Question 5. What would you say is most urgent in terms of future research needs? Answer. It is useful to summarize here IPCC 2001 's statements on future research needs. These are an appropriate recognition of the needs in the present times, based on considerations stemming from the current assessment of climate change science. Note that IPCC itself does not make any recommendations on prioritization or funding plans, nor is it associated with or endorses any national/ international programs.

First, systematic observations and reconstructions of past climates need to be sustained and improved wherever possible. Observations include those that are designed to understand the processes, as well as those that are specifically geared towards long-term monitoring of key climate variables. The elements include: arresting and reversing the decline of observational networks; sustaining and expanding the observational foundation of climate studies by providing accurate, long-term, highly reliable and consistent data, including implementation of strategies for integrated and well-coordinated global observations; enhancing development of reconstruction of past climate periods; improving observations of the spatial and temporal distributions of greenhouse gases and aerosols; sustaining measurements that monitor forcing agents and climate feedback processes; improvements in observations of the world's oceans including ocean thermal changes (this may prove to be an optimal item to measure the increasing heat content of the climate system).

Second, improvements in modelling and process studies are needed to improve the quantitative realism of the simulated climate system. These include: improved understanding of the physical and chemical mechanisms that lead to a forcing of climate change; understanding and characterizing the important unresolved processes, and physical and biogeochemical feedbacks in the climate system; improved methods to quantify uncertainties of climate projections and scenarios, including long-term ensemble simulations using complex but well-understood models; improving the integrated hierarchy of global and regional climate models, with a focus on the simulations of climate variability, regional climate changes and extreme events; linking more effectively models of the physical climate and the biogeochemical system, and in turn improving the coupling with other factors intrinsically associated with human activities.

There is a vital research element to be added to the above viz., an appropriate synthesis of the observations and model simulations leading to a scientifically, wellgrounded picture of climate change and its causes. Rigorous diagnostic analyses of observations and model simulations are critically needed in unravelling the evolution of climate change. Lastly, in the sequence, it cannot be overemphasized enough that each successive piece of knowledge gained, whether in modeling, observations or diagnostic analyses, needs to be gainfully used to plan better observational strategies and to improve further upon the model simulations/projections of climate change.

It is vital that there be a balanced approach that weighs in both observations and modeling studies. In particular, the build-up of the infrastructure and funding plans must recognize this point. For instance, observations should guide the science of what forcings are operating, what are the feedbacks, how should we be modeling these, what are the results of the simulations, how robust are they, how do they compare with various climate parameters, why is there a disagreement or why is there a good agreement, what can we relay back to the observational infrastructure so that they can receive better guidance. The idea should be to continually enhance the confidence in the climate forcings, feedback mechanisms, and responses, consistent with the central focus of understanding climate variations and changes. Question 6. You have mentioned that the best agreement between observations and model simulations over the past 140 years is found when both human-related and natural climate-change agents are included in the simulations. Why is it important for the model simulation to include both?

Answer. In order to investigate the long-term climate change, model simulations of climate change have considered four different possibilities: (a) unforced internal variability of the nonlinear coupled atmosphere-ocean system i.e., the climate variations that occur even in the absence of any forcing; (b) climate change due to the introduction of “natural” factors such as solar irradiance changes and volcano-induced enhancement of stratospheric aerosol concentrations; (c) climate changes when only "anthropogenic" factors (e.g., emissions of greenhouse gases and aerosols) are considered; and (d) when all the factors are considered in unison. This modus operandi enables the identification of specific causal factors and aids in framing the detection-attribution analyses.

The climate model simulations performed indicate that it is very unlikely that internal variability of the climate system alone can explain the past 140 years' observed surface temperature record. Three different models (one of them from NOAA)

are in agreement on this finding. The models' surface temperature interdecadal variation is not inconsistent with that observed over the past 140 years. A model simulation without consideration of the water vapor feedback yields far less variability than evidenced in the observations, suggesting that the manner in which this feedback is represented in the models may be qualitatively consistent with reality. Owing to the lack of a long record in atmospheric observations, there tends to be a reliance on climate models for estimates of the unforced climate variability. Although this is a limitation, there are tests that climate models have successfully met in this regard.

