Global Warming: State of the Science

Testimony before the House Committee on Science

Subcommittee on Energy and Environment

October 7, 1997

Professor Alan Robock

Department of Meteorology
University of Maryland
College Park, Maryland 20742
Phone: 301-405-5377

Scientific Consensus on Global Warming

I agree with the conclusions of the 1995 IPCC Working Group I report (Houghton et al., 1996) that "the balance of evidence suggests that there is a discernible human influence on global climate." Note that this is the balance of evidence, NOT unambiguous proof. The report points out that "our ability to quantify the human influence on global climate is currently limited because the expected signal is still emerging from the noise of natural variability, and because there are uncertainties in key factors. These include the magnitude and patterns of long term variability…." [Both these quotes are from p. 5 of the Summary for Policymakers.] I agree with this part of the assessment, too.

The evidence which supports a human influence on climate includes observations that the concentrations of "greenhouse gases" produced by human activity, especially carbon dioxide, are increasing and that these gases warm the surface by enhancing the natural greenhouse effect. These facts are undisputed. But these gases are not the only cause of climate change. When the most recent climate model experiments, done since the latest IPCC report, include the effects of greenhouse gases, aerosols (particles in the atmosphere), volcanic eruptions, ozone depletion, solar variations, and El Niño in their calculations, they produce simulations of climate change of the past 100 years that agree quite well with the past surface temperature record. For example, Haywood et al. (1997) describe calculations made with the climate model of NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) at Princeton University. When they attempt to simulate the climate change of the past 130 years taking into consideration just the effects of CO2 increases, the model produces too much warming as compared to observations (Figure 1). When they now also include the direct effects of tropospheric sulfate aerosols (haze), their simulation almost exactly matches the past climate change (Figure 2). The enhanced industrialization following World War II rapidly increased aerosol and CO2 output. But the average aerosol lifetime is only 4 or 5 days as compared to the 100-year lifetime of CO2. Therefore the total aerosol effect is felt immediately, retarding for a couple decades the anthropogenic warming. This is an example of more realistic results from climate models produced by using more realistic scenarios of climate change.

In addition to the above results, there are many other examples of phenomena explained and predicted by climate models that we can observe, demonstrating the fingerprint of human causation of climate change. Observations show that stratospheric temperatures are decreasing, sea level is rising, sea ice is retreating, snow cover is decreasing, and glaciers are melting, all in agreement with these calculations. Stratospheric cooling can be well simulated by observed CO2 increase and ozone depletion. In the stratosphere the human impact is much easier to detect, as it has much less weather variability to disguise the human signal. Near the surface, weather noise is much larger and includes the effects of El Niños, so the greenhouse warming is harder to detect. Nevertheless, it is impossible to explain the warming of this century in any other way. Solar variations are speculative and much too small. Even the temporary cooling after the 1991 Pinatubo volcanic eruption, the largest of the past 250 years, was not enough to cool the climate to 19th century levels.

The disagreement between satellite (MSU - Microwave Sounding Unit; Spencer et al., 1990) and surface temperature records (Jones et al., 1986a,b, updated) for the past 20 years are well understood and do not argue against global warming. In fact, if the climate changes caused by the 1982-83, 86-87, and recent El Niños, ozone depletion, and the 1982 El Chichón and 1991 Pinatubo volcanic eruptions are accounted for, the MSU record shows a warming during this period, in agreement with climate model simulations.

It is the same models described above that we use for projections of future climate. Figure 3 shows the GFDL model predictions until near the end of the next century. While the predicted warming is less when including the more realistic effects of aerosols, significant warming is still predicted from now on. The additional CO2 being put in the atmosphere is already overwhelming the aerosol effect. These calculations show global average warming of more than 7°F by the end of the next century with "Business as Usual." Other model experiments described in the 1995 IPCC report forecast that the global average temperature will rise by 2 to 6°F and sea level will rise by 1 to 3 feet by the end of the next century. Even for the smallest increase projected, "the average rate of warming would probably be greater than any seen in the last 10,000 years." [p. 6 of the IPCC Summary for Policymakers.] The IPCC goes on to say, "actual annual to decadal changes would include considerable natural variability. Regional temperature changes could differ substantially from the global mean value." This means that certain regions of the globe are apt to warm much more than the global average. Some areas would not warm much at all or could even cool slightly. We do not now understand where each of these regions will be.

