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growth in atmospheric abundance of carbon dioxide. Measurements, using air bubbles trapped within accumulating layers of snow, show that atmospheric carbon dioxide has increased by more than 30 percent over the industrial era, compared to the relative constant abundance that it had over the previous 750 years.

The predominant cause of the increase in carbon dioxide is the combustion of fossil fuels and burning of forests. Other heat-trapping gases are also increasing as a result of human activities. The increase in heat-trapping greenhouse gases due to human activities are projected to be amplified by feedback effects, such as changes in water vapor, snow cover, and sea ice. So as atmospheric concentrations of carbon dioxide and greenhouse gases increase, the resulting increase in surface temperature leads to less sea ice and snow, thereby reducing the amount of the Sun's energy reflected back into space, resulting in a higher temperature.

As greenhouse gases increase, evaporation increases, which leads to more atmospheric water vapor. The additional water vapor acts as important feedback to increase temperature. Our present understanding is that these two feedbacks account for about 60 percent of the warming. The exact magnitude of the feedback effects and others, such as changes in clouds, remain a significant source of uncertainty related to our understanding of the impact of greenhouse gases.

Increases in evaporation water vapor affect global climate in other ways besides increasing temperature, such as increasing rainfall and snowfall rates. The increase in greenhouse gas concentration implies a positive radiative forcing and has a tendency to warm the climate. Particles or aerosols in the atmosphere resulting from human activities can also affect climate. Aerosols vary considerably from region to region. Some aerosol types act, in a sense, opposite to the greenhouse gases and cause a negative forcing or cooling of climate, as Dr. Hansen's chart shows.

There may also be other natural factors that exert an influence on climate: Changes in the sun's energy, and changes in volcanic eruptions. These effects, however, such as volcanic eruptions, are short-lived. The forcing estimates in the case of greenhouse gases are substantially greater than those for these other two forcing agents. What do the changes imply? First off, there is a growing set of observations that yields a collective picture of a warmer world. There is just simply no question the climate of the last 100 years is increasing the temperature. We have ample evidence: Widespread retreat of glaciers in non-polar regions; snow cover, and sea ice extent has decreased; thickness of sea ice has decreased; and duration of ice on lakes and rivers also all have decreased.

It is also likely that the frequency of extreme events have increased as global temperatures have risen. This is particular evident in areas where precipitation has increased, primarily mid- and high-latitudes of the Northern Hemisphere. Other extremes have decreased, such as the frequency of extremely cold weather, and the frequency of frost during the period of instrumental record. There is a new and stronger evidence that most of the warming

of future human activities indicate continued changes in atmospheric composition throughout the 21st Century.

Based on these scenarios and the estimated uncertainties in climate models, resulting projections of global temperature increase by the year 2100 range from 2.3 to 10.1 degrees Fahrenheit. Such a projected rate of warming would be much larger than observed over the 20th Century and would very much likely be without precedent over the past 10,000 years. It is important to emphasize that greenhouse gas warming could be reversed only very slowly. The quasi-irreversibility arises because of the slow rate of removal from the atmosphere of greenhouse gases and because of the slow response of oceans to thermal changes.

It is presently not possible to generally define a safe level of greenhouse gases. There are still large uncertainties related to the projected rate and magnitude of climate change. The determination of an acceptable concentration of greenhouse gases depends on narrowing this range, as well as the knowledge and risk of vulnerabilities to climate change. Analysis reveals that sectors and regions vary in their sensitivity to climate change, but generally those societies and systems least able to adapt and those regions with the largest changes are at greatest risk. This includes the poor nations and sectors of our society, natural ecosystems-those regions that are likely to see the largest changes, for example, in the Arctic.

In terms of our understanding, there is still considerable uncertainty of how the natural variability of the climate system reacts to emissions of greenhouse gases and aerosols. Current estimates of the magnitude and impacts of future warming are subject to future adjustments either up or down. To address these uncertainties in several areas, we think it is important that we embark on understanding the complex climate system. Progress in this area will be limited by the weakest link in the chain. At the present time, there are several weak links that need to be addressed.