"Natural" factors alone cannot account for the observed warming over the past 140 years, although there are suggestions that over the first half of the 20th century, these factors may have contributed to the warming occurring at that time. In particular, solar irradiance changes may have contributed to the observed warming during the first half of the 20th century. Although episodic volcanic eruptions exert impacts during the 1-2 years that they enhance stratospheric aerosol concentrations, their effects over the past century are less relative to those due to the secular changes in greenhouse gases. Model simulations with "anthropogenic" factors alone indicate that, despite uncertainties in the quantitative estimates of the forcing, their influence in the model simulations can be associated with the rapid rise in the observed warming over the latter half of the 20th century.

When considering the entire modern instrumental surface temperature record, it becomes clear that both “natural” and “anthropogenic" factors need to be considered for the simulation of the observed temperature record. This includes the Sun's output changes as well as the particularly active volcanic period in the 1880-1920 and 1960-1991 time periods. For a proper explanation of climate change, and to distinguish between the natural factors and anthropogenic species, these factors must be juxtaposed with the internally generated variability.

RESPONSE TO WRITTEN QUESTIONS SUBMITTED BY HON. JOHN MCCAIN
TO JAMES J. MCCARTHY

Question 1. Why would climate changes in the 21st century be 2-10 times faster than those of the 19th century?

Answer. On pp. 30-31 of the oral testimony transcript I am correctly quoted as having made a statement like this in comparing rates of climate change between the 21st century and the 20th (not the 19th) century.

More specifically, this comparison is between the rates of global mean temperature change. For the 20th century this rate was 0.6C (1.0F) per century. For the 21st century, the scenarios project a range of increases between 1.4C (2.5F) and 5.8C (10F). This comparison is the root of the 2-10 fold comparisons.

Question 2. Your written testimony states that even the most optimistic scenarios for mitigating future climate change are unlikely to prevent significant damage from occurring. What type of events would qualify as significant damage?

Answer. Extrapolating from the changes that have occurred in the last few decades in the distributions and timing of seasonal biological phenomena, accelerating some of these by 2-10 times in the current century may push some species over the edge. Prime examples are tropical and Arctic systems, where temperature limits for some species like coral may be exceeded, and the ice habitat for many organisms, like pregnant polar bears needing the high fat nourishment of seals, may be lost. Most problematic, though, are the impacts on human systems related to extreme climate events. Table 1 in the Working Group I SPM indicates levels of confidence in extreme weather and climate observations over the past 50 years and projections in the next 50 years. Table 1 in the Working Group II SPM lists representative examples of projected impacts from these extreme events. Extrapolating from the tolls in lives, livelihoods, and properties caused by the flood and mudslide disasters in the past 5-10 years to the projected future provides good examples of likely significant damage.

Question 3. There has been and continues to be a major discussion on how to reduce emissions. How can we best prepare people and systems for the disruption that will ensue with the climate change that is now projected for the 21st century?

Answer. This is in my estimation one of the most critical questions that we face. The scenarios mentioned above that yield the range of 1.4-5.8C increases are representatives of classes of scenarios (35 were used) that have several variable components. These include the projections for human population numbers over the next century, our standard of living and socioeconomic conditions in the developed and developing world, and the fossil-fuel intensity of our energy producing activities. The

last of these is the one that is most easily altered with minimal impact on the other conditions.

