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more energy efficient economy are sufficient to make this path attractive to most countries. It is likely that the shape of the U.S. and global CO2 emissions curves will continue to be fundamentally congruent. In any case, any strategy for achieving a climate change "soft landing”, whether pursued unilaterally or otherwise, surely requires that the downward change in the U.S. CO2 emission growth rate be at least comparable to the change needed in the global average. There are many reasons for the United States to aggressively pursue the technology needed to achieve reduced CO2 emissions, including potential economic benefit and reduced dependence on foreign energy sources.

It is not our task to suggest specific policies. However, there are options for achieving the slower CO2 growth rate. Otherwise, the alternative scenario is not viable.

In the short-term, a case can be made that pent-up slack in energy efficiency (14), if pursued aggressively, can help achieve a zero or slightly negative CO2 emissions growth rate. Renewable energy sources, even though their output is relatively small, also can contribute to slowing the growth rate of emissions. There has been resistance of some industries to higher efficiency requirements. In that regard, the experience with chlorofluorocarbons is worth noting. Chemical manufacturers initially fought restrictions on CFC production, but once they changed their position and aggressively pursued alternatives they made more profits than ever. Similarly, if substantially improved efficiencies are developed (for air conditioners, appliances, etc.), such that there is a significant gap between operating costs of installed infrastructure and available technologies, that could facilitate increased turnover. Perhaps government or utility actions to encourage turnover also might be considered. Corporations will eventually reap large profits from clean air technologies, energy efficiency, and alternative energies, so it is important for our industry to establish a leadership position.

In the long-term, many energy analysts believe it is unlikely that energy efficiency and alternative energy sources can long sustain a global downtrend in CO2 emissions. Lovins (15) argues otherwise, pointing out the cost competitiveness of efficient energy end-use, gas-fired cogeneration and trigeneration at diverse scales, wind power and other renewable sources. Certainly it makes sense to give priority to extracting the full potential from efficiency and renewable energy sources. Holdren (16) concludes that meeting the energy challenge requires that we maximize the capabilities and minimize the liabilities in the full array of energy options.

Many (my impression is, most) energy analysts believe that the requirement of a flat-to-downward trend of CO2 emissions probably would require increasing penetration of a major energy source that produces little or no CO2. Our task is only to argue that such possibilities exist. It will be up to the public, through their representatives, to weigh their benefits and liabilities. We mention three possibilities.

(1) Nuclear power: if its liabilities, including high cost and public concern about safety, waste disposal and nuclear weapons proliferation, can be overcome, it could provide a major no-CO2 energy source. Advocates argue that a promising new generation of reactors is on the verge of overcoming these obstacles (17). There does not seem to be agreement on its potential cost competitiveness.

(2) Clean coal: improved energy efficiency and better scrubbing of particulate emissions present an argument for replacing old coal-fired power plants with modern designs. However, CO2 emissions are still high, so an increasing long-term role for coal depends on development of the "zero emissions" plant, which involves CO2 capture and sequestration (18).

(3) Others: Oppenheimer and Boyle (19) suggest that solar power, which contributes very little of our power at present, could become a significant contributor if it were used to generate hydrogen. The hydrogen can be used to generate electricity in a fuel cell. Of course the other energy sources can also be used to generate hydrogen.

In Holdren's (16) words: there are no silver bullets (in the array of energy options) nor are there any that we can be confident that we can do without. This suggests the need for balanced, increased public and private investment in research and development, including investments in generic technologies at the interface between energy supply and end use (20). The conclusion relevant to the alternative scenario is that, for the long-term, there are a number of possibilities for energy sources that produce no CO2.

7. Benchmarks.

The alternative scenario sets a target (1 W/m2 added climate forcing in 50 years) that is much more ambitious than IPCC business-as-usual scenarios. Achievement of this scenario requires halting the growth of non-CO2 climate forcings and slightly declining CO2 emissions. Climate change is a long-term issue and strategies surely must be adjusted as evidence accumulates and our understanding improves. For that purpose it will be important to have quantitative measures of the climate forcings.

