STATEMENT OF DR. ROBERT T. WATSON, DIRECTOR, PROCESS STUDIES PROGRAM, EARTH SCIENCE AND APPLICATIONS DIVISION, NATIONAL AERONAUTICS AND SPACE ADMINISTRATION Dr. WATSON. Thank you, Mr. Chairman, members of the committee. It is a pleasure to be here today to testify. Originally I was asked to just testify on the IPCC scientific assessment, but I have also been asked to comment on the Marshall report.

The IPCC assessment was a document written by 100 scientists and reviewed by over 400 scientists. In my opinion it represents the large majority opinion of not only the meteorologists, but the oceanographers, the ecologists, and atmospheric chemists. The document in China was actually a consensus document by more than 50 countries represented by scientists and representatives from scientific agencies, environmental agencies, energy agencies, and natural resource agencies. This is a heavily peer-reviewed document. On the other hand, the Marshall Institute report was written by three scientists, maybe four, totally un-peer-reviewed. I actually think we have spent enough time on the Marshall report, but I will refer to it in my testimony.

From the IPCC, what can we conclude? Greenhouse gases are increasing due to human activities. CO2 has increased by 25 percent. Methane has more than doubled. Nitrous oxide has gone up between 8 and 10 percent. All of these gases are radiatively active. All general circulation models predict a significant change in climate, and there is a general consensus that for a doubling of CO2, the atmospheric temperature, globally and annually, average will increase by 1.5 to 4.5 degrees Centigrade. Associated with this would be changes in precipitation patterns.

The general circulation models of today are doing a better job of simulating today's climate, and they are starting to do a good job of simulating the inter-annual variability observed in the natural record.

I can also tell you that the global temperatures have increased by between 0.3 and 0.6 degrees Centigrade, and there has been a retreat of land glaciers over the last 100 years consistent with the increase in temperature.

The models also predict that the magnitude and rate of change of climate will be greater than anything we have observed in the last 10,000 years.

Now, at this stage, it would be very easy to say this is a very serious problem, major action is needed today. That is what the IPCC report says. The IPCC report also says the following, though, and that is why I believe it is a very balanced document. It says, "our confidence in predicting the magnitude, the timing, and the regional patterns of climate change is low." The magnitude is governed by our understanding of greenhouse gases, largely carbon dioxide. The magnitude is also governed by the role of clouds. The timing is largely governed by the role of the oceans and the exchange of energy and chemicals with the atmosphere, and the regional patterns are governed by the role of the oceans and land surface processes.

I could also tell you that the cause of the observed global increase of temperature has not been established. There is no question about that.

I can also tell you that the magnitude of the observed change is at the low end of the model predictions, and that little or no change has occurred in the USA, and that the models predict a large gradient in temperature changes in the northern versus southern hemisphere, and it has not been observed.

I can also tell you that the satellites in the 1980's saw no statistical change in temperature.

From the second half of my conversation you conclude this is an overblown subject and we should not waste time on it.

Now, let me bring these two rather disparate viewpoints together. First, the Marshall Institute makes a very big deal about the fact that the satellite data has not shown a significant increase in the last 10 years. There are several points.

One, it is a value of 0.6, plus or minus .14. It can accommodate a temperature increase of 0.2, totally consistent with the models. It is also consistent with the surface and balloon data that shows about 0.2, plus or minus 0.1. There is no discrepancy between the satellite data and the ground-based and balloon data when you start to look at it correctly.

Also, when you take a 10-year record, and you say that the satellite data is only one-fifth of the general circulation models, it is totally and patently absurd. One, they did not take into account ozone depletion over the last 10 years, and they did not take into account the changes in sulfur emissions averaged over the northern hemisphere.

Also, we all recognize there is natural variability. So, to compare a 10-year record with a model is absolutely and patently unprofessional and intellectually incorrect.

How can I bring together the fact that, indeed, the model predictions are on the low end of the observed temperature change? As I have just said, ozone depletion may well have offset to some extent in the last two decades global warming as much as 10 to 20 percent of the effect. The increase of sulfur in the northern hemisphere may well have also offset to some extent, in the northern hemisphere primarily, some of the greenhouse warming. However, the atmospheric lifetime of sulfur is only days; the atmospheric lifetime of CO2 is a century or more. Hoping that sulfur will continue to offset global warming in the future is a dangerous policy. We cannot allow for the offset of sulfur to be part of our policy formulation.

As far as the record is concerned, it is also at the low end as I said. It is also consistent that natural variability has largely masked the real greenhouse effect. I think when we look at uncertainty, there is no question there is significant uncertainty. Uncertainty goes in two directions.

One of the Senators has said we may have overestimated the impact of acid deposition. Let me remind you, we significantly underestimated the effect of ozone depletion. We took prudent action in 1978 in the USA, and then internationally we took prudent action in the mid-1980's. Unfortunately, we have now proven beyond doubt that ozone depletion in Antarctica is due to human

activities, and in most likelihood the global observed decrease in ozone, except for the tropics, but throughout the year is most likely in large measure due to human activities.

