UNITED STATES SENATE

Mr. Chairman:

My name is Jerry Mahlman. I am the Director of the Geophysical Fluid Dynamics Laboratory of NOAA. 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 buildups of atmospheric carbon dioxide and other gases have the potential to warm earth's climate, through the so-called "greenhouse" effect. Today, I will discuss modeling the projections of climate changes due to these increasing greenhouse gases for a time around the middle of the century.

Because I speak with credentials as a physical scientist, I do not offer personal opinions on what society should do about these projected climate changes. Societal actions in response to greenhouse warming involve value and policy judgements that are beyond the realm of climate science.

At the onset, please recognize that a major international effort to assess climate warming was completed in 1996. This is "The Intergovernmental Panel on Climate Change Assessment" (IPCC). The IPCC was established in 1988 by the United Nations Environment Programme and the World Meteorological Organization to assess the available information on climate change and its environmental and economic impacts. This was the most widely accepted assessment ever on climate change. The 2001 IPCC Assessment will be completed soon. I expect only small changes in its major conclusions, mainly concerning some important increases in scientific confidence.

I strongly recommend your use of the IPCC 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.

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 and in computer power. However, significant progress is expected over the next 10 years.

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 by setting 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 projected; "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 analysis is presented in decreasing levels of confidence.

* Human-Caused Increasing Greenhouse Gases (virtually certain)

There is no 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.

* A Doubling of Carbon Dioxide Expected (virtually certain)

Atmospheric carbon dioxide amounts are expected to double over pre-industrial levels in this century. Current emissions growth is on track to quadruple atmospheric carbon dioxide.

* Long Time to Draw Down Excess Carbon Dioxide (virtually certain)

We know that it takes decades to centuries to produce a large 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 measured 20^th century warming in the surface temperature records of over one degree fahrenheit is undoubtedly real. Its cause is very probably due mostly to added greenhouse gases. 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 2 to 6 fahrenheit, with continued increases for the rest of the century. The largest uncertainty is due to the effects of clouds.

* Increased Summertime Heat Index (very probable)

In warm, moist subtropical climates the summertime heat index effect is expected to magnify the warming impact felt by humans by an additional 50%.

* Rise in Global Mean Sea Level (very probable)

A further rise of 4-12 inches in mean sea level by the year 2050 is estimated due to thermal expansion of warmer sea water. Continued sea level rise is expected for many centuries, probably to much higher levels.

* Summer Mid-Continental Dryness and Warming (probable)

Model studies predict a marked decrease of soil moisture over summer mid-latitude continents. This projection remains sensitive to model assumptions.

* Increased Tropical Storm Intensities (probable)

A warmer, wetter atmosphere will likely lead to increased intensities of tropical storms, such as hurricanes. We still know little about changes in the number of hurricanes.

* Increased Numbers of Weather Disturbances (uncertain)

Although many speak of more large-scale storms, there is still no solid evidence for this.

* Global and Regional 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, especially so on regional scales.

Even though these uncertainties are daunting, important advances have already been achieved in observing, understanding, and modeling the climate. Today's models can simulate many aspects of climate and its changes. Although major progress has been made, much more needs to be learned. More efforts are needed world-wide to provide a long-term climate measuring system. Focussed research into climate processes must be continued. Theories must be formulated and re-evaluated 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 has already made important progress on these fronts. However, patient, 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. That concludes my testimony.