Non-CO2 forcings. The reason commonly given for not including O, and soot aerosols in the discussions about possible actions to slow climate change is the difficulty in quantifying their amounts and sources. That is a weak argument. These atmospheric constituents need to be measured in all countries for the sake of human health. The principal benchmark for these constituents would be their actual amounts. At the same time, we must develop improved understanding of all the sources of these gases and aerosols, which will help in devising the most cost-effective schemes for reducing the climate forcings and the health impacts.

Methane, with an atmospheric lifetime of several years, presents a case that is intermediate between shortlived air pollutants and CO2. Measurements of atmospheric amount provide a means of gauging overall progress toward halting its growth, but individual sources must be identified better to allow optimum strategies. Improved source identification is practical. In some cases quantification of sources can be improved by regional atmospheric measurements in conjunction with global tracer transport modeling.

Carbon Dioxide. Is it realistic to keep the CO2 growth rate from exceeding that of today? The single most important benchmark will be the annual change of CO2 emissions. Figure 11 shows the United States record in the 1990s. The requirement to achieve the "alternative scenario" for climate forcings is that these annual changes average zero or slightly negative. It is apparent that CO2 emissions grew at a rate that, if continued, would be inconsistent with the alternative scenario.

We suggest in the discussion above that it is realistic to aim for a lower emission rate that is consistent with the alternative scenario. This particular benchmark should receive much closer scrutiny than it has heretofore. The climate simulations and rationale presented above suggest that, if air pollution is controlled, the trend of this CO2 benchmark, more than any other single quantity, can help make the difference between large climate change and moderate climate change.

8. Communication.

Our paper on the alternative scenario (1) was reported with a variety of interpretations in the media. As I discuss in an open letter (21), this may be unavoidable, as the media often have editorial positions and put their own spin on news stories. Overall, the media correctly conveyed the thrust of our perspective on climate change. Furthermore, I suggest in my open letter that the Washington Post editorial on our paper (23) represented an astute assessment of the issues.

A basic problem is that we scientists have not informed the public well about the nature of research. There is no fixed "truth" delivered by some body of "experts". Doubt and uncertainty are the essential ingredient in science. They drive investigation and hypotheses, leading to predictions. Observations are the judge.

Of course, some things are known with higher confidence than others. Yet fundamental issues as well as details are continually questioned. The possibility of finding a new interpretation of data, which provides better insight into how something in nature works, is what makes science exciting. A new interpretation must satisfy all the data that the old theory fit, as well as make predictions that can be checked.

For example, the fact that the surface of the Earth has warmed in the past century is well established, and there is a high degree of confidence that humans have been a significant contributor to this warming. However, there are substantial uncertainties about the contributions of different forcings and how these will change in the future.

In my open letter (21) I note the potential educational value of keeping an annual public scorecard of measured changes of (1) fossil fuel CO2 emissions, (2) atmospheric CO2 amount, (3) human-made climate forcing, and (4) global temperature. These are well-defined quantities with hypothesized relationships. It is possible to make the science understandable, and it may aid the discussions that will need to occur as years and decades pass. It may help us scientists too.

9. Summary: A Brighter Future.

The "business-as-usual” scenarios for future climate change provide a useful warning of possible global climate change, if human-made climate forcings increase more and more rapidly. I assert not only that a climatically brighter path is feasible, but that it is achievable via actions that make good sense for other reasons (22, 24). The alternative scenario that we have presented does not include a detailed strategic plan for dealing with global warming. However, it does represent the outline of a strategy, and we have argued that its elements are feasible.

It is impractical to stop CO2 from increasing in the near term, as fossil fuels are the engine of the global economy. However, the decline of the growth rate of CO2 emissions from 4 to 1%/year suggests that further reduction to constant emissions is feasible. The potential economic and strategic gains from reduced energy imports themselves warrant the required efforts in energy conservation and development of alternative energy sources. It is worth noting that global CO2 emissions declined in 1998 and again in 1999, and I anticipate that the 2000 data will show a further decline. Although this trend may not be durable, it is consistent with the alternative scenario.