While an optimist will suggest that it is unlikely that we will climb steeply up the highest of these slopes, a realist will also suggest that it is unlikely that we be able to stay close to the lowest of these slopes. Partly this is due to the socioeconomic and geophysical inertia in our energy systems. While it is easier to modulate the use of fossil fuel, and especially to switch to alternative sources of energy, than it is to reduce the world's human population numbers, the difficulties in changing human behavior and human institutions are enormous. At the same time, since CO2 emitted today will be still be in the atmosphere a century from now, everything we do now to reduce rates of emission will pay increasing dividends in the future. This having been said, it is clear that we must also prepare for the sort of increasing prospect of damage mentioned in #2 above by enhancing adaptation. This is particularly critical in the regions hardest hit where adaptive capacity is the least (tropics and subtropics). Serious attention must be given to the potential impacts on the availability of safe water, subsistence agriculture, and human health.

How the scenarios mentioned above play out will greatly influence the rate of sea level rise. A large component of sea level rise is due to the expansion of the ocean as it warms. The convection of heat from the surface ocean to deeper waters is a slow process. A greater rate of atmospheric warming early in this century followed by a slower rate of warming later in the century will have a stronger effect on sea level rise within the next 100 years than a slow warming followed by a fast warming that would have atmospheric temperature at the same point 100 years from now. Coastal zones and small island states are vulnerable to this aspect of climate change and even more so with increases in peak storm wind and precipitation intensities. Planning for coastal human settlements, their infrastructures, and resources (like ground water) must be prepared to consider adaptive strategies that can minimize these impacts. Indigenous communities may in some instances be especially vulnerable, such as in the case mentioned for Alaska by Senator Stevens.

Question 4. Can you discuss some of the impacts of climate change on public health?

Answer. Impacts of potential climate change on human health are given a full chapter in the Working Group II report, and this is summarized in section 3.5 of the SPM. Broad categories include negative consequences of increasing thermal stress, the impacts of storms, and increases in the areal extent or seasonal duration of certain infectious diseases. In some areas there may be positive aspects of climate change for human health, such as with diminished winter mortality, but it is important to emphasize that the negative aspects will disproportionately hit the tropical and subtropical regions. An obvious adaptive strategy would be to enhance public health institutions and resources. Since these are woefully inadequate in many areas today, successful adaptation will take a concerted effort the likes of which is without any obvious precedent.

Question 5. How significant was last summer('s) passage of a ship through the Northwest Passage without touching ice? Has shipping traffic increased?

Answer. There is something symbolic and sobering about this observation. Had it occurred any time before in the last 150-200 years it would have been evident in the accounts of sealing and exploring vessels. It is possible that the thinning and loss of areal extent of summer ice in the Arctic Ocean and adjacent regions may be the result of a long term natural cycle, but the period of such a cycle must be longer than a few hundred years, and no known or hypothesized mechanism has this potential. Climate models have forecast diminished Arctic summer ice with continued greenhouse gas-forced warming, but the rates were less than has been observed in the last few years.

At this moment there are probably many commercial enterprises that are exploring options for capitalizing on the diminished ice in the Northwest Passage. Canadian claims regarding access through its Arctic archipelago are certainly an issue that that will require careful consideration by nations wishing to anticipate increased shipping potential through the Northwest Passage.

Question 6. You have mentioned how some species are being driven from their natural habitats because of changing environmental conditions due to increasing temperatures. How many species have been declared extinct because of these weather patterns changes?

Answer. As I stated in my testimony, it is not clear that any of the changes in distribution of species and the timing of biological processes (that can be plausibly liked to local climate change) have led to the loss of any species. Habitat destruction and the intentional and accidental introduction of invasive species have caused several extinctions, especially on islands. These may continue to be larger factors than climate change with regard to extinctions, but in the Arctic and the tropical ocean

this condition may not hold-climate change may dominate. There are synergistic interactions among some of these factors, such as climate change prompting relocation of species, which is then hindered by land-use change that has interrupted migration corridors.

