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Research Features

Global Warming in the 21st Century: Page 2 of 2

Discussion of "An Alternative Scenario"

Expanded from an MIT Workshop at Johns Hopkins University, Oct. 16, 2000

The original article which led to this "discussion" was by James E. Hansen, Makiko Sato, Reto Ruedy, Andrew Lacis, and Valdar Oinas and appeared in Proc. Natl. Acad. Sci..

Alternatively, a description of the "Alternative Scenario" study can be read here on the GISS website.

I appreciate this opportunity to clarify our paper. Some thoughtful people did not understand very well what we were trying to say, so I accept the blame for not being clear enough. Even usually reliable sources such as The New York Times had either inaccuracies or emphases in their description of the paper that were misleading about its thrust. I have found it difficult to correct the mischaracterizations. For example, I wrote a letter to Nature, but they edited my letter so as to change its meaning. It made me realize that they had an editorial position on the matter. That is their right, but they should have published the article discussing our paper on the editorial page, not under a "News" banner.

It is impossible for me to keep up with all of the commentary. Therefore, over the weekend I wrote a draft "open letter" in which I try to clarify our paper. I finished that draft late last evening and did not have time to make a separate talk, so I will use pieces of that draft for my talk today. (Open letter available in PDF, 205 kB).

BAU and Alternative Scenarios

Our paper is about the forcings that drive climate change. A climate forcing is an imposed perturbation of the Earth's energy balance with space. Human-made "greenhouse" gases now in the atmosphere cause a forcing of more than 2 Watts per square meter. In other words the gases that people have added to the atmosphere cause a heating of the Earth's surface as if two miniature one-Watt Christmas tree bulbs had been placed over every square meter of the Earth's surface. This is equivalent to increasing the brightness of the sun by about 1 percent.

That is the forcing due to past emissions. For the future, most climate modelers employ business-as-usual (BAU) scenarios for future greenhouse gas amounts. In BAU scenarios it is assumed that greenhouse gases will increase at an accelerating rate in the 21st century. This leads to an additional human-made climate forcing of about 3 Watts by 2050. It is assumed that about 2/3 of this forcing is caused by increasing CO2 and about 1/3 by other gases, but for the sake of simplicity the BAU forcing is usually represented as a 1 percent per year increase of CO2.

BAU provides a valuable warning, should the world follow a course with accelerating growth rates of greenhouse gases. Paleoclimate history indicates that a forcing of the BAU magnitude would cause global warming of several degrees with substantial practical impacts.

Our thesis is that the BAU scenario should be accompanied by an alternative scenario with a much smaller climate forcing, which would lead to only moderate climate change. Contrary to what Nature says, we neither predict that the "alternative scenario" will occur nor assume that it will occur. It is simply a scenario that we hope can help stimulate discussion about the nature of the actions required to yield only a modest climate forcing, instead of a large forcing.

Fig 1

Fig. 1: Climate forcings between 2000 and 2050 in "business-as-usual" and "alternative" scenarios.

Our "Alternative Scenario" adds a human-made forcing of 1 Watt in the next 50 years. This is obtained by keeping the net increase of non-CO2 forcings at 0 Watts and the increase of the CO2 forcing at 1 Watt, as summarized in Figure 1. The non-CO2 and the CO2 aspects of this scenario are equally important in the next 50 years, but on the longer run, especially if we stop the growth of the non-CO2 forcings, CO2 will be the issue.

Non-CO2 Forcings: "Air Pollution"

We suggest that it is plausible to keep the non-CO2 forcing, which we can loosely describe as air pollution, no larger in 2050 than it is today. We note that there will be multiple benefits of a climate strategy that focuses on non-CO2 forcings as well as on CO2. I will mention the three major non-CO2 forcings.

Black carbon (soot) aerosols. This is a major pollutant that has received inadequate attention. Aerosols are tiny solid or liquid droplets in the air. Black (elemental) carbon aerosols are soot formed by incomplete combustion, which often can be seen emanating from diesel-powered trucks and buses. Coal and biomass burning are also major sources.

Black carbon aerosols cause a positive climate forcing that is very uncertain in magnitude, but we estimate that it is probably of the order of 0.5 to 1 Watt per square meter on global average. It causes warming in several ways: by absorbing sunlight, by warming the lower layer of the atmosphere and thus reducing cloud cover, by making clouds slightly "dirty", and by darkening snow and sea ice surfaces when it is deposited there.

Black carbon aerosols are a principal component of air pollution. In the words of Tony Hansen, microscopic soot aerosols are like tiny sponges that absorb other substances including toxic organic particles. A recent study in France, Austria and Switzerland puts the annual cost of particulate pollution in those countries at 40,000 lives, 500,000 asthma attacks, and medical costs amounting to 1.7% of their gross domestic products. Air pollution in developing countries, such as India and China, is even more severe.

