Heat exchange: the global warming debate mixes daunting complexity with high political stakes, a toxic brew that continues to test dispassionate science

Natural History, April, 2004 by Robert Ehrlich

The Discovery of Global Warming by Spencer R. Weart Harvard University Press, 2003; $24.95

The complex mix of science and politics bundled together under the label "global warming" usually prompts one of two generic responses in books for the general public. One is a call to action, as in Al Gore's Earth in the Balance. The second is a call to inaction, as in the wittily titled The Satanic Gases, by Patrick J. Michaels and Robert C. Balling Jr. In The Discovery of Global Warming, Spencer R. Weart brings a welcome third perspective to the subject: the historical point of view.

Weart, as it happens, is both a physicist and a historian of science, and his diverse activities make him well qualified for the task he sets out to do (and accomplishes!) in his book. His aim is to describe the many converging strands of science that led to the "discovery" of global warming--by which he means the emerging scientific consensus that people are discernibly (and probably quite significantly) affecting Earth's climate through the greenhouse effect.

As Weart makes clear, the "discovery" announced in his title is quite different from the usual discoveries in physics. The latter can often be traced to a particular moment in time and to the work of a particular person or group. But the consensus among scientists about the effects of global warming developed gradually, Weart tells us. And by "connecting the dots among roughly a thousand of the most important papers in the science of climate change," he shows how that happened. (The Web site that supplements Weart's published volume, hosted by the American Institute of Physics at www.aip.org/history/climate/, includes roughly three times more material than the book does, but it is both clearly organized and surprisingly easy to navigate.) In some places the book does indeed read almost like an elaborate mystery story, with all the attendant false clues and twists of plot.

Some of the plot thickeners are quite instructive. Take for instance the view that emerged during the 1960s and early 1970s, according to which the planet was probably facing a new ice age. That view was embraced by some of the same investigators who today are quite certain the Earth is headed for some serious warming. To be fair, however, that shift is by no means just a reversal of opinion. Rather, it represents a refinement of a still-evolving climate theory--an outcome that should come as no surprise to anyone familiar with the way science really works. In fact, the current understanding among most climate scientists is that both views (warming and cooling) are right. We are facing both near-term global warming, because of a buildup of greenhouse gases, and an eventual serious cooling (another ice age), probably as a consequence of small shifts in the Earth's axis of rotation. Weart's approach, then, gives just the kind of treatment that places the history of ideas in the context they demand.

Weart is probably correct in asserting that the consensus today is more reliable and more robust than the ice-age predictions of thirty years ago. The belief in a human-induced warming trend is more widely held within the scientific community than the belief in a coming ice age ever was. The evidence for an anthropogenic cause of recent warming also rests on firmer foundations, given that climate-modeling tools are far more sophisticated now than they were in the 1970s. Still, science is not an enterprise in which "majority rules." And Weart reports that though nearly all the computer models now agree in "predicting" the present climate--that is, taking into account conditions and trends in the past, their computational results are in accord with today's climate--the critics of global warming point out that the models have been "laboriously 'tuned' to match [current climate] by adjusting a variety of arbitrary parameters."

None of these caveats imply that the climate modelers are wrong; science generally makes gradual progress toward the truth. But the caveats ought to remind people that when climate models are projected into the future, their predictions should be viewed with some degree of caution, particularly when it comes to the assumptions built into the models. In most places Weart is appropriately cautious in his assertions, reminding his readers that

scientists rarely label a proposed answer to a scientific question "true" or "false," but rather consider how likely it is to be true. Normally a new body of data will shift opinion only in part, making the idea seem a bit more likely or less likely.

Weart also is forthcoming about the many uncertainties about global warming that remain, despite the present consensus.

Weart's decision to write from the historical perspective--understandable as that is for a historian of science--does present some structural problems for the reader. For one thing, information on specific topics is spread throughout the book. Readers who want to know about the role of aerosols in the global climate, for instance, have to read about the subject in dribs and drabs. They won't find out whether, on balance, aerosols tend to warm or to cool the planet until they are three-quarters of the way through the book. That's a more serious shortcoming than it may at first appear, because aerosols are one example of what global-warming skeptics call a "fudge factor": one whose effects can be arbitrarily adjusted in the computer models.