RESPONSE TO WRITTEN QUESTIONS SUBMITTED BY HON. JOHN MCCAIN
TO DR. JAMES E. HANSEN

Question la. You mentioned that your alternative scenario assumes that air pollution is not allowed to get any worse than it is today and that global use of fossil fuels will continue at about today's rate. It also assumes no net growth of the other forcings. What are those other forcings?

Answer. They are included in Figure 2 of my submitted testimony. Chief among them are methane, tropospheric ozone and black carbon (soot) aerosols.

Question 1b. Does the IPCC business as usual scenario assume that air pollution is stable?

Answer. No, They have ozone and methane increasing substantially. In addition, they grossly underestimate the climate forcing by black carbon, and thus their scenarios tend to ignore it. Since air pollution is excluded from the Kyoto Protocol, it receives little attention in the IPCC scenarios.

Question 1c. Do these differences in assumption account for the differences in expected temperature increases in the next 50 years for the two scenarios? And again what are the temperature differences?

Answer. As shown in Figure 5 of my submitted testimony the additional warming in the next 50 years is about 1.6C in the business-as-usual scenario and about 0.75Č in our alternative scenario. Moreover, the business-as-usual scenario "builds in" a much larger later warming, which will appear in the latter half of the century.

The smaller warming in the alternative scenario is due to the two assumptions: (1) it will be possible to stop further growth of non-CO2 forcings (loosely labeled "air pollution"), particularly ozone, black carbon and methane, (2) it will be possible to keep the growth of atmospheric CO2 to about 75 parts per million in the next 50 years, which would require that CO2 emissions remain roughly similar to today's rate or decline slightly.

Question 2. You mentioned in your statement that the judge of science is observations. You also mentioned the potential educational value of keeping an annual public scorecard of measured changes. Can you elaborate on this idea?

Answer. It is briefly elaborated upon in reference 22 of my submitted testimony, where I mention an annual public scorecard of (1) fossil fuel CO2 emissions, (2) atmospheric CO2 amount, (3) human made climate forcing, (4) global temperature. I will try to write a paper with a more a more comprehensive discussion in the near future. One obvious addition would be an annual measure of CH4 emissions and atmospheric amounts. However, the single most important benchmark for the United States is probably an annual update of the bar graph in Figure 11 of my testimony. i.e., the annual growth of CO2 emissions the annual growth needs to be reduced to zero or slightly negative.

Question 3. Do you feel that your results were reviewed and properly considered as part of the IPCC process?

Answer. No. IPCC's size and review procedures make it inherently lethargic, so responding to a mid-2000 paper is difficult. However, the real problem is probably the close binding between IPCC and the Kyoto Protocol discussions. Kyoto excludes consideration of air pollution (such as tropospheric ozone and black carbon), for example, so IPCC basically ignores these topics and downgrades them. The only IPCC "review" of our paper was by the IPCC leaders (as reported in the New York Times, for example), who saw our paper as potentially harmful to Kyoto discussions. They received the backing of organizations (such as the Union of Concerned Scientists, who commissioned a criticism of our paper that I respond to in reference 22) and publications (particularly Nature), who had previous editorial positions favoring the Kyoto Protocol. When I had difficulty publishing a response in Nature, I wrote an open letter that is available at http://naturalscience.com/ns/letters/ns-let25.html. Question 4. You mentioned that the climate cannot respond immediately to a forcing because of the long time needed to warm the oceans. How would we measure the real impact of reducing the amount of greenhouse gases in the atmosphere in the short term?

Answer. We should of course measure the individual greenhouse gases as the best measure of short-term effectiveness of any attempts to reduce emissions. However, the best measure of the impact of the net climate forcing is likely to be heat storage in the ocean. Natural variations of this rate will occur because of the dynamics of

the system. but if the measurements are accurate and maintained for years they will soon begin to provide us with a great tool for understanding where the future climate is heading.