Uncertainty since 1985 has gone in one direction repeatedly. We underestimated the impact of human activity, and the problem is, just like the CFC's, CO2 and nitrous oxide have an atmospheric lifetime longer than a century. If we allow the experiment to continue and we find we don't like our changed environment, you cannot reverse that damage for many, many decades, possibly even centuries.

Consequently, policy formulation will have to be made now and in the near future with significant scientific uncertainty. But we have to recognize what the long atmospheric lifetimes mean. If you wait for cause and effect to be established, it will be a long while to reverse that damage.

Therefore, any policy has to be based on two factors: prudence in reducing the emissions of greenhouse gases and a very comprehensive research program in order to obtain the scientific understanding we need in order to have better predictions of changes in climate especially at the regional level. If the models are anywhere near correct, we cannot mitigate completely against climate change. We will also have to learn to adapt. To learn to adapt, we need to understand changes at the local and regional level. We do not have confidence in our ability to predict those today.

Thank you.

[The prepared statement of Dr. Watson follows:]

PREPARED STATEMENT OF DR. ROBERT T. WATSON, DIRECTOR, PROCESS STUDIES PROGRAM, EARTH SCIENCE AND APPLICATIONS DIVISION, NATIONAL AERONAUTICS SPACE ADMINISTRATION

Mr. Chairman and Members of the Subcommittee, it is a pleasure to be here today to discuss the current scientific understanding of global climate change. I will summarize the key findings of the Intergovernmental Panel on Climate Change (IPCC) 1992 Supplement: Scientific Assessment, which in my view represents the large majority opinion of the national and international scientific community.

There are two parts to the 1992 IPCC scientific assessment: (a) the IPCC (i.e., government) approved supplement of about 20 pages, and (b) a document of about 150 pages that provides the background scientific information for the IPCC supplement. The background document, which was co-authored by about 118 scientists from 22 countries (22 lead authors and 92 contributors), and peer-reviewed by 380 scientists from 63 countries and 18 UN or non-governmental organizations, has three main chapters: (i) Greenhouse Gases; (ii) Climate Modelling, Climate Prediction and Model Prediction; and (iii) Observed Climate Variability and Change. While every attempt was made by the 22 lead authors to incorporate the comments of the peerreviewers into their chapters, in some cases these formed a minority opinion which could not be reconciled with the larger consensus. Therefore, there are some scientists who still have points of disagreement with areas of the background report. The IPCC 1992 Supplement: Scientific Assessment was prepared by the lead authors of the background document (Working Group I: Scientific Assessment of Climate Change), reviewed by several hundred scientists by mail, and then discussed and finalized at a three day meeting of 130 government representatives, lead authors and other experts from 47 countries and a number of UN and non-governmental organizations in Guangzhou, China in January, 1992.

This testimony has three parts: the major conclusions of the 1992 IPCC Scientific Supplement are reproduced verbatim (Part 1); a description of the current state of knowledge concerning greenhouse gases and their impact on the radiative budget; (Part 2); and summary (Part 3).

PART 1: THE MAJOR CONCLUSIONS OF THE IPCC REPORT

Findings of scientific research since 1990 do not affect our fundamental understanding of the science of the greenhouse effect and either confirm or do not justify alteration of the major conclusions of the first IPCC Scientific Assessment, in particular the following:

Emissions resulting from human activities are substantially increasing the atmospheric concentrations of the greenhouse gases: carbon dioxide, methane, chlorofluorocarbons, and nitrous oxide;

The evidence from the modelling studies, from observations and the sensitivity analyses indicate that the sensitivity of global mean surface temperature to doubling CO2 is unlikely to lie outside the range 1.5 to 4.5 °C;

There are many uncertainties in our predictions particularly with regard to the timing, magnitude and regional patterns of climate change due to our incomplete understanding;

Global mean surface air temperature has increased by 0.3 to 0.6 °C over the last 100 years;

The size of this warming is broadly consistent with predictions of climate models, but it is also of the same magnitude as natural climate variability. Thus the observed increase could be largely due to this natural variability; alternatively this variability and other human factors could have offset a still larger human-induced greenhouse warming;

The unequivocal detection of the enhanced greenhouse effect from observations is not likely for a decade or more.

There are also a number of significant new findings and conclusions which we summarize as follows:

Gases and Aerosols

Depletion of ozone in the lower stratosphere in the middle and high latitudes results in a decrease in radiative forcing which is believed to be comparable in magnitude to the radiative forcing contribution of chlorofluorocarbons (CFCs) (globallyaveraged) over the last decade or so.

The cooling effect of aerosols resulting from sulphur emissions may have offset a significant part of the greenhouse warming in the Northern Hemisphere (NH) during the past several decades. Although this phenomenon was recognized in the 1990 report, some progress has been made in quantifying its effect.

The Global Warming Potential (GWP) remains a useful concept but its practical utility for many gases depends on adequate quantification of the indirect effects as well as the direct. We now recognize that there is increased uncertainty in the calculation of GWPs, particularly in the indirect components and, whilst indirect GWPs are likely to be significant for some gases, the numerical estimates in this Supplementary Report are limited to direct GWPS.