First and foremost, a climate observing system is needed to monitor decade-to-century scale changes for basic variables needed to describe the climate system. Current observing systems yield large uncertainties in several key parameters, especially on regional and local scales. Although we have been able to link observed changes to human activities, it is not possible to quantitatively identify the specific contribution of each forcing factor, which is required for the most effective strategy to prevent large or rapid climate change. This will require better understanding in several areas: The feedbacks of the climate system; the future usage of fossil fuels; carbon sequestration on land and in the ocean; details of regional climate change; and natural climate variability.

Finally, we found that no matter how good our understanding of future climate change might be, we ultimately must understand how this impacts natural and human systems. To achieve this understanding will require first an interdisciplinary research that couples physical, chemical, biological, and human systems, improved capability to integrate scientific knowledge, including its uncertainty, into effective decision support systems, a better understanding of the impact of multiple stresses on human and natural

Thank you, and I look forward to working with you on these issues, and thank you again for inviting me to appear today.

Chairman LIEBERMAN. Thanks, Mr. Karl. Let me begin questioning. Although we asked you here to discuss the science of climate change, I think it would be interesting to ask if you have any response, having the expertise you do, to the Byrd-Stevens proposal that is the focus of our hearing today, and to the coordination of the response to climate change that it would enact. Do either of you have a response?

Mr. Karl.

Mr. KARL. One thing I would highlight is, as I indicated in my testimony, this is an extremely complex issue, one which encompasses many areas of science. It encompasses areas of social science, as well as the physical sciences. So, to move forward, it is very clear a coordinated effort is clearly needed, and I think that is one of the highlights of the Byrd-Stevens bill.

Chairman LIEBERMAN. Thank you. Dr. Hansen.

Mr. HANSEN. I was delighted to hear the discussions by the several Senators. I agree with Mr. Karl. It is a very complicated issue and we need a broad approach to look at it.

Chairman LIEBERMAN. Do you think that the Byrd-Stevens proposal, as you understand it, meets that standard?

Mr. HANSEN. I do not think it is appropriate for me to take a position with regard to it, but certainly the discussions we heard today seem to be right on the mark.

Chairman LIEBERMAN. Understood. It is my impression that there is not really remaining dispute regarding whether climate change is occurring. In fact, I noticed last week that our colleague, Senator Hagel, who was one of the co-authors, obviously, of the Byrd-Hagel resolution, was quoted in USA Today as saying that, "There is no question there is climate change. We are beyond that debate." Would you agree with Senator Hagel, Dr. Hansen?

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Mr. HANSEN. Yes, I was one of the authors, as was Mr. Karl, of the recent National Academy of Science's report in which we reaffirmed the reality of global warming and that there is the possibility of disruptive climate change later this century. I think we also took pains to stress some caveats about what will happen. It depends very much on how these climate forcing agents develop, and it is certainly within our capability to influence that and to influence the amount of climate change that will occur.

Chairman LIEBERMAN. Mr. Karl.

Mr. KARL. Yes, there is no question that the climate is changing in ways which we have now seen from the observational record and our past paleoclimate data. One of the important attributes of climate, though, is much broader than just changes in temperature, and as I indicated, there are some unsettling things we do not know about-for example, changes in some of the extreme precipitation events in all areas of the world.

So I think it is really going to be key, as we continue to change atmospheric composition, to look at changes in all the elements of the climate system, particularly for potential surprises, accelerated changes. That is one of the areas I would like to emphasize. Although we are sure climate is changing in significant ways, we do

Chairman LIEBERMAN. In other words, there are questions about whether some of the extreme precipitation or extreme weather that people are experiencing is related to the climate change that we know is a reality.

Mr. KARL. Part of the difficulty we have, if you look at our observing system, is that in the mid-latitudes and some of the higher latitudes, we have enough data to make what we think are reasonably confident statements. But if you look at the rest of the world, the observing systems really are not capable of delivering that kind of information which we so badly need.

Chairman LIEBERMAN. One area of focus of the Byrd-Stevens bill, S. 1008, which is, I thought, very interesting, was the need to help us-Americans-adapt to the already inevitable consequences of climate change, or at least that is the way I read one of their four goals. I wanted to ask you to what-perhaps you have answered it already, but just to come at it in a different way-to what extent do you believe that some climate change is already inevitable? In other words, that there will be consequences already. And what measures would you recommend to help adapt to that change?