Methane. Methane (CH4) causes a larger forcing than is generally recognized. As shown in Figure 2, the forcing due to increased CH4 (0.7 Watts) is fully half as large as the forcing by CO2 (1.4 Watts).

Fig 2

Fig. 2: Estimated climate forcings between 1850 and 2000. (From Hansen et al. 2000)

The observed growth rate of atmospheric CH4 has declined by at least two-thirds since the 1970s (Figure 3). The reasons are not well understood, but probably include decreases in the growth rate of methane sources and changes in atmospheric chemistry (and thus the methane "sink").

There are numerous opportunities to reduce human emissions of CH4. We suggest that it may be practical to keep the CH4 of 2050 from exceeding that of today.

One claim that has greeted our paper, that an increased emphasis on methane would be seen as shifting the blame for global warming to developing countries, makes no sense, in my opinion. On the contrary, interest in methane could be framed as an opportunity for developing countries, if they were given some modest economic incentive to reduce emissions. Such an approach has been remarkably successful in dealing with CFCs.

Fig 3

Fig. 3: Climate forcings by individual greenhouse gases: (From Hansen et al. 2000)

Ozone. Tropospheric ozone is estimated to have caused the 3rd largest climate forcing among the greenhouse gases (Figure 2). Ozone is an oxidant that is damaging to the respiratory system and to agricultural productivity, with estimated annual costs in the United States of about $10B/year.

Recent global trends of tropospheric ozone are unclear, as some countries have taken steps that reduce ozone, while ozone has increased in other places. Ozone and its precursors in one country are believed to affect ozone levels in other countries, even across the ocean.

There are complex interactions among atmospheric gases, and actions that decrease one gas may increase another. However, by comparing the pre-industrial atmosphere with that of today we see that human emissions have greatly increased both O3 and CH4. We suggest that, with a focus on reducing air pollution, it will be possible to have no further growth, or even a decrease, in the sum of O3 and CH4 global climate forcings in the next 50 years.

CO2 Forcings

Our alternative scenario aims to limit added CO2 forcing to 1 Watt in the next 50 years. This would require the CO2 growth rate to average about the same in the next 50 years as it did in the past 20 years, which was 1.5 ppm per year. This means, to first approximation, that CO2 emissions from fossil fuel use would need to remain about the same as today or begin to decline slightly.

How does this compare with recent history? From 1945 until the oil price shock of the 1970s, fossil fuel use increased 4% per year, in lock step with economic growth. But since then fossil fuel use has increased only 1% per year (Figure 4), as economic growth has been substantially decoupled from energy growth. Our scenario requires an additional decline of the emissions growth rate, from 1% per year to zero.

Fig 4

Fig. 4: CO2 emissions from fuel use (from Hansen et al 2000; estimate for wood from N. Makarova)

Therefore, in this scenario, incremental growth of energy demand needs to be met by some combination of (1) increased energy efficiency in present uses, and (2) continued trend toward decarbonization of energy sources, including the introduction of renewable energy sources such as the wind, sun, and other processes that produce little or no CO2. We note that within 50 years renewable energy sources may include technologies that we don't yet know about.

On the other hand, let's say that by 2050 we have failed to develop any alternatives to coal as the successor to oil as the oil resource inevitably declines. In that case, we point out in our paper, if climate considerations demand even slower CO2 growth, sequestration of CO2 provides a viable back-up strategy. Substantial sequestration should be practical, because it is expected that a larger fraction of energy will be generated at power plants, where capture of CO2 emissions is relatively easy.

It is not our task to prescribe policies that would keep the added CO2 forcing in the next 50 years at 1 Watt. We only note that such a flat growth rate of CO2 forcing is plausible and nearly has been obtained already. However, common sense suggests that achievement of both energy efficiency gains and increasing contributions from renewable resources would be aided if the price of fossil fuels reflected their true cost. Thus governments may want to consider limiting subsidies for fossil fuels and perhaps gradually adding an assessment to fuel price to cover the cost of its impact on human health. Sudden jumps in energy costs tend to fall unfairly on some consumers and they are ineffective at promoting energy efficiency and alternative energies. However, gradual long-term changes, which allow introduction of viable choices for improved efficiencies, could be effective and also limit hardships. This issue should be considered carefully by political leaders as scientific understanding advances and empirical evidence increases. The ability of the public to grasp the issues and support policies that husband valuable resources should not be underestimated.

Summary of Forcings

In summary, our "alternative scenario" tries to keep the added forcing in 50 years at 1 Watt, by keeping the added CO2 forcing at 1 Watt and getting all the other forcings to add up to 0 Watts. A big uncertainty in this equation is caused by aerosols. If we reduce sulfates, which we should do to reduce acid rain, we will cause a positive forcing. On the other hand, if we would go after black carbon aggressively, we could push the change of aerosol forcing back to zero or negative. (Figure 5)

Fig 5

Fig. 5: A scenario for additional climate forcings between 2000 and 2050. Reduction of black carbon moves the aerosol forcing to lower values. (From Hansen et al. 2000)

It's important to note that, unlike greenhouse gases, the aerosol forcing is not being measured. We just had a workshop at the Geophysical Fluid Dynamics Laboratory to define the measurement needs, and we wrote a letter to Jim Baker of NOAA (letter in PDF, 230 kB) in an attempt to get him to advocate accurate measurements of the aerosol climate forcing from the satellite that provides the next opportunity, the NPOESS Preparatory Project.

