Cloudy forecast; Climate debate must avoid extremes.

COLUMN: IN OUR OPINION

When it comes to global warming - or, to use its latest public persona, climate change - about the only thing the true believers and the skeptics agree upon is that the Earth has warmed appreciably over the last century. Exactly how much it has warmed and where, and what the implications may be for agriculture, coastal cities and the planet's future, remains a matter of sharp and serious debate.

Non-scientists and taxpayers - that is to say, the majority of us - are the ones who will bear the brunt of the burdens, both physical and fiscal, that could arise from major policy actions or inactions. Thus, it is important that the public recognize the dangers of extremism on both sides of the climate debate. To deny that human industrial activity has any impact whatsoever on the planet's climate seems to us a foolish and long-discredited position. At the same time, predictions of rapidly rising sea levels, runaway concentrations of carbon dioxide, widespread crop failures and ever-stronger killer hurricanes seem to us the work of alarmists, not prophets.

A careful examination of the climate record and current trends offers plenty of evidence that should evoke concern and lead to reasonable steps to reduce emissions of greenhouse gases.

However, the data do not yet conclusively point to doomsday scenarios that would justify the most radical cures on offer.

For example, observed rates of global temperature increases from 1975 to 2007 project a 2 degree Celsius increase in global temperatures by 2100. While significant, that projection is below nearly every computer model being employed by the United Nations' Intergovernmental Panel on Climate Change (IPCC), which has become the main organ for those who accept the notion of dangerous anthropogenic global warming.

The Obama administration has been widely lauded in the scientific community for its renewed commitment to science, rather than ideology, as a driver of public policy. But scientists are not immune from extremism themselves, and recent news that the administration's science adviser, John P. Holdren, is mulling plans for possible geo-engineering - such as seeding the clouds with pollution particles in order to cool the Earth - should give thoughtful parties on either side of the debate pause.

In theory, such an approach could work, just as major volcanic eruptions throughout history have acted to cool the planet. The June 1991 eruption of Mount Pinatubo in the Philippines, for example, blocked some 10 percent of the sun's light from reaching Earth, resulting in a cooling of about 1 degree Fahrenheit throughout the Northern Hemisphere. It remains far from certain, however, that such radical intervention by humans would achieve its desired effect without triggering other, unanticipated consequences.

To his credit, Mr. Holdren acknowledged that geo-engineering is an idea to be approached with caution. But the administration's full-court press for carbon taxes, alternative-energy mandates and cap-and-trade auctions for pollution credits may prove to be a massive and costly overreaction to still-uncertain science.

We urge Mr. Holdren, the administration, and all Americans, to remember that even our recent history is littered with predictions of disaster - the population bomb, mass starvation, the disappearance of oil, and an impending ice age - that have not come to pass. The conventional wisdom on global warming - which these days seems often to be a matter of shared alarm - may yet prove to be conventional folly.