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
NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION
U.S. DEPARTMENT OF COMMERCE

BEFORE THE

SUBCOMMITTEE ON ENERGY AND ENVIRONMENT
COMMITTEE ON SCIENCE

U.S. HOUSE OF REPRESENTATIVES

HEARING ON:

CLIMATE MODELS AND PROJECTIONS OF POTENTIAL IMPACTS
OF GLOBAL CLIMATE CHANGE

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

oxide, human-produced chlorofluorocarbons, and their substitutes. The amounts of

all these gases are increasing at a rate sufficient to have substantial climatic

Today, I will emphasize only the estimated climatic effects of these increasing greenhouse gases for a time around the year 2050. The year 2050 is roughly the time that current projections lead to an equivalent doubling of carbon dioxide levels. However, please note that such projections indicate that carbon dioxide amounts would continue to increase well beyond the year 2050, perhaps even to a quadrupling.

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 judgements that are beyond the realm of climate science. Indeed, I would encourage your skepticism whenever you hear a climate scientist's prediction being accompanied by a policy opinion.

At the onset, please recognize that a major international effort to assess climate warming was completed in 1990. This is "The IPCC Assessment". That report involved nearly two years and 170 scientists from 25 countries. Also, it has recently been updated in the 1992 and 1994 Supplements. These Supplements reinforce the general conclusions of the 1990 Assessment. They have reduced our uncertainty in

several important areas. These are the most widely accepted statements ever on climate change. A new IPCC Assessment 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 a 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.

Occasionally, my own judgements will differ somewhat from these IPCC Assessments. Overwhelmingly, however, I agree with them. I emphatically support their highest priorities for climate change research.

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 current climate and its natural variability. Unfortunately, important uncertainties remain due to deficiencies in our scientific understanding and in computer power. However, significant progress is expected over the next 10 years. The uncertainty in such areas as ocean circulation, cloud processes, and regional effects will gradually diminish.

However, let me say at the outset: None of the uncertainties I will discuss 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 will give my evaluation of current model predictions of climate change in the middle of the next century using simple "betting odds". By "Virtually Certain", I mean that there is no plausible alternative; in effect, the bet is off the books. "Very Probable" means I estimate about a 9 out of 10 chance that this will happen within the range predicted; "Probable" implies about a 2 out of 3 chance. "Uncertain" means a plausible effect, but which lacks appropriate evidence. Essentially, I set the odds; you choose your bet. My predictions follow in decreasing levels of confidence.

Human-Caused Increasing Greenhouse Gases (virtually certain)

There is no real remaining doubt that increasing greenhouse gases are due to human activities. Clear contributors are the burning of fossil fuels and the use of chlorofluorocarbons.

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.