illuminate many of the questions raised here.