Whilst the rates of increase in the atmospheric concentrations of many greenhouse gases have continued to grow or remain steady, those of methane and some halogen compounds have slowed.

Some data indicate that global emissions of methane from rice paddies may amount to less than previously estimated.

Scenarios

Steps have been taken towards a more comprehensive analysis of the dependence of future greenhouse gas emissions on socio-economic assumptions and projections. A set of updated scenarios have been developed for use in modelling studies which describe a wide range of possible future emissions in the absence of coordinated policy response to climate change.

Modelling

Climate models have continued to improve in respect of both their physical realism and their ability to simulate present climate on large scales, and new techniques are being developed for the simulation of regional climate.

Transient (time-dependent) simulations with coupled ocean-atmosphere models (CGCMs), in which neither aerosols nor ozone changes have been included, suggest a rate of global warming that is consistent, within the range of uncertainties, with the 0.3 °C per decade warming rate quoted by IPCC (1990) for Scenario A of greenhouse gas emissions.

The large-scale geographical patterns of warming produced by the transient model runs with CGCMs are generally similar to the patterns produced by the earlier equilibrium models except that the transient simulations show reduced warming over the northern North Atlantic and the southern oceans near Antarctica.

CGCMs are capable of reproducing some features of atmospheric variability on intradecadal time-scales.

Our understanding of some climate feedbacks and their incorporation in the models has improved. In particular, there has been some clarification of the role of upper tropospheric water vapor. The role of other processes, in particular cloud effects, remains unresolved.

Climate Observations

The anomalously high global mean surface temperatures of the late 1980s have continued into 1990 and 1991 which are the warmest years in the record.

Average warming over parts of the Northern Hemisphere mid-latitude continents has been found to be largely characterized by increases in minimum (night-time) rather than maximum (daytime) temperatures.

Radiosonde data indicate that the lower troposphere has warmed over recent decades. Since meaningful trends cannot be assessed over periods as short as a decade, the widely reported disagreements between decadal trends of air temperature from satellite and surface data cannot be confirmed because the trends are statistically indistinguishable.

The volcanic eruption of Mount Pinatubo in 1991 is expected to lead to transitory stratospheric warming. With less certainty, because of other natural influences, surface and tropospheric cooling may occur during the next few years.

Average warming over the Northern Hemisphere during the last four decades has not been uniform, with marked seasonal and geographic variations; this warming has been especially slow, or absent, over the extratropical north west Atlantic.

The consistency between observations of global temperature changes over the past century and model simulations of the warming due to greenhouse gases over the same period is improved if allowance is made for the increasing evidence of a cooling effect due to sulphate aerosols and stratospheric ozone depletion.

The above conclusions have implications for future projections of global warming and somewhat modify the estimated rate of warming of 0.3 °C per decade for the greenhouse gas emissions Scenario A of the IPCC 1990 Report. If sulphur emissions continue to increase, this warming rate is likely to be reduced, significantly in the Northern Hemisphere, by an amount dependent on the future magnitude and regional distribution of the emissions. Because sulphate aerosols are very short-lived in the atmosphere their effect on global warming rapidly adjusts to increases or decreases in emissions. It should also be noted that while partially offsetting the greenhouse warming, the sulphur emissions are also responsible for acid rain and other environmental effects. There is a further small net reduction likely in the rate of global warming during the next few decades due to decreases in stratospheric ozone, partially offset by increases in tropospheric ozone.

Research carried out since the 1990 IPCC Assessment has served to improve our appreciation of key uncertainties. There is a continuing need for increased monitoring and research into climate processes and modelling. This must involve, in particular, strengthened international collaboration_through the World Climate `Research Programme (WCRP) the International Geosphere Biosphere Programme (IGBP) and the Global Climate Observing System (GCOS).

PART 2: CURRENT STATE OF KNOWLEDGE CONCERNING GREENHOUSE GASES

(i) Influence of Greenhouse Gases and Aerosols on the Earth's Climate System Increases in the concentration of the greenhouse gases will reduce the efficiency with which the Earth cools to space and will tend to warm the lower atmosphere and surface. The amount of warming depends on the size of the increase in concentration of each greenhouse gas, the radiative properties of the gases involved, and the concentration of other greenhouse gases already present in the atmosphere. It also can depend on local effects such as the variation with height of the concentration of the greenhouse gas, a consideration that may be particularly germane to water vapor which is not uniformly mixed throughout the atmosphere. The effect is not a simple one and the balance which is struck between these factors depends on many aspects of the climate system.

Aerosols (small particles) from volcanoes, emissions of sulphates from industry and other sources can absorb and reflect radiation. Moreover, changes in aerosol concentrations can alter cloud reflectivity through their effect on cloud properties. In most cases aerosols tend to cool climate. In general, they have a much shorter lifetime than greenhouse gases so their concentrations respond much more quickly to changes in emissions.

Any changes in the radiative balance of the Earth, including those due to an increase in greenhouse gases or in aerosols, will tend to alter atmospheric and oceanic