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Q21.4 CFCs?

Q21.5 other greenhouse gases?

A21. It is true that most of the "greenhouse effect", i.e. the atmospheric absorption of radiant energy emitted in the infrared portion of the spectrum by the Earth's surface, is due to water vapor, with carbon dioxide being the next most important contributor. Methane, CFCs, nitrous oxide, ozone and other trace constituents are much less important than the first two in the overall energy balance of the atmosphere. Small changes in this balance on the order of a few percent, however, have the potential for causing widespread changes in surface climatology. It is for this reason that researchers are actively investigating the role of increased greenhouse gas concentrations in altering the atmospheric energy balance.

Q22. Dr. Spencer testified that “if all the atmosphere's carbon dioxide were removed, we would still have over 98 percent of the earth's greenhouse effects." Do you agree with that statement and if not, why not?

A22. Although we have not performed the calculation suggested by Dr. Spencer, we have no reason to doubt its result. Unfortunately, the cumulative greenhouse effect of water vapor and carbon dioxide is not simply additive. For example, if all of the water vapor were removed, carbon dioxide would still absorb about a quarter of the radiant energy absorbed by the atmosphere as a whole. As was mentioned above, a 2% change in the Earth's greenhouse effect would produce dramatic climate changes.

Q23. How much would a doubling of carbon dioxide increase the overall greenhouse effect?

A23. Best estimates of the effect of doubling the concentration of carbon dioxide (from preindustrial levels) range between 1 and 2%. The uncertainties result from an incomplete understanding of all of the cloud and water vapor feedbacks that would result from increased carbon dioxide concentrations.

Is Climate Change Underway?

Q24. Did the 1995 report of the Intergovernmental Panel on Climate Change (IPCC) conclude that climate change is already underway, and if so, what specific climate change?

A24. The 1995 Scientific Assessment Summary for Policymakers stated "The balance of evidence suggests a discernable human influence on global climate." The report's statement was based on statistical comparisons between observed and modeled largescale changes in the global temperature fields. The ability to quantify the human influence on climate is still limited and is the focus of continuing research. More studies are needed before we can either strengthen or abandon this very tentative conclusion. We require a more definitive estimate of natural climate variability, a more complete understanding of the role of aerosols and better verification of models against data.

HEARING OF THE SUBCOMMITTEE ON ENERGY AND ENVIRONMENT
COMMITTEE ON SCIENCE

U.S. HOUSE OF REPRESENTATIVES

on

Countdown to Kyoto-Part 1: The Science of a Global Climate Change Agreement

Tuesday, October 7, 1997

Post-Hearing Questions and Answers

Dr. Ronald G. Prinn

TEPCO Professor of Atmospheric Chemistry and Director,
Center for Global Change Science
Massachusetts Institute of Technology

Climate Model Imperfections

Q1.

Al.

Q2.

A2.

You stated in your testimony that “the imperfections of current climate models make them both inadequate tools for defining natural variability and uncertain · predictors of the climate response to human forcing." Please explain why you think this is so.

Concerning natural variability, current climate models fail to simulate important decadeto-decade variations such as the North Atlantic Oscillation and very important millenniumto-millennium variations such as the glacial-interglacial oscillations. Concerning uncertain predictions, there are large uncertainties in feedbacks involving water vapor and clouds, the rate of penetration of heat into the ocean, and the direct and indirect effects of aerosols to name a few. These uncertainties are in part the reason for significant differences between models, and lead to the need for arbitrary adjustments in these models in order to simulate current climate.

What improvements need to be made to improve the precision and usefulness of the
GCMs?

There are many improvements needed, but I would highlight here the much needed advances in treatments of convection and clouds, ocean-atmosphere and biosphereatmosphere interactions, and aerosol effects in these models. A key near-term goal for climate scientists is to understand and simulate observed natural climate variability without

Arctic Ice and GCMs

Q3.

A3.

A recent article in the Journal of Climate by D.S. Battisti and others suggests that current climate models do not represent sea ice in the Arctic regions very well. The authors cite the variability in Arctic sea ice and suggest that it is not captured adequately in GCMs. As GCMs predict the greatest warming will occur over the polar latitudes in winter, how much confidence can we place in these predictions in light of the research by Battisti et al.?

The effect of sea ice on albedo is one of the most important feedbacks in climate after water vapor. It is also involved in driving the deep ocean (thermohaline) circulation and controlling ocean-air heat exchanges. Hence poor simulation of sea-ice is yet another imperfection in models leading to the need for improvements before predictions can be regarded as reliable.

Climate Model Uncertainties

Q4.

A4.

Chapter 5 of the 1995 IPCC scientific notes that, “Clouds, the hydrological cycle and the treatment of the land surface remain the largest areas of uncertainty in the climate models." Do you agree with this assessment and could you explain some of the major areas of uncertainty where you think the models need to be improved?

The list is definitely incomplete. I would add, with equal emphasis, the ocean circulation, ocean-atmosphere interactions, and aerosols to the list. The role of the ocean is large in determining both regional and global climate. It is also suspected to play major roles in the North Atlantic Oscillation and the glacial-interglacial oscillations. The forcing by aerosols could range from slightly positive (adding to the greenhouse gases) to significantly negative (canceling much of the effects of recent increases in long-lived greenhouse gases).