A BRIGHTER FUTURE-BY DR. JAMES E. HANSEN

Contrary to Wuebbles' thesis (2002), most of the media did not misunderstand the thrust of our recent paper (Hansen et al., 2000). We do indeed assert that a scenario is feasible in which the rate of global warming declines. We also posit that, with an understanding of the significant climate forcings, it is possible to achieve such a climatically brighter path with actions that are not "economically wrenching", indeed, actions that make economic sense independent of global warming.

Our paper does not denigrate the "business-as-usual" (BAU) scenario that has been popular in global climate model simulations. The BAU scenario provides a valuable warning of potential climate change if the world follows a path with climate forcings growing more and more rapidly. Our aim was to present a companion scenario that stimulates discussion of actions that help avoid a gloom and doom scenario. I tried to clarify our objectives in an "Open Letter", which is made available from Climatic Change I summarize here key points of discussion.

Black Carbon (BC). One of our assertions is that BC (soot) plays a greater role in climate change than has been appreciated. We believe that the forcing due to BC is of the order of 1 W/m2, rather than of the order of 0.1 W/m2, as assumed by IPCC (1996).

My present estimate for global climate forcings caused by BC is: (1) 0.4±0.2 W/ m2 direct effect, (2) 03±015 W/m2 semi-direct effect (reduction of low-level clouds due to BC heating; Hansen et al., 1997), (3) 0.1±0.05 W/m2 "dirty clouds" due to BC droplet nuclei, (4) 0.2±0.01 W/m2 snow and ice darkening due to BC deposition. These estimates will be discussed in a paper in preparation. The uncertainty estimates are subjective. The net BC forcing implied is 1±0.3 W/m2.

Air Pollution. Aerosols and tropospheric ozone (O3) are not addressed by the Kyoto protocol. They should be. A reason proffered for excluding ozone is that its chemistry is so complex that "most scientists” eyes glaze over” (Revkin, 2000). Perhaps the latter assertion is true. But it is not adequate reason to exclude air pollution from international climate negotiations. Our estimated anthropogenic global climate forcing due to BC (1 W/m2) and O3 (0.4 W/m2) is comparable to the CO2 forcing (1.4 W/m2). One thesis in our paper is that halting the growth of air pollution can make a significant contribution to slowing global warming.

Effects of air pollution on humans are large in the developed world and staggering in the developing world. A recent study (Kunzli et al., 2000) estimates that particulate air pollution in France, Austria and Switzerland takes 40,000 lives annually with health costs equal to 1.6% of the gross national products. An example for the developing world is the estimate (Smith, 2000) that 270,000 Indian children under 5 years old die annually from acute respiratory infections caused by air pollution. Most of the pollution in this latter case arises from indoor combustion for cooking and heating, a primary source of the cloud of pollutants now mushrooming from India and China. Aerosols and ozone also reduce agricultural productivity with costs of many billions of dollars.

Practical benefits of air pollution reduction accrue immediately, not in 100 years. We assert in our paper that this offers an opportunity to reduce the climate problem with a cooperative approach that has immediate clear benefits to both developing and developed countries.

Methane. We conclude that climate forcing by CH4 is 0.7 W/m2, fully half as large as the forcing by CO2. Observed growth of CH4 is not accelerating, contrary to assumptions in many climate scenarios. Indeed, the growth rate has declined by twothirds in the past 20 years. However, future trends are uncertain.

The task of understanding CH4 should be jumped on, like a chicken on a June bug. Yet research support has been minuscule. We need quantitative understanding of CH4 sources and sinks to define optimum policies. It may be possible to find practices that reduce methane emissions while saving money. Farmers want cows and beasts of burden to produce milk, meat, and power, not methane. Rice growers seek food and fiber, not methane, but we must also compare impacts of altered practices on N2O production. There is much potential for methane capture via improved mining and waste management practices.

Scenarios. Science works via iterative comparison of theory and observations. Differences found are not a problem-on the contrary, only by discovering and investigating these can our understanding advance. One problem with the IPCC reports is that each report produces new (and more numerous) greenhouse gas scenarios