A clearer global climate forecast

By 2100, retired snowbirds will be joined by "sun birds" – who flee north to escape oppressively hot, humid summers not just in Miami, but Milwaukee as well. In the US West, deep mountain snows – currently a key natural reservoir for fresh water – will virtually vanish. And while the growing season will expand by about a month, urban gardeners will spend more time indoors as higher temperatures help boost smog at ground level.

Welcome to a world