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The threat of midlatitude drought, and resulting crop failures in the breadbaskets of the world, is a significant potential danger. The food supply of a planet that will have many more mouths to feed is threatened. It is difficult to quantify this threat. While IPCC studies (Watson et al., 1996) show possible large increases and decreases in crop productivity in different regions of the world, with no net large changes in current production, much more work is needed.

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Other potential impacts on humans include stronger and more violent storms, coastal flooding and erosion, forest declines, spreading of deserts, increased pest outbreaks, increased fire frequency and intensity, more intense droughts and floods, spread of tropical diseases, poorer winter skiing and snowboarding, increased human mortality and illness from heat, and increased economic and geographical dislocations. The distribution of impacts is not uniform around the world. Ironically, while the developed nations of the world produce the majority of greenhouse gases, it appears that developing countries will be more severely affected. However, quantified estimates of total damage to society are currently quite uncertain.

What Should We Do?

Here I give you my professional opinion based on my scientific and political knowledge. Policy responses will have to made in an environment of uncertainty, but not in an environment of ignorance.

The basic question we have to address now is:

1. Would slower climate change be better for humans than rapid change?

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2. Must we act now to stop the warming, or will continued change be OK for a while?

3. Is there some threshold we must avoid that would result in vastly greater harm to society?

I do not think we can answer questions 2 or 3 now. If the answer to 1. is "No," then indeed we should not change anthropogenic activities which inadvertently produce greenhouse gases (predominantly burning fossil fuels and deforestation), and perhaps should even intentionally produce these gases. If the answer to 1. is "Yes," and I maintain that it is, then clearly we should take action now. It seems clear that many current human systems and ways of living, from agricultural to choosing building sites, are designed with the implicit assumption that climate will not change. Rapid climate change, at a rate faster than ever before experienced by our species, will exact substantial societal costs for adaptation. Rapid climate change could also have benefits and produce opportunities, but I see no evidence that they will outweigh the costs.

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If the answer to any of the questions is "We don't know," then we must act as if the answer is "Yes," as a matter of insurance against finding out too late that it is "Yes."

In the considerations here, it is important to point out that there is a great time lag in the global warming problem. In the first place, the atmospheric lifetime of the major anthropogenic greenhouse gases is decades or centuries. In the second place, it takes decades for the climate system to respond to imposed forcings (such as greenhouse gas and aerosol changes) due to the large mass of water in the upper ocean that must be heated or cooled. This means that all the greenhouse gases we have put into the atmosphere by human activities during the past 50 years will continue to warm the climate system for decades to come in the future, even if we stop emissions today. The implication of this inherent time lag is that if we wait to act until we are sure we have identified a clear anthropogenic signal in the climate system or until we unambiguously understand all the potential human impacts, we will have inadvertently subjected the world to a massive climate change that cannot be reversed in less than a century, if at all. Any solution will have to be a long-term one, and the sooner we get started, the less the eventual maximum climate change.

Our response to the threat of global warming at this time should be one of adaptation, improved knowledge, and mitigation. "No regrets" responses should be strongly pursued. I will briefly comment on each of these.

Adaptation. No matter what our response, the planet will warm. The most we can hope to achieve is to slow the rate of warming in the next century. Therefore, in the case of each threat to society listed above, all the threats not mentioned, and the threats that will appear in the future that we are not smart enough to imagine now, we will have to adapt to minimize the negative impacts. This adaptation will require much better information and technological innovations. This represents a significant business opportunity in the United States to develop the necessary devices and products and to market them to the world.

Improved knowledge. We need better data, better models, better computers, and more trained scientists and engineers to address the problems presented by global warming. Investing in the nation's scientific research establishment is a very inexpensive, and very rewarding, allocation of the nation's resources. The current U.S. Global Change Research Program budget (Our Changing Planet, 1997) of $1.81 billion, consisting of $1.12 billion of hardware (satellites) and $688 million in research funding, is barely adequate to answer all the pressing questions. We have to know where and when temperature, precipitation, storm, and sea level changes will take place. We need to know the biological response of agricultural and natural ecosystems to the changed climate. Only then can we gauge the impacts of our actions, and help to adapt precisely to the changes. As quoted in Our Changing Planet (1997) from the original 1989 report,

"The national goal of developing a predictive understanding of global change is, in its truest sense, science in the service of mankind."

