model the effects of clouds, which have the greatest potential of all feedback mechanisms to amplify or moderate global warming. Insufficient Computing Power The accuracy of general circulation model estimates are further limited by computers' insufficient capacity to store and analyze the vast quantity of data required to accurately simulate global climate changes on a regional scale. In an effort to overcome these limitations, modelers introduce assumptions into their models that deliberately simplify some operations in order to free the models' capacity and time for other, more critical operations. One such simplification used in certain models, referred to as the cold start error, assumes that the oceans did not absorb greenhouse gas emissions before 1985. As a result, these models do not adequately account for the oceans' reduced capacity to absorb these emissions in the future. In fact, the oceans will reach their capacity possibly decades sooner than the models have predicted. They will then deflect more of the heat-trapping emissions to the atmosphere, thereby enhancing global warming more rapidly than the models predict. Scientists estimate that cold start error causes models to underestimate temperature changes, resulting in projections of average annual temperatures that differ by as much as 0.7 degrees after 50 years. Months of additional computer time would be required to address just this one existing limitation. Another simplification that modelers use to deal with insufficient computer capacity is to divide the earth into large segments, or grids, for analytical purposes. Although the grids, which cover an area about the size of South Carolina, enable the models to depict large-scale regional effects in relatively large, homogeneous regions, they do not allow modelers to incorporate detailed regional features to forecast climatic changes for smaller, less homogeneous regions. Improving Models' Estimates A number of efforts are under way to improve the accuracy of general circulation models' estimates. Scientists are working to develop models that incorporate more of the processes affecting the climate system-particularly cloud formation processes-and to better reflect interactions among various components of the climate system, including interactions between or among the ocean and the atmosphere. Scientists are also developing larger and faster computers to manipulate data for longer periods of time and to better understand regional effects resulting from using smaller grids. In addition, they are collecting more data and conducting more research on the processes affecting climate and improving the international exchange of such data. Various international programs, such as the World Climate Research Programme and the Global Climate Observing System, also currently have research programs under way to improve the accuracy of the models' estimates. CONCLUSIONS Although the accuracy of general circulation models has improved over the past decade, these models are still limited by incomplete and inaccurate representations of the processes affecting climate and by insufficient computing power. While these limitations do not change the likelihood that the climate will change in response to increased greenhouse gas emissions, they do limit the ability to predict with certainty how the climate will respond-how much warming will occur, how soon it will happen, and what regional impacts will occur. Efforts are under way to collect and analyze data, improve representations of various climatic processes, and develop and use more powerful computers. These efforts, which are anticipated to take a decade or more, should improve the accuracy of the models. Mr. Chairman, this concludes my prepared statement. We will be pleased to respond to any questions you or other Members of the Subcommittee may have. Mr. ROHRABACHER. Dr. Mahlman. "This program was established as a joint undertaking of the International Council of Scientific Unions and the World Meteorological Organization, to foster an improved understanding of the climates variability and predictability. "This international organization was established in 1992 to monitor climatic changes and obtain data for application to national economic development. STATEMENT OF DR. JERRY MAHLMAN, DIRECTOR, GEOPHYSICAL FLUID DYNAMICS LABORATORY, NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION Dr. MAHLMAN. Mr. Chairman, I am the Director of the Geophysical Fluid Dynamics Laboratory of NOAA. For over 30 years, our laboratory has been a world leader in modeling the earth's climate. Today, I will evaluate scientific projections of climate change, as well as their current uncertainties. We have long known that buildups of atmospheric carbon dioxide and other gases have the potential to warm earth's climate through the so-called greenhouse effect. I will emphasize only the estimated climatic effects of these greenhouse gases for a time around the year 2050. Because I speak with credentials as a physical scientist, I do not offer personal opinions on what society should do about these predicted climate changes. Societal actions in response to greenhouse warming involve value judgments that are beyond the realm of climate science. Indeed, I would encourage your skepticism whenever you hear a climate scientist's prediction that is accompanied by a policy opinion. At the onset, please recognize that a major international effort to assess climate change was completed in 1990. This is the IPCC Assessment. Also, it has been recently updated in this 1994 supplement. These are the most widely accepted statements ever on climate change. A new IPCC statement will be completed in the next few months. I expect only small changes in its major conclusions, mainly concerning some increases in scientific confidence. I strongly recommend your use of these scientific assessments as the foundation for your own evaluations. I also recommend their use as a