Mr. HANSEN. I think that we have evidence that some additional warming is on the way. There has been warming already of about half-a-degree Celsius or one degree Fahrenheit in the past century, and I think that there is about another half-a-degree Celsius, which is already in the pipeline, because of the greenhouse gases that we have added to the atmosphere and which the system has not yet responded to, due to the long time constant of the ocean. It takes a long time for the ocean to warm up in response to this forcing.

If we can slow down the growth rate of these climate forcing agents, then I think the additional warming in the next 50 years will be less than one degree. That is a magnitude which we could adjust to probably without a great deal of difficulty, although even now climate fluctuations are a major factor that we need to pay more attention to, making ourself less vulnerable to those fluctuations.

Chairman LIEBERMAN. How serious would the steps be that we have to take to control or contain climate change within the next 50 years, to the degree that you describe?

Mr. HANSEN. Well, there are two things that we need to do: One is, as I mentioned, stop the growth of these non-CO2 forcings. I think there are very good reasons to do that anyhow, which to a large degree could pay for themselves. They are not going to happen automatically. We have to see that they happen. They are basically air pollution and they affect everybody-I gave numbers for people that die from it—but there are even more people who do not die, but suffer consequences of air pollution.

The CO2 part: How do we keep the rate of emissions of CO2 from increasing? Again, that is debatable. There are people who feel that just from conservation, energy efficiency and renewable energy sources, we can keep the emissions similar to what they are today. Most energy experts, however, believe that we will need some clean energy sources such as-I gave you examples: Nuclear power, which has disadvantages; or capture the CO2 from coal-that is

that appear practical-but they will require a real effort to do them.

Chairman LIEBERMAN. Mr. Karl, how about your reaction to the extent to which climate change is already inevitable, perhaps also your evaluation of Dr. Hansen's alternative scenario?

Mr. KARL. Yes, I would like to address that and emphasize as well, one of the great problems we face, as Dr. Hansen said, which I agree with, we already have in the pipeline some additional warming, something on the order of half-a-degree, and it is clear that greenhouse gas concentrations are likely to continue to increase. One of the real difficulties we have is trying to ensure that new systems that are expected to have a lifetime of many decades now begin to incorporate, not just the past climate, but projected changes in climate, to ensure that their design efficiency is as good as it could possibly be.

Chairman LIEBERMAN. How do you mean new systems?

Mr. KARL. For example, we have noticed that the design standards for buildings are being exceeded in many parts of the country and engineers are using climatologies based on earlier records in the 20th Century. So in order to ensure that we have efficiency in our energy systems, we would really need to think about how we use the climate of the past and what we might expect into the future, and that is a very important area of adaptation, because quite frankly, at this time, people are a bit scrambling, trying to decide exactly what to do.

Chairman LIEBERMAN. Are we seeing elsewhere, in your experience, the rather dramatic examples that Senator Stevens gave us about what is happening in Alaska and the Arctic region, of the effects of climate change?

Mr. HANSEN. The Arctic region-it is not the entire Arctic. For example, Greenland has actually cooled in the last 50 years. So there is a change in the long-wave patterns at the high latitudes, such that the region around Alaska and the center of Siberia warm substantially. Those are the regions where we have seen the largest warming. I do not think there is a comparable warming in other parts of the world. As we said, the average warming is about half-a-degree Celsius, but in those regions it has been significantly larger than that.

Chairman LIEBERMAN. Go ahead, Mr. Karl.

Mr. KARL. I think it would be worth emphasizing that the expectations of warming are larger over land areas compared to the ocean areas, and large over places like North America and midand high-latitudes, significantly larger than the average temperatures that you hear being discussed in terms of projected change.

Chairman LIEBERMAN. Why is that?

Mr. KARL. The oceans are a great reservoir of heat, and we have just conducted some research in our agency which showed that the ocean heat content has increased. So part of the warming being taken up into the oceans is being transported down to deep layers in the ocean.

Chairman LIEBERMAN. But why more of an impact in North

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