Mitigation. If climate change is slowed down and more gradual, society will have more time to learn to live in this new world. This means stopping the global growth in the emission of carbon dioxide, and slowly reducing it. The only way to do this is to include burning less coal and oil in the response. Reforestation and reduction of other greenhouse gases will have some impact, but any meaningful combination of conservation, energy efficiency, energy tax, and public transportation enhancements will result in less gasoline being burned and less coal being burned.

"No regrets" policies. Reduced usage of energy will have many positive benefits to society, even if projected global warming turns out to have been exaggerated (which is just as likely as that the warming turns out to have been underestimated). We would have cleaner air, less acid rain, greater visibility in the atmosphere, cooler central regions of cities, more trees, and less dependence on foreign oil supplies (currently about half of our usage).

Discussion and Conclusions

There will be those who say we should wait until we understand the exact nature of the problem, and until we have had time to develop more efficient and cost-effective responses to the problem. However, it will be possible to say exactly the same thing for quite a while into the future. Unlike the ozone depletion problem, uncertainty about detection of climate change, and the human impact of the climate change will be around for decades. We must act in the light of this uncertainty. The sooner we begin to attempt to mitigate the problem, the sooner we will be spurred to develop efficient and cost-effective responses. The longer we wait, the greater the chance of unforeseen, potentially irreversible, and very costly consequences.

The science of global change suggests possible serious problems for humanity during the next century due to the inadvertent effects of our industrialized society. The problem involves long time scales of decades and centuries, in that the effects of yesterday's and today's greenhouse gas pollution will not go away in our lifetimes. Therefore it is prudent to act immediately to reduce our pollution while we simultaneously enhance our efforts to better understand and predict the impacts, so that we may better adapt to the changes and calibrate future remediation efforts. The particular mitigation response of the United States and other nations, and whether the costs of such responses are justified, given the uncertainty described above, is a political decision that must be made by an informed public.

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References

Broecker, W. S., D. Peteet, and D. Rind, 1985: Does the ocean-atmosphere have more than one stable mode of operation? Nature, 315, 21-25.

Haywood, J. M., R. J. Stouffer, R. T. Wetherald, S. Manabe, and V. Ramaswamy, 1997: Transient response of a coupled model to estimated changes in greenhouse gas and sulfate concentrations. Geophys. Res. Lett., 24, 1335-1338.

Houghton, J. T., L. G. Meira Filho, B. A. Callander, N. Harris, A. Kattenberg, and K. Maskell, Eds., 1996: Climate Change 1995-The Science of Climate Change, Cambridge Univ. Press, Cambridge, 584 pp.

Jones, P. D., S. C. B. Raper, R. S. Bradley, H. F. Diaz, P. M. Kelly, and T. M. L. Wigley, 1986a: Northern Hemisphere surface air temperature variations: 1851-1984. J. Clim. Appl. Meteor., 25, 161-179.

Jones, P. D., S. C. B. Raper and T. M. L. Wigley, 1986b: Southern Hemisphere surface air temperature variations: 1851-1984. J. Clim. Appl. Meteor, 25, 1213-1230.

Our Changing Planet, 1997: The FY 1998 U.S. Global Change Research Program. National Science and Technology Council, Washington, 118 pp.

Repetto, Robert and Ducan Austin, 1997: The Costs of Climate Protection: A Guide for the Perplexed. World Resources Institute, Washington, 51 pp.

Robock, Alan, Richard P. Turco, Mark A. Harwell, Thomas P. Ackerman, Rigoberto Andressen, Hsin-shih Chang and M. V. K. Sivakumar, 1993: Use of general circulation model output in the creation of climate change scenarios for impact analysis. Climatic Change, 23, 293-335.

Spencer, R. W., J. R. Christy, and N. C. Grody, 1990: Global atmospheric temperature monitoring with satellite microwave measurements: Method and results 1979-1984, J. Climate, 3, 1111-1128. (Updated; pers. comm.)

Watson, R. T., M. C. Zinyowera, and R. H. Moss, Eds., 1996: Climate Change 1995-Impacts, Adaptations and Mitigation of Climate Change: Scientific-Technical Analyses, Cambridge Univ. Press, Cambridge, 880 pp.

Acknowledgments. I thank Jim Haywood for the data used in the figures.

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Figure 1. Comparison between observed global average climate change since 1865 with a climate model simulation using only CO2 changes of the IS92a IPCC scenario (Haywood et al., 1997).

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