Hot enough for you? The state of the global-warming debate, and politicking.

IT has been four years since President Bush declared the Kyoto Protocol a dead letter, but the campaign to impose industrial greenhouse-gas emission controls on the American economy shows no signs of letting up. Although Sen. John McCain's bid last month to include concrete emission controls in the pending energy bill attracted only 38 votes, the Senate subsequently passed a resolution calling on Congress to "enact a comprehensive and effective national program of mandatory, market-based limits on emissions of greenhouse gases that slow, stop and reverse the growth of such emissions." An attempt to table the resolution was opposed by Republican senators Lamar Alexander, Lincoln Chafee, Susan Collins, Mike DeWine, Pete Domenici, Lindsey Graham, Judd Gregg, Richard Lugar, John McCain, Olympia Snowe, Arlen Specter, and John Warner. Apparently, even red-state Republicans are having doubts about Bush's position on climate change.

One might think that the increased political buzz around global warming is driven by science. One would be wrong. The scientific case for alarm is no more compelling today than it was yesterday.

The first (and sometimes last) stop in the global-warming debate is the question, Is it real? The answer seems to be yes. Ground-based and oceanic temperature records show warming of about three-quarters of a degree Celsius in the last century. About half of that warming, however, occurred before World War II and is widely thought to be related to solar activity. Satellite and weather-balloon records, which do not go back as far, show less warming in the late 20th century than the land-based stations.

What's causing this warming? We don't know. As the vice president of the U.N.'s Intergovernmental Panel on Climate Change (IPCC), Yury Izrael, wrote bluntly last month, "There is no proven link between human activity and global warming." Given the extreme variability of global temperature, warming might simply be statistical noise. It might result from solar and/or volcanic activity. It might be caused by industrial emissions. And it might come from some combination of the three.

What do most scientists suspect is going on? The best way to ascertain the "scientific consensus" is to look at the latest report of the IPCC (released in 2001), which purports to summarize the state of scientific knowledge on global warming. Here's what it says: "Most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations." The report finds that it is "unlikely (bordering on very unlikely) to be entirely the result of internal variability," and that "natural forcing alone [i.e., solar and/or volcanic activity] is unlikely to explain the increased rate of global warming since the middle of the 20th century."

The promiscuous use of such vague terms as "likely" and "unlikely" by scientists who are trained in precision speaks volumes about how much is unknown. At the very least, such language makes it impossible to accept the Greens' claim that "the debate is over," particularly given all the uncertainty--fully discussed in the IPCC report--regarding long-term climate records and important data on atmospheric feedbacks. In fact, uncertainty about future climate conditions is greater in the 2001 IPCC report than it was in the 1995 IPCC report.

Do other reviews of the scientific literature tell a different story? It depends on whom you ask. An article by Naomi Oreskes in Science last December examined 1,000 scientific papers published since the early 1990s. Oreskes concluded that 75 percent of those papers either directly or implicitly supported the argument that industrial emissions are driving global warming, and none directly argued to the contrary. Asubsequent review of the same articles by Benny Peiser, a senior lecturer on the science faculty at Liverpool John Moores University, found nothing of the kind. Peiser concluded that only one-third of the papers reviewed by Oreskes actually supported the "consensus view," and only 1 percent did so explicitly.

In any case, debating what constitutes the mainstream thinking on climate change is not particularly enlightening. Regardless of how one defines "the consensus," scientific truth is not revealed by a show of hands. As Thomas Kuhn demonstrated in The Structure of Scientific Revolutions, the history of scientific progress is a history of once-solid consensuses being overthrown by minority skeptics. In short, today's consensus proves nothing.

Even more heated than the debate about the cause of climate change is the debate about its likely effects. In fact, most of the so-called skeptics who publish in peer-reviewed literature accept the contention that mankind is probably responsible for most present-day warming. They argue, however, that the warming has been and will continue to be quite modest, and that the pattern of warming we're seeing does not suggest that a parade of horribles awaits us.

The skeptics are on solid ground here because the atmosphere simply has not proven to be as sensitive to industrial greenhouse-gas emissions as some theorists have feared. Unless some temporary phenomenon is masking the effect of such emissions, atmospheric physics suggests that warming will occur at a linear rate--a conclusion affirmed by almost all the computer climate models in existence. This insight suggests a simple exercise: Plot temperature data over the last 50 years and draw a trend line to see what the future has in store. Doing so suggests that warming will likely be at the low end of the IPCC's projections--about 1.5 degrees Celsius by the year 2100.

Should we worry about such modest warming? From an ecological perspective, probably not. Because water vapor is responsible for 94 percent of the natural greenhouse effect, industrial greenhouse gases have a greater impact in dry air masses than in wet ones. Fully 78 percent of the warming has been concentrated in the driest air masses, which are primarily found during the winter (when 69 percent of the warming has occurred), at night, and in the northern latitudes.

The fact that winter nighttime lows in the Northern Hemisphere aren't quite as cold as they used to be need not cause anyone to panic--and there seems not to be an increased incidence of the destructive weather events that would follow from warming in wet air masses. According to the IPCC, "[T]here is little sign of long-term changes in tropical storm intensity and frequency," and "no compelling evidence" that local severe-weather events are on the rise. Most important, "no significant acceleration in the rate of sea level rise during the 20th century has been detected." Precipitation in the northern hemisphere has likely increased by a meager 0.5-1 percent a decade, but "no comparable systematic changes in precipitation have been detected in broad latitudinal averages over the Southern Hemisphere."

There are good reasons to think that a warmer world might be a better world. Agronomists, for instance, are fairly convinced that heavier concentrations of carbon dioxide in the atmosphere, as well as the longer and somewhat wetter growing seasons that follow from the greenhouse warming pattern, have already increased crop yields and will continue to do so. Warmer weather also leads to declines in energy use, and probably fewer weather-related deaths.

Not surprisingly, economists who have examined the implications of the warming projections offered by the IPCC have had a hard time proving the existence of net negative effects. In fact, Yale forestry economist Robert Mendelsohn has demonstrated that nations north of the equator will probably benefit from global warming, and that warming will likely prove an economic wash for the world as a whole.

Both sides in the global-warming debate contend that "sound science" should dictate public policy. For the foreseeable future, though, it's unlikely that scientists will be able absolutely to prove or disprove the proposition that industrial greenhouse-gas emissions are ushering in a dangerous warming trend. Even if scientists could prove this, they have no particular expertise at choosing among competing policy responses. Nor are scientists' levels of risk aversion, or their choices about how to hedge against risk, necessarily superior to those of anyone else.

Scientists cannot tell us how best to handle the threat posed by global warming, no matter how much we, or they, wish otherwise. They can help inform the debate--but they have less to contribute than most people think.

Mr. Taylor is director of natural-resource studies at the Cato Institute.