A parking lot effect?

Dr. Pielke and others argue that land-use changes in a region may have significant effects thousands of miles away - not unlike the El Niño effect in which warming zones of the Pacific Ocean force droughts and weather changes worldwide.

That's still theory, of course. Skeptics point out that only 29 percent of the earth's surface is land - and only 1 to 2 percent is urbanized. Another 40 percent of land has been modified by agriculture and deforestation, Pielke says. So can the regional land-use tail really wag the global dog?

Not according to Alan Robock, an editor of the Journal of Geophysical Research: Atmospheres. He's also seen no uptick in studies of land-use impact on climate in scientific literature he's read.

"Everybody realizes modifying land surface is important locally," says Dr. Robock. "If you're talking globally, though, CO2 is the dominant way humans cause climate change."

Still, the view that human changes to the landscape are a factor driving climate, too, is gaining some traction in powerful corners of the scientific community. A report by the National Academy of Sciences due later this year will examine the warming effects of non-CO2 agents: aerosols, solar variability, and land-use changes.

"The public does not hear too much about this, because all the climate-change treaties have been focused on CO2," says Daniel Jacob, a Harvard University professor of atmospheric chemistry who chairs the panel writing the report. "For a long time it's been really hard to communicate these other factors to the policymakers, mainly because it's difficult to find the proper currency for them."

The impact of such change would begin first with global climate modelers, like Robert Dickinson, president of the American Geophysical Union. Dr. Dickinson is working to include more detailed effects from land-surface changes, aerosol, and soot in his climate model. He says one of his graduate students is pursuing a surface-temperature study of China. But like many, he still maintains that CO2 is the dominant force in climate change.

Kalnay's research is providing ammunition for some private groups to argue that global warming is a myth. In an editorial last June, the Center for the Study of Carbon Dioxide and Global Change argued that Kalnay's work showed that the impact of CO2 was overstated. "The warming of the past century or so was nothing more nor less than the natural recovery of the earth from the global chill of the Little Ice Age," the Tempe, Ariz., nonprofit reported.

Such conclusions irk Peter Frumhoff, global environment program director for the Union of Concerned Scientists in Cambridge, Mass. "Just because new research shows there are other factors to pay attention to [beside CO2, that] doesn't mean there's any less reason to pay attention to greenhouse gases," he says.

Kalnay is undeterred. Having completed her temperature study of the US, she is working on a global analysis of 50 years of temperature data. Already, early results from South America support her conclusions. "Greenhouse gases are undoubtedly very important," she says. "But the second cause for climate change is the way we are using the land surface."

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