New view on global warming

The science of global warming changed course last October, even if the politics of global warming did not.

"Our most fateful new challenge is the threat of global warming," declared U.S. President Bill Clinton last month in his state-of-the-union address to Congress, holding climate-change politics stubbornly on an errant heading. "Nineteen ninety-eight was the warmest year ever recorded. Last year's heat waves, floods, and storms are but a hint of what future generations may endure if we do not act now."

There's no sign of change here. These are the words of a national leader certain enough about the existence of a global-warming problem to want to raise taxes and spend public money to solve it. They are certain words anchored in a simple view: that greenhouse gases, especially carbon dioxide emitted by human activity, are building up in the atmosphere, trapping solar heat, and raising average temperatures with no compensation elsewhere in the system. So governments have to do something, namely raise taxes and spend money.

Holy certainty

It is a good thing for science that not everyone interested in the matter acts so willing to spend the wealth of others on their holy certainty. And it is a good thing for the politics of global warming that not everyone clings to the simple view when complexity emerges.

In October, a pioneer alarmist in the field of global warming changed his message in an important way. A decade earlier, James Hansen of the National Aeronautical and Space Administration's Goddard Institute for Space Studies created global neurosis when he told a Senate committee that computer models showed human intensification of the greenhouse effect was changing the climate. Global warming thus leapt to the head of environmentalists' lists of reasons to make people cut their use of fossil energy to improve their lives and livelihoods.

Unlike many participants in the global-warming debate, however, Hansen didn't quit thinking once a simple explanation was at hand. He apparently kept studying the climate. And like most scientists who have pressed the search for better understanding of the subject, he found problems with the simple view.

Hansen was lead author of an October article in the Proceedings of the National Academy of Science addressing climatological influences, or "forcings." An abstract of the article begins with this remarkable observation: "The forcings that drive long-term climate change are not known with an accuracy sufficient to define future climate change." The world is perhaps not warming, after all.

Hansen and his coauthors assert that greenhouse gases caused by human activity are well-measured and cause warming. Other, less well-measured forcings caused by people exert opposing influences that tend to offset greenhouse warming. They include changes of atmospheric aerosols, clouds, and land-use patterns.

As a result, the natural forcing of changes in solar irradiance might be more important to long-term climate change than what observers infer when they look at greenhouse gases of human origin alone. And the greenhouse gas contribution to warming is less than popularly assumed, Hansen and the other authors conclude.

'Paradigm change'

"The summary implication," they say, "is a paradigm change for long-term climate projections: Uncertainties in climate forcings have supplanted global climate sensitivity as the predominant issue."

Hansen's candor and accommodation of his highly public position to climatological complexity are indeed refreshing. They stand in telling contrast to the simplistic certainty on this difficult subject expressed by politicians looking for new claims to other people's money. And it should serve as useful guidance to industry leaders inclined to default to the simplest available explanation on global warming in deference to the popular mood.