The other requirement in our alternative scenario is to stop the growth of non-CO2 forcings, which means, primarily, air pollution and methane. The required actions make practical sense, but they will not happen automatically and defining the optimum approach requires research.

A strategic advantage of halting the growth of non-CO2 forcings is that it will make it practical to stop the growth of climate forcings entirely, in the event that climate change approaches unacceptable levels. The rationale for that claim is that an ever-growing fraction of energy use is in the form of clean electrical energy distributed by electrical grids. If improved energy efficiency and non-fossil energy sources prove inadequate to slow climate change, we may choose to capture CO2 at power plants for sequestration.

Climate change is a long-term issue. Strategies will need to be adjusted as we go along. However, it is important to start now with common-sense economically sound steps that slow emissions of greenhouse

gases, including CO2, and air pollution. Early emphasis on air pollution has multiple immediate benefits, including the potential to unite interests of developed and developing countries. Barriers to energy efficiency need to be removed. Research and development of alternative energies should be supported, including a hard look at next generation nuclear power. Ultimately strategic decisions rest with the public and their representatives, but for that reason we need to make the science and alternative scenarios clearer.


1. Hansen, J., M. Sato, R. Ruedy, A. Lacis and V. Oinas, Global warming in the twenty-first century: an alternative scenario, Proc. Natl. Acad. Sci., 97, 9875-9880, 2000.

2. Intergovernmental Panel on Climate Change, Climate Change 1995, J.T. Houghton, L.G. Meira Filho, B.A. Callandar, N Harris, A. Kattenberg and K. Maskell (eds.), Cambridge Univ. Press, Cambridge, England, 572 pp., 1996; Intergovernmental Panel on Climate Change, Climate Change 2000, editors........2001.

3. Hansen, J., R. Ruedy, A. Lacis, M. Sato, L. Nazarenko, N. Tausnev, I. Tegen and D. Koch, in General Circulation Model Development, ed. D. Randall, Academic Press, New York, pp. 127-164, 2000.

4. Hoffert M.I. and C.Covey, Deriving global climate sensitivity from paleoclimate reconstructions, Nature, 360, 573-576, 1992. 5. Hansen, J., A. Lacis, D. Rind, G. Russell, P. Stone, I. Fung, R. Ruedy and J. Lerner, Climate sensitivity: analysis of feedback mechanisms, Geophys. Mono., 29, 130-163, 1984.

6. Hansen, J. M. Sato, L. Nazarenko, R. Ruedy, A. Lacis, D.Koch, I. Tegen, T. Hall, D. Shindell, P. Stone, T. Novakov, L. Thomason, R. Wang, Y. Wang, D. Jacob, S. Hollandsworth, L. Bishop, J. Logan, A. Thompson, R. Stolarski, J. Lean, R. Willson, S. Levitus, J. Antonov, N. Rayner, D. Parker and J. Christy, Climate forcings in the GISS SI2000 model, submitted to J. Geophys. Res., 2001.

7. Levitus, S., J.I. Antonov, T.P. Boyer and C. Stephens, Warming of the world ocean, Science, 287, 2225-2229, 2000. 8. Kunzil, N., R. Kaiser, S. Medina, M. Studnicka, O. Chanel, P. Filliger, M. Herry, F. Horak, V. Puybonnieux-Texier, P. Quenel, J. Schneider, R. Seethaler, J.C. Vergnaud and H. Sommer, Public health impact of outdoor and traffic-related air pollution: a European assessment, The Lancet, 356, 795-801, 2000.

9. Smith, K.R., National burden of disease in India from indoor air pollution, Proc. Natl. Acad. Sci., 97, 13286-13293, 2000. 10. Streets, D.G., S. Gupta, S.T. Waldhoff, M.Q. Wang, T.C. Bond and B. Yiyun, Black carbon emissions in China, Atmos. Envir., in press, 2001.

11. Jacob, D.J., J.A. Logan and P.P. Murti, Effect of rising Asian emissions on surface ozone in the United States, Geophys. Res. Lett., 26, 2175-2178, 1999.

