by Dr. Kerry Emanuel and Dr. Susan Solomon, (Professors, Massachusetts Institute of Technology, Cambridge, MA), The Union of Concerned Scientists, October 16, 2014
Stephen Koonin’s recent Wall Street Journal op‐ed illustrates the importance of distinguishing scientific fundamentals from numerical details, and keeping the distinction between science and values clear in discussions of risk.
Koonin notes several key scientific fundamentals. He does not deny that climate is changing, that human activities are at least partly responsible for it, or that policy formulation should take climate change into account. But the headline statement—that not enough is known about climate to warrant significant action given the risks—is a statement of values and does not follow from the scientific substance of the essay. The phrase in the essay’s title, “Settled Science,” repeated elsewhere, is often used by journalists and environmentalists, but we have yet to hear it used by an actual climate scientist. Although the first sentence refers to its use in “popular and policy discussions,” later in the essay Koonin argues that uncertainty “should not be confined to hushed sidebar conversations at academic conferences.”
This claim amazed us, because uncertainty is a major and perhaps the major focus of scientific research on climate change and on attempts to formulate sensible policy. In the Summary for Policymakers of the latest IPCC Assessment Report, the word uncertain or uncertainty appears 36 times, and graphs pertaining to climate projections have hefty uncertainty limits and error bars. It is hard to claim that uncertainty is being hushed up. “Settled Science” is a red herring when it comes to the actual practice and publication of climate science.
We are used to hearing that “climate is always changing” as a means of downplaying current climate change; rather like a murder defendant telling the judge that “people are always dying.” Climate changes on many time scales, and if that were not the case, there would be little basis for worrying about our own influence, because the evidence would point to a large intrinsic stability of the climate system. There is no significant doubt that the earth’s past climate has responded dramatically to relatively small changes in Earth’s energy balance in the past, such as changes in sunlight and its distribution with latitude, as well as to changes in atmospheric composition. This is hardly comforting. Koonin gets it wrong when he states that the impact of human activity appears comparable to natural intrinsic variability; in point of fact, many studies using independent data sets have demonstrated that human signals have clearly emerged from the noise on global and regional scales over the past fifty years. What is at issue here is not just the total global temperature change, but the rate at which we are changing the energy balance of our planet, which over the last half century has been far in excess of anything evident in paleo climate data for many thousands of years, at least.
Elsewhere, Koonin falls prey to some of the most common misconceptions about climate science. One of the most widely held is the idea that predictions of climate change rest solely on highly complex computer models. This is far from the case; basic physics and very simple models all show that increasing greenhouse gas concentrations lead to nontrivial warming. In 1906, the Swedish chemist Svante Arrhenius estimated that doubling CO2concentrations would increase average global surface temperature by around 4 oC; his calculations were done with paper and pencil. If the computer had never been developed, climate science would still have identified the substantial risk incurred by changing by hundreds of percent the concentrations of long-lived greenhouse gases. That is what we are on track to do within this century if current rates of growth of human greenhouse gas emissions continue.
Koonin states that the human impact on the greenhouse effect is a small percentage of the total greenhouse effect. While strictly correct, Koonin’s use of this information in his essay is deeply misleading. While the most important greenhouse gas in the atmosphere is water vapor, the residence time of a water molecule in the atmosphere is about two weeks. Global water vapor concentration therefore responds to changes in global temperature rather than forcing them. The water vapor greenhouse effect is a fast feedback driven by other changes in the system, as is universally recognized (going back at least to Arrhenius), and as Koonin acknowledges later in his essay. By contrast, a portion of the human addition of CO2 to our atmosphere lasts hundreds to thousands of years; thus on such time scales it is properly considered a forcing. Despite a nominally small contribution to the total planetary greenhouse effect, it is well established that eliminating all atmospheric CO2 would drop the Earth’s mean surface temperature to below freezing, demonstrating its key role; doubling its concentration can similarly be expected to influence our climate.
The hiatus in the upward trend of surface temperature is a fascinating and important puzzle for climate science, and is not entirely consistent with our present understanding of natural variability. Uncertainties in forcing by changes in atmospheric pollution and volcanic particles may also play an important role. Whole workshops are devoted to this issue. In dealing with it, one must be conscious of the fact that roughly 90% of the planetary energy imbalance caused by upward trends in long-lived greenhouse gases is used to heat the oceans, rather than the atmosphere, and the best available measurements show that ocean heat content has continued its inexorable climb even while the atmospheric temperature has leveled off. It does not take much heat exchange between the oceans and atmosphere to cause large swings in the temperature of the latter. We do not fully understand the natural causes of such exchanges.
Finally we come to what we regard as the most egregious part of Koonin’s essay; namely, his implication that scientific uncertainties dictate that we do nothing. Outside the radical environmental movement and lesser media outlets, no one is claiming that uncertainty in climate projections is small. In spite of their flaws, projections based partially on the ensemble of complex climate models run by many groups internationally is well grounded in basic physics going back to the time of Arrhenius, and represent civilization’s current best shot at the problem; anything else is mere conjecture. These projections portray time-evolving probability distributions of climate variables such as precipitation and global mean surface temperature, which experts in risk analysis convert to probability distributions of various kinds of risk. These risk analyses are taken very seriously by business enterprises (e.g., the reinsurance industry) and many government organizations (e.g., the Department of Defense; the city of New York) and are used constructively in planning.
While it is challenging, many citizens are beginning to understand how to think about risk and uncertainties in climate change too. The median CO2-doubling temperature increase of about 2.5 oC already poses serious risks, while the not-so-improbable higher tail of the distribution would likely be very serious, even catastrophic, for civilization. And if nothing is done to curb emissions, we are on our way to tripling CO2 by the end of this century, and quadrupling it not long after. Even low-end estimates of the climate response to such forcing entail very serious risk. Barring major improvements in technology for extracting CO2 from the atmosphere and sequestering it, decisions on whether and how much to reduce emissions cannot be postponed owing to the very long residence time of anthropogenic CO2 in the atmosphere.
Koonin’s point that decisions on how to deal with this important problem should be made democratically seems obvious to us, but the events of the last decade or so show that the main threat to democratic decision making is not overweening climate scientists (yes, there are some!) but rather wealthy interests vested in the status quo, that seem to exercise undue influence in today’s politics. The rising oligarchy naturally tries, as a means of diversion, to alarm the public with the specter of a technocracy.
Reasonable people may disagree about the best course of action given an uncertain but potentially very serious threat. It should be up to citizens to decide how much to spend now to mitigate such a threat. But ignoring or slanting the risks does a disservice to science and the public.
About the authors:
Dr. Kerry Emanuel is the Cecil and Ida Green professor of atmospheric science at the Massachusetts Institute of Technology, where he has been on the faculty since 1981, after spending three years on the faculty of UCLA. He is a co-director of MIT’s Lorenz Center, a climate think tank devoted to basic, curiosity-driven climate research.
Dr. Susan Solomon is the Ellen Swallow Richards Professor of atmospheric chemistry and climate science at the Massachusetts Institute of Technology and the Founding Director of the MIT Environmental Solutions Initiative. Prior to that, she was a scientist at NOAA in Boulder, Colorado, from 1981-2011, and an adjunct professor at the University of Colorado.
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