Surprises are also possible, including rapid warming once the climate has passed a certain threshold, by a mechanism previously unknown. This is exactly what happened when the ozone hole suddenly appeared over the South Pole during the previous decade. It was not predicted and came as a complete surprise. Broecker et al. (1985) have suggested possible rapid shifts in ocean circulation with resulting rapid climate change on land in certain regions, but surprises, by definition, cannot now be described or quantified.

It is important to point out that many human impacts of climate change depend on climate elements other than temperature. For example, "ocean currents, frequency and strength of oceanic storms, winds, frequency of fog, sea-ice distribution and thickness, and sea level will all be important for impacts on ocean transportation. For agriculture, temperature, precipitation, cloudiness, wind, carbon dioxide concentration, intensity of ultraviolet light, frequency of severe storms, and soil moisture" (Robock, 1993, p. 301) will all be important.
Table 1. Areas of Human Endeavor That Could be Affected by Global Warming
Water Resources
Human Health
Rivers and Lakes
Air Pollution
Land Transportation
Air Transportation
Ocean and River Transportation
Electricity Demand
Wind Energy Generation
Solar Energy Generation
Hydroelectricity Generation
Societal Systems
Political Systems 

In order to determine the net impact of greenhouse warming on humans, we must first examine each potential area of impact (Table 1). Then for each activity, we must determine the future changes of the mean, variability, and extreme values of each important element for that activity, for every region of the earth where it would have an impact, for all times of the year. If this cannot be done, then we must at least determine the sensitivity to each element by varying it over a range of possible values. Next, we must evaluate the direct impact of climate change on the activity, taking into consideration future technological, sociological, economic, political, and military responses to each impact, singly, and in all combinations. Finally we assign probabilities to each choice and result, and determine the net human impact (Robock, 1993). It is unlikely that this will be done with any confidence for the foreseeable future, say the next 10 or 20 years. In light of this, what can we say about impacts of global warming on society? What does the latest research tell us?

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.

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?

Related questions include:

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.

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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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.

Figure Captions

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).

Figure 2. Comparison between observed global average climate change since 1865 with a climate model simulation using CO2 and tropospheric aerosol changes of the IS92a IPCC scenario (Haywood et al., 1997).

Figure 3. Haywood et al. (1997) climate simulations from Figures 1 and 2, including predictions into the next century. Curves are smoothed with a 3-year running mean.

Curriculum Vitae of Alan Robock

I earned a Ph.D. in Meteorology at the Massachusetts Institute of Technology in 1977. Since then I have been on the faculty of the Department of Meteorology of the University of Maryland, where I am now a Professor and the State Climatologist of Maryland. My research involves many aspects of climate change, including the greenhouse effect, impacts of climate change and satellite observations. I have published more than 125 articles on my research, more than half of these in the peer-reviewed literature. I conduct both observational analyses and climate model simulations.

A detailed list of my publications is available, but here I list only those directly related to the global warming question. I have published papers on the creation of regional climate change scenarios for impact analysis (Robock, 1993) and on the effects of climate change on corn production in Venezuela (Maytín et al., 1996). I recently co-authored a paper (Vinnikov et al., 1996) which showed that the cooling of the stratosphere which has been observed during the past 30 years has a very small chance of having happened due to natural climate fluctuations, and is most likely a signal of human impacts on the climate.