12. Lee, S.H., H. Akimoto, H. Nakane, S. Kurnosenko and Y. Kinjo, Lower tropospheric ozone trend observed in 1989-1998 at Okinawa, Japan, Geophys. Res. Lett., 25, 1637-1640, 1998.

13. Simmonds, P.G., S. Seuring, G. Nickless and R.G. Derwent, Segregation and interpretation of ozone and carbon monoxide measurements by air mass origin at the TOR station Mace Head, Ireland from 1987 to 1995, J. Atmos. Chem., 28, 45-59, 1997.

14. Brown, M.A., The role of CO2 gases in climate policy, workshop of United States Association for Energy Economics, Washington, DC, October 16, 2000.

15. Lovins, A.B. and L. Hunter Lovins, Climate: Making Sense and Making Money, Rocky Mountain Institute, Snowmass, CO,; Hawken, P.G., A.B. Lovins and L.H. Lovins, Natural Capitalism, Little Brown, NY, 1997,

16. Holdren, J.P., Meeting the energy challenge, Science, 291, 945, 2001.

17. Wald, M.L., Industry gives nuclear power a second look, New York Times, April 24, 2001.

18. Ecoal, Harnessing energy with reduced emissions to atmosphere - the pace of research to generate dynamic solutions for coal, World Coal Institute Newsletter, 36, December, 2000; also see Williams, R.H., presentation at symposium at Nuclear Control Institute, Washington, DC, April 9, 2001..

19. Oppenheimer, M. and R.H. Boyle, Dead Heat, Basic Books, New York, 1990.

20. Nakicenovic, N., A. Grübler and A. McDonald, Global Energy Perspectives, Cambridge Univ. Press, Cambridge, U.K., 1998. 21. Hansen, J., An open letter on global warming,

22. Hansen, J., A brighter future, Clim. Change, in press, 2001.

23. Anonymous, Hot news on global warming, Washington Post, page A18, August 28, 2000.

24. Hansen, J., Try a common-sense response to global warming, International Herald Tribune, Nov. 16, 2000.


Figure 1. Climate forcing during the Ice Age 20,000 years ago relative to the current interglacial period. This

forcing of -6.6! 1.5 W/m2 and the 5°C cooling of the Ice Age imply a climate sensitivity of 0.75°C per W/m2. Figure 2. Estimated change of climate forcings between 1950 and 2000, based on (1) with five principal aerosols delineated.

Figure 3. Climate forcings in the past 50 years, relative to 1950, due to six mechanisms (6). The first five forcings

are based mainly on observations, with stratospheric H2O including only the source due to CH oxidation. GHGs include the well-mixed greenhouse gases but not O3 and H2O. The tropospheric aerosol forcing is uncertain in both its magnitude and time dependence.

Figure 4. Simulated and observed climate change for 1950-2000 (6). These simulations with the GISS climate model (3) employ empirical mixing rates and fixed horizontal heat transports in the ocean (5). Climate forcings are those in Figure 3.

Figure 5. Simulated temperatures and planetary energy imbalance for the forcings in Figure 3 (6). The business-asusual scenario (1% CO2/year) adds 2.9 W/m2 forcing in 2001-2050. The alternative scenario adds a

greenhouse gas forcing of 1.1 W/m2 in that period and includes volcanoes similar to those during 1951-2000. Figure 6. Cartoon depicting approximate added climate forcings in an extreme "business-as-usual" scenario and the

"alternative" scenario (8).

Figure 7. Measured greenhouse gas amounts and "alternative scenario" extensions to 2050. IS92a scenarios of

IPCC (2) for CO2, CH, and N2O are illustrated for comparison. The sum of CFC and “other trace gas" forcings is constant after 2000 in the alternative scenario.

Figure 8. Annual emissions of CO2 from fossil fuels in the United States (principal data source: Oak Ridge

National Laboratory, Department of Energy).

Figure 9. Annual emissions of CO2 from fossil fuels in the world (principal data source: Oak Ridge National
Laboratory, Department of Energy).

Figure 10. Percentage of world fossil-fuel CO2 emissions produced in the Untied States.
Figure 11. Annual change of United States fossil-fuel emissions.

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