point of departure for evaluating the credibility of opinions that disagree with them. My information is derived from the strengths and weaknesses of climate models, climate theory, and widespread observations of the climate system. Climate models have improved in their ability to simulate the climate and its natural variability. Unfortunately, important uncertainties remain due to deficiencies in our scientific understanding, as well as in our computer power. However, I expect significant progress over the next decade. However, let me say at the outset none of the uncertainties I will discuss here can make current concerns about greenhouse warming go away. This problem is very real and will be with us for a very long time. I give my evaluation of current model predictions of climate change in the middle of the next century by using very simple betting odds. When I say virtually certain, I mean that there is no plausible alternative. In effect, the bet is off the books. Very probable means I estimate a nine out of ten chance that this will happen within the range predicted. Probable implies about a two out of three chance. When I label something uncertain, I mean that there is a plausible effect but it lacks appropriate scientific evidence. Essentially I set the odds, you choose your bet. My analysis follows in decreasing levels of confidence. Human caused increasing greenhouse gases: virtually certain. There is no real remaining doubt that the increasing greenhouse gases that have been observed are due to human activities. Yes? Mr. ROHRABACHER. Will you repeat that? Dr. MAHLMAN. Yes. There is no real remaining doubt that increasing greenhouse gases are due to human activities. Radiative effect of increased greenhouse gases: virtually certain. Greenhouse gases absorb and reradiate infrared radiation. Independent of other factors, this property acts to produce an increased heating effect on the planet. Long time to draw down excess carbon dioxide: virtually certain. We know that it takes decades to centuries to produce the buildup of greenhouse gases. Much less appreciated is that a return to normal from high carbon dioxide levels would require many additional centuries. Global surface warming over the past century: virtually certain. The observed warming in the surface temperature records of about one degree Fahrenheit cannot yet be unambiguously ascribed to greenhouse warming. However, no other hypothesis is nearly as credible. Future global mean surface warming: very probable. For the middle of the next century, global-mean surface warming is estimated to be in the range of two to six degree Fahrenheit. The largest uncertainty by far is due to the effects of clouds. Rise in global-mean sea level: very probable. A further rise of four to 12 inches in global mean sea level by the year 2050 is estimated, mainly due to the thermal expansion of warmer sea water. Continued sea level rise is expected for many centuries, probably to much higher values. Summer midcontinental dryness and warming: probable. Two out of three. Model studies predict a marked decrease of soil moisture and elevated temperatures over summer mid-latitude continents. This important result is still sensitive to model assumptions. Increased tropical storm intensities: uncertain. A warmer, wetter atmosphere could indeed lead to increased intensities of tropical storms, such as hurricanes. Presently, this problem has not been properly addressed due to inadequate computer power and to uncertainties in regional climate change predictions. Regional and temporal details of the next 25 years: uncertain. The predicted warming up to now is not yet large compared to natural climate fluctuations. On these shorter time scales, the natural fluctuations can artificially reduce or enhance apparent measured greenhouse warming signals. Even though these uncertainties are daunting, important advances have already been achieved in observing, understanding, and modeling the earth's climate. Today's models can simulate many aspects of climate and its changes. More efforts are needed worldwide to provide a better long-term climate measuring system. Focused research into climate processes must be continued. Theories must be reformulated and reevaluated in the light of newer data. Climate modeling efforts must receive resources that are in balance with the broader scientific programs. The U.S. Global Change Research Program is making excellent progress on these fronts. However, sustained efforts will be required in the years ahead. Through long-term research and measurements, uncertainties will decrease and confidence for predicting climate changes will increase. In summary, the greenhouse warming effect is quite real. The state of the science is strong, but important uncertainties remain. Finally, it is a virtually certain bet that this problem will refuse to go away no matter what is said or done about it over the next five years. Thank you, Mr. Chairman. [The prepared statement of Dr. Mahlman follows:] TESTIMONY OF J.D. MAHLMAN, DIRECTOR GEOPHYSICAL FLUID DYNAMICS LABORATORY BEFORE THE SUBCOMMITTEE ON ENERGY AND ENVIRONMENT U.S. HOUSE OF REPRESENTATIVES HEARING ON: CLIMATE MODELS AND PROJECTIONS OF POTENTIAL IMPACTS November 16, 1995 UNCERTAINTIES IN CLIMATE CHANGE MODELING Mr. Chairman and Members of the Subcommittee: My name is Jerry Mahlman. I am the Director of the Geophysical Fluid Dynamics Laboratory of NOAA in Princeton, New Jersey. For over thirty years our Laboratory has been a world leader in modeling the earth's climate. I will evaluate scientific projections of climate change as well as their current uncertainties. We have long known that a buildup of atmospheric carbon dioxide has the potential to warm earth's climate, through the so-called "greenhouse" effect. We now know that other greenhouse gases are currently contributing nearly as much to climate warming as does increased carbon dioxide. These include methane, nitrous |