I am a member of the American Meteorological Society, the American Geophysical Union, and the American Association for the Advancement of Science (AAAS). I serve on the Scientific Advisory Board of the National Institute for Global Environmental Change, Great Plains Regional Center, at the University of Nebraska in Lincoln, and have since its inception in 1992. This center is funded by DOE. I am the Associate Editor for Meteorology of Reviews of Geophysics. I serve on the International Climate Commission of the International Association for Meteorology and Atmospheric Science and the American Meteorological Society Committee on Climate Variations. I was awarded a AAAS Congressional Science Fellowship in 1986, and served as Legislative Assistant to Congressman Bill Green (R-NY) and as a Research Fellow with the Environmental and Energy Study Conference from September, 1986, through August, 1987, where I authored the report The Greenhouse Effect: Global Warming Raises Fundamental Issues. During the 1994-95 academic year I was a Visiting Research Scientist at Princeton University in the Atmospheric and Oceanic Sciences Program, conducting climate research at NOAA's Geophysical Fluid Dynamics Laboratory.

I am a contributing author to 4 of the 11 chapters of the most recent IPCC 1995 Working Group I report, including Chapter 8, "Detection of Climate Change and Attribution of Causes." The work I did in contributing information to these chapters, and in reviewing these and other chapters, was done as a volunteer, at night and in my spare time, with no compensation. I currently have grants from the National Science Foundation, the National Aeronautics and Space Administration (NASA), the National Oceanic and Atmospheric Administration (NOAA), and the U.S. Department of Energy (DOE) that support my scientific research. The views expressed here are my own and do not represent those of any organization or funding agency.


Maytín, Carlos E., Miguel Acevedo, Ramón Jaimez, Rigoberto Andressen, Mark A. Harwell, Alan Robock, and Aura Azócar, 1995: Potential effects of global climatic change on the phenology and yield of maize in Venezuela. Climatic Change, 29, 189-211.

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.

Vinnikov, Konstantin Ya., Alan Robock, Ronald J. Stouffer, and Syukuro Manabe, 1996: Vertical patterns of free and forced climate variations. Geophys. Res. Lett., 23, 1801-1804.

Sources of Federal Government Funding for Alan Robock

During the current and two preceding fiscal years (October 1, 1995 - present) I have conducted research supported by portions of the following federal research grants. These grants paid for 25% of my salary, 100% of the salary of a Senior Research Scientist working with me, about 80% of the salary of an Associate Research Scientist working with me, 100% of the tuition and salary of about 4 Ph.D. students per year working with me, computer equipment, software, supplies, scientific data, travel to professional conferences, page fees for publishing journal articles, and office expenses of conducting research, such as telephones, faxes, and copying.

1. NASA, NAG 5-1835, "Climate Model Calculations of The Effects of Volcanoes on Global Climate," December 1, 1991 - November 30, 1996, $439,000.

2. DOE Office of Energy Research, DE-FG02-93ER61691.A000, "Validation of Soil Moisture in GCMs - AMIP Diagnostic Subproject 11," September 1, 1993 - August 31, 1997, $148,500.

3. NOAA Climate and Global Change Program, NA56GPO212, "Midlatitude Land Surface Processes: Modeling and Analysis in Support of GCIP Using American, Russian, and Chinese Data," May 1, 1995 - April 30, 1998, $385,000.

4. NSF Climate Dynamics Program, ATM-9528201, "Climatic Effects of Volcanic Eruptions," March 1, 1996 - February 28, 1999, $165,000.

5. NASA, NAGW-4912, NAG53739, "Climatic Effects of Volcanic Eruptions," December 1, 1995 - February 28, 1999, $165,000.

6. NOAA Climate and Global Change Program, NA66GPO438, "Limits of Natural Variations in Global and Regional Climate as Compared to Observed Climatic Trends," July 1, 1996 - June 30, 1998, $150,000.

7. DOE Great Plains National Institute for Global Environmental Change, 92-0294-NEBR, Subcontract LWT 62-123-07506, "The Diurnal Cycle over the Great Plains in the Future: Mechanisms and Spatial Distribution," July 1, 1996 - June 30, 1999, $215,000.

8. NASA Mission to Planet Earth, NAGW5227, "Global Soil Moisture Data Set From Satellite and Gravimetric Observations for Climatic Studies and Evaluation of the Hydrological Aspects of Climate Models," July 1, 1996 - June 30, 1998, $197,300.