Hot Stuff.

Mr. Green, director of the environmental program at the Reason Public Policy Institute in Los Angeles, is author of A Plain English Guide to the Science of Climate Change.

Greenhouse: The 200-Year Story of Global Warming, by Gale E. Christianson (Walker, 305 pp., $25)

'Hot enough for you?" So went the panting title of a recent New York Times column on global warming. The hot summer of 1999, following the hot summers of 1998 and 1997, has got lots of folks talking about how hot it is, as well as about what certain other folks must have done to bring on this heat. Of course, 1998's high temperatures were caused primarily by El Nino. As for the present scorching summer, even a heat wave lasting ten years wouldn't mean a whole lot in the context of a climate record of 4 billion years. But that doesn't stop doomsayers like Vice President Gore from intimating that recent heat waves are harbingers of still more intense heat waves to come, the heating trend in general being the result of human activities.

Some conservatives would have us ignore it, but the truth is that the home planet does indeed seem to be warming up a bit. Over the past 100 years, the global average temperature has risen by about one degree centigrade, although this warming has not been uniform in chronology or distribution. More of the warming has been over land than over water, more at night than during the day, and more in winter than in summer. But why is it getting warmer? That is the question of the hour.

This book, by science historian Gale Christianson, offers an answer by reviewing the history of industrialization and fuel use over the past two centuries. The book is well written and rich in detail, full of fascinating stories about figures from science, technology, and industry. Christianson's exposition of the lives and labors of such great scientists as Fourier, Arrhenius, Darwin, and Lyell are particularly enthralling.

But Greenhouse, like other populist writings on climate change, works backwards from an assumption that human activities must have been the principal cause of recent warming. Like many others, Christianson relies on a simple correlation of industrial development with climate change. Such a correlation says nothing about causation, but Christianson brushes over this fact-a big mistake: A simple (and imperfect) cor- relation of industrialization and industrial pollution with still-rough estimates of climate change omits so many of the complexities of climate as to render the observation virtually meaningless.

What's omitted? Well, the sun, for one thing, and clouds, for another. Though Christianson barely mentions it, fluctuations in solar output may be responsible for 40 percent of the net warming that scientists have estimated since 1850. Meanwhile, changes in the quantity and reflectivity of the earth's cloud cover may have offset half or more of whatever warming effect humanity's release of greenhouse gases may have caused. Aerosols released by volcanoes may have canceled out another 18 percent of the estimated human-warming im- pact. In other words, much of the warming that mankind might have produced was probably offset by these independent factors. So the question remains: Why does it seem to be getting hotter around here?

James Hansen, lauded by Christian-son as a heroic pioneer in the study of climate change, acknowledges in his 1998 article "Climate Forcings in the Industrial Era" that "the forcings that drive long-term climate change are not known with an accuracy sufficient to define future climate change." "Forcings" are the various factors that contribute to heating or cooling, including solar fluctuations, greenhouse gases, changes in cloud reflectivity, and so on. In fact, the most recent of Hansen's findings neatly refute the scary, guilt-inducing prophecies of Al Gore et al. Human effects on the climate have often been considered as though they all led in one-needless to say, harmful-direction: as though every single one edged the temperature upward. Yet common sense suggests that any human actions that might affect the climate-being undirected by any master plan or conspiracy-would be as likely to cancel each other out as they would to add up to mischief. Indeed, as Hansen acknowledged last year, 78 percent of the warming potential of the "well-mixed greenhouse gases" released by industry since 1850 have been offset by cooling potentials from tropospheric aerosols, human- produced cloud changes, vegetation and land-use changes, and changes in stratospheric ozone concentration. Curiously, Christianson ignores Hansen's recent work. Instead he focuses on the scientist's earlier congressional testimony, even giving us a photograph of Hansen to gaze upon.

Of course, environmentalists rightly point out that many of mankind's activities that have had a cooling effect have been forms of pollution. Several of these have been sharply reduced by laws against sulfur pollution and ozone-depleting chlorofluorocarbons. Human activity is thus now actually being regulated so as to produce a net warming effect. Still, in order to cool the earth, few would argue that we should go back to producing sulfuric-acid clouds. (Admittedly, in an environmental context, there is a depressing tendency among conservatives to favor anything that liberals oppose: Climate change? Bring it on! Acid rain? It's good for the soil!) On the other hand, some might well suggest that if human beings can unintentionally offset global warming, perhaps they could do so intentionally as well-through changes in forestry, agricultural practices, and so on. Hence the demand of environmentalists to do something, virtually anything, and right away, to halt the warming trend.

A more sober approach seeks to delay government regulatory action until we fully understand what's causing global warming. Is this just a waste of precious time? "Better safe than sorry!" is the oft-sung refrain of environmentalists. But "Look before you leap" may be a wiser guiding principle in a world beset by many risks, where every regulation may potentially have irreversible unintended consequences.

The question of causation does indeed matter because one thing is certain: Global warming has many causes. Understanding the relative importance of each of them will allow us to prioritize our responses. It will also let us make the best use of our resources in addressing the problem-whether we ultimately do so by acting preemptively, heading off global warming at the pass, or by adapting after the fact to the undesirable effects of climate change. What response we choose is crucial for an additional reason about which Christianson's book is silent-namely the benefits of living in an industrialized society. Though you'd never know it from this book, developed countries have the longest life expectancies, the highest standard of living, and the greatest resources available to preserve and protect their environments.

So yes, the earth seems to be warming, and, yes, the history of climate change makes for a fascinating story. But a selective account of "what happened"-of the climate changing while society industrialized -tells us very little about why it all happened, and what to do now that it has. Historians like Gale Christianson like to remind us that those who forget the lessons of the past may be doomed to repeat its worst aspects. What they don't often talk about is the danger of learning spurious lessons from a history imperfectly related.