One other summary comment. Two of the four primary forcings (black carbon and ozone) are not yet included in existing or proposed protocols. I believe that it is feasible to halt further growth of these non-CO2 forcings, which can loosely be termed air pollution. Again this is a topic for experts to debate, but it seems worthwhile to stimulate that discussion.

Interpretations and Representations

I have time for only a few comments about the interpretations of our paper, in addition to my previous comment about Nature. I discuss them further in my "open letter".

Union of Concerned Scientists (UCS). The UCS leadership sent to its members an "Information Update" discussing our paper. The essence of the update seems to be that our paper is controversial, potentially harmful to the Kyoto Protocol, and not a helpful contribution to the climate change discussion as it "may fuel confusion about global warming among the public". They describe "first reactions from within the scientific community", which perhaps are accurate but they seem a bit like commissioned criticisms.

The UCS leaders say that our team proposes to "focus first on the reduction of non-CO2 GHGs - essentially to buy time — and then, by mid-century, shift attention back to CO2 and fossil fuel sources". However, on the contrary, we recommend immediate attention to improved energy efficiency, continuing decarbonization of the fuel source, and development of renewable energy sources, as required to achieve a flat or declining CO2 emissions growth rate.

The UCS leaders state that "the author team seems unduly convinced that reducing fossil fuel emissions would be economically wrenching to the United States". On the contrary, it is our opinion that the actions we recommend (removing barriers to energy efficiency, development of clean fuels, development of renewable energy sources that produce little or no CO2) will stimulate the economy, create jobs, reduce reliance on foreign sources of energy, improve our economic competitiveness, and create a more healthful environment, while at the same time slowing the growth rate of CO2. Apparently the UCS leadership did not read our paper, or did not understand it.

New York Times. The first New York Times article on our paper (August 19) was not far off, although the first sentence implied that I had changed my opinion about global warming, and that I now said "emphasis on carbon dioxide may be misplaced". First, we are not de-emphasizing CO2, and, second, we have long championed the importance of the other forcings. In fact, in 1976 five of us at GISS published a paper in Science pointing out for the first time that CH4, N2O and several other gases together provided a forcing that was considerable compared to that of CO2.

A second article (October 3) aggravated the misunderstanding by discussing only the non-CO2 forcings. Their description of me as an "inveterate pot-stirrer" can be taken as a compliment, but I suspect that the people wishing to hang that appellation on me intend it to discredit my opinions.

Washington Post and Rolling Stone. The most accurate summaries of our paper, in my opinion, were an article in Rolling Stone by Chuck Sudetic (October 12) and an editorial in the Washington Post (August 28). The Rolling Stone article was based mainly on a recorded interview and the Post editorial followed a substantive phone conversation and reading of our paper. The Post editorial concluded:

"... Some of the gases involved are already in decline because of other international restrictions; going after others amounts to an attack on air pollution, which the scientists argue should be attractive action in all parts of the world, independent of concerns about warming, because of health benefits of cleaner air.

"That optimistic scenario immediately caused some environmentalists to worry that the report would become a weapon for those who are skeptical about warming — who oppose any action. Dr. Hansen himself said it undoubtedly will be used that way, but that would be a misreading of the study. The new report does not challenge either the evidence that surface temperatures are going up or the growing consensus that human activities are contributing to the increase. It continues to cite the need for reductions in carbon dioxide emissions. There is no suggestion, nor should there be, that response to global warming should wait until the science is more certain.

"What it does do is remind us that climate issues are complex, far from fully understood and open to a variety of approaches. It should serve as a caution to environmentalists so certain of their position that they're willing to advocate radical olutions, no matter what the economic cost. It suggests that the sensible course is to move ahead with a strong dose of realism and flexibility, focusing on approaches that are economically viable, that serve other useful purposes such as cutting dependence on foreign oil or improving public health, and that can help support international consensus for addressing climate change. If the Hansen report pushes the discussion in that direction, it will turn out to be good news indeed."

Constructions and the Bottom Line

Negative construction. The negative construction that has been placed on our article is that it is harmful, because it can be used by global warming skeptics to argue against the need to slow CO2 growth rates.

Positive construction. A positive construction would be something along the lines of the last paragraph of the Washington Post editorial. One might even recognize in the alternative scenario a strategy to slow global warming quickly.

The bottom line. Our job is not to place any construction on the paper. Our bottom line is that in our letter to Nature: "Our aim is to produce the most objective quantitative analysis that we can. In the end, that is likely to serve the public best."