Flux Adjustments in GCMs

Q5.

A5.

It appears that one problem with GCMs is the widespread use of flux adjustments— what laymen would call “fudge factors”—to make the models comport with reality. In a recent article in Science, Robert Kerr makes the observation that, "Climate modelers have been 'cheating' for so long it's almost become respectable." Mr. Kerr goes on to say that this a practice most researchers do not like. Would you explain what these arbitrary adjustments are and why they are necessary?

These adjustments consist of deliberate changes in the model's predictions of the flows of heat and water between the atmosphere and various parts of the world's oceans. In some models, and in some areas, the adjustments can be a significant fraction of the total observed flows! They are needed to correct the model predictions of current regional and global climate so that they agree with observations. They are a symptom of the failure of these models to simulate fundamental climate processes. They are an embarrassment to

Parameterization in GCMs

Q6.

A6.

Dr. Spencer mentioned that current GCMs parameterize certain processes, like cloud formation, rather that incorporate the physics in them. Please explain the difference between these two approaches. Which is better?

Parameterizations are attempts to simulate small-scale processes in terms of large-scale variations. Basically, any climate process that occurs with space or time scales too small to be resolved by the models must be parameterized. Convection and clouds are important examples-the scales of their physics range from one millionth of a meter (cloud droplets) to 10 kilometers (convection), while the scales resolved by climate models are typically 100 kilometers. Simulations using parameterizations are fundamentally inferior to simulations at the actual scales of the process.

Human Influence on Global Climate

Q7.

A7.

Q8.

A8.

Concerning the IPPC statement regarding a “discernible human influence on global climate," your testimony suggests that it still may be many years before a definitive signal of human activity is found. In your view, was the IPCC statement premature?

The statement by the IPCC Summary for Policymakers which I quote in my testimony is a misleading summary of the work actually reviewed in the relevant part (Chapter 8) of the full IPCC Report. Taken in isolation (as it has largely been), the quote is a premature statement. Part of the problem here results from a poor summary leading to poor communication. I wonder if the IPCC summarizers realized the extent to which this quote would be taken out of the context of Chapter 8.

Your testimony describes the “current view of experienced climate scientists on this issue as ‘equivocal'." Could you please explain what you mean by "equivocal” in this context?

By "equivocal" I mean that due to current uncertainties, you cannot determine whether observed climate changes over the past 120 years have been caused by predominantly natural or predominantly human influences. The uncertainties involve the failure of current models to simulate natural variability (the "noise" from which the "signal" would arise), the current leeway which modellers have to add the amounts of aerosol forcing needed to provide a semblance of agreement with observations, the neglect of tropospheric ozone forcing which counters the aerosols, and so on. Under these circumstances, it is not yet possible to determine that the balance of evidence is either for or against human influence. I refer you also to the last paragraph of an article by Dr. Klaus Hasselmann in Science, vol. 276, pgs. 914-915, May 9, 1997.

Models and Regional-Scale Predictions

Q9.

A9.

In your view, are the models we have today sophisticated enough to make regional predictions with any confidence, and if not, how long do you think it will take to make these kinds of predictions with any confidence?

My confidence in regional predictions is very low. If scientists focus more on explaining and simulating observed natural variability, then maybe in ten years the accuracy of such predictions will be much improved.

Role of Aerosols

Q10. There has been a great deal of controversy concerning the impact of aerosols on the results of climate models. A recent GCM by J. Hansen and a paper in Science by S. Tett et al. indicate that the impact of sulfates is generally slight or neutral. It has also been suggested that the impact of aerosols could be offset by carbon particles in the atmosphere, which absorb heat, from fossil fuel burning. Please discuss the uncertainties regarding our understanding of aerosols in GCMs.

A10.

Here are the major uncertainties in approximate order of importance: (1) the effect of aerosols on the albedo of clouds (the so-called "indirect" effect), (2) the absorption (warming) versus reflecting (cooling) properties of aerosols, (3) the production and loss processes for aerosols, (4) the sources of the gases which are converted to aerosols (sulfur dioxide, dimethyl sulfide).

Q11. Among the findings of a 1996 National Research Council report, Aerosol Radiative Forcing and Climate Change, were the following: “(1) anthropogenic aerosols reduce the amount of solar radiation reaching the Earth's surface, (2) anthropogenic aerosols provide a negative climate forcing function for large regions, (3) global models suggest that sulfate aerosols produce a direct forcing in the Northern Hemisphere of the same magnitude as that from anthropogenic greenhouse gases, but opposite in sign, and (4) there is substantial uncertainty about the magnitude and spatial distribution of the radiative forcing by aerosols.” Are these findings still pertinent?

A11. I think (1), (2), and (3) all presume the aerosols are good reflectors as opposed to good absorbers. Some recent measurements by Dr. Peter Hobbs and colleagues at the University of Washington may cause one to doubt this presumption. Point (4) is still very pertinent.

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