The Global Warming Debate
In my view, we are not doing as well as we could in the global warming debate. For one thing, we have failed to use the opportunity to help teach the public about how science research works. On the contrary, we often appear to the public to be advocates of fixed adversarial positions. Of course, we can try to blame this on the media and politicians, with their proclivities to focus on antagonistic extremes. But that doesn't really help.
The fun in science is to explore a topic from all angles and figure out how something works. To do this well, a scientist learns to be open-minded, ignoring prejudices that might be imposed by religious, political or other tendencies (Galileo being a model of excellence). Indeed, science thrives on repeated challenge of any interpretation, and there is even special pleasure in trying to find something wrong with well-accepted theory. Such challenges eventually strengthen our understanding of the subject, but it is a never-ending process as answers raise more questions to be pursued in order to further refine our knowledge.
Skepticism thus plays an essential role in scientific research, and, far from trying to silence skeptics, science invites their contributions. So too, the global warming debate benefits from traditional scientific skepticism.
I have argued in a recent book review that some "greenhouse skeptics" subvert the scientific process, ceasing to act as objective scientists, rather presenting only one side, as if they were lawyers hired to defend
a particular viewpoint. But some of the topics focused on by the skeptics are recognized as legitimate research questions, and also it is fair to say that the injection of environmental, political and religious perspectives in midstream of the science research has occurred from both sides in the global warming debate.
So, what to do? Most scientists are willing to spend part of their time communicating with the public about how science works. And they should be: after all, the financial support for most research is provided ultimately by the public. But one quickly learns that such communication is not easy, at least not for many of us.
Recently I was asked to debate the well-known greenhouse skeptic Dr. Patrick Michaels of the University of Virginia. I summarize here some key points in the debate, "A Public Debate on the Science of Global Warming", held at the New York Hilton, Nov. 20, 1998, and organized by the American Association for the Rhetoric of Science and Technology. A copy of my entire contribution may be downloaded as a PDF document (note: this document is 597 kB).
I agreed to participate in this debate with Dr. Michaels after learning that he had used (or misused) a figure of mine in testimony to the United States Congress. The figure showed the first predictions made with a 3-D climate model and time-dependent climate forcings -- it was a figure from a paper that we had published in the Journal of Geophysical Research in 1988 and it had been a principal basis for testimony that I gave to the United States Senate in 1988.
The figure that we published is reproduced here as Fig. 1. It shows the simulated global mean temperature for three climate forcing scenarios. Scenario A has a fast growth rate for greenhouse gases. Scenarios B and C have a moderate growth rate for greenhouse gases until year 2000, after which greenhouse gases stop increasing in Scenario C. Scenarios B and C also included occasional large volcanic eruptions, while scenario A did not. The objective was to illustrate the broad range of possibilities in the ignorance of how forcings would actually develop. The extreme scenarios (A with fast growth and no volcanos, and C with terminated growth of greenhouse gases) were meant to bracket plausible rates of change. All of the maps of simulated climate change that I showed in my 1988 testimony were for the intermediate scenario B, because it seemed the most likely of the three scenarios.
But when Pat Michaels testified to congress in 1998 and showed our 1988 predictions (Fig. 1) he erased the curves for scenarios B and C, and showed the result only for scenario A. He then argued that, since the real world temperature had not increased as fast as this model calculation, the climate model was faulty and there was no basis for concern about climate change, specifically concluding that the Kyoto Protocol was "a useless appendage to an irrelevant treaty".
Although scientists have a right to express personal opinions related to policy issues, it seems to me that we can be of more use by focusing on the science and carrying that out with rigorous objectivity. That approach seems to be essential for the success, as well as the "fun", of scientific research.
Fig. 1 is a good case in point. We now know (Hansen et al. 1998a, 1998b) that the growth rate of greenhouse gases in the period 1988-1998 has been flat, very similar to scenarios B and C (which are nearly the same until year 2000). Thus we can compare real world temperature changes in the past decade (filled circles in Fig. 1) with model calculations for the B-C scenarios. Taking account of the fact that the real world volcano occurred in 1991, rather than 1995 as assumed in the model, it is apparent that the model did a good job of predicting global temperature change. But the period of comparison is too short and the climate change too small compared to natural variability for the comparison to provide a meaningful check on the model's sensitivity to climate forcings. With data from another decade we will be able to make a much clearer evaluation of the model.
As the opinions in the global warming debate do not seem to be converging, it seems to me that one useful thing that can be done is to clearly delineate the fundamental differences. Then, as our scientific understanding advances over the next several years, we can achieve more convincing evaluations of the global warming issue. (Stated less generously, this is a way to pin down those who keep changing their arguments.)
Table 1 summarizes chief differences that I delineated for the sake of a discussion with Richard Lindzen, who has provided the intellectual underpinnings for the greenhouse skeptics, in October 1998. I also used this list (Table 1) as the principal fodder for my "affirmative closing argument" in the debate with Pat Michaels.
Differences 1 (reality of global warming) and 2 (climate sensitivity) are very fundamental. From my perspective, strong evidence is already accumulating that weighs heavily against the skeptics contentions that there is no significant global warming and that climate sensitivity is low. These issues will become even clearer over the next several years.
Difference 3 (water vapor feedback) is related to climate sensitivity, but is so fundamental that it deserves specific attention. The topic has resisted definitive empirical evaluation, because of the poor state of water vapor measurements and the fact that tropospheric temperature change has been small in the past 20 years. Ozone depletion, which affects upper tropospheric temperatures, has also complicated this problem. This situation will change if, as I would anticipate, ozone depletion flattens and global temperature continues to rise.
Difference 4 has an academic flavor, and is perhaps not worth special efforts. But it illustrates a lack of understanding of the basic greenhouse mechanism by Lindzen.
Difference 5 is fundamental because substantial efforts to curb global warming may require that climate change first be apparent to people. If our assessments are right, we are in fact on the verge of warming being noticeable to the perceptive person-in-the-street. (See related material Global Temperature Trends: 1998 summation and the Common Sense Climate Index.)
Difference 6, concerning the planetary "disequilibrium" (imbalance between incoming and outgoing radiation) is the most fundamental measure of the state of the anthropogenic greenhouse effect. The disequilibrium should exist if climate sensitivity is as high (and thus the ocean thermal response time so long) as we estimate, and if increasing greenhouse gases are the dominant climate forcing mechanism. We have presented evidence (Hansen et al. 1997) of a disequilibrium of at least 0.5 W/m2. This imbalance is the basis by which we could predict that record global temperatures would occur within a few years, that the 1990s would be warmer than the 1980s, and that the first decade of next century will be warmer than the 1990s, despite the existence of natural climate variability. I do not know of a reference where Lindzen specifically addresses planetary radiation imbalance, but his positions regarding climate sensitivity and the ocean response time clearly imply a smaller, negligible imbalance.
The important point is that the planetary radiation imbalance is measurable, via the ocean temperature, because the only place this excess energy can go is into the ocean and, probably to a less extent, into the melting of ice. If our estimates are approximately right, this heat storage should not escape detection during the next several years.
In summary, all of these issues are ones that the scientific community potentially can make progress on in the near future, if they receive appropriate attention. The real global warming debate, in the sense of traditional science, can be resolved to a large extent in a reasonable time.
Last updated: Friday, 06-Sep-2002 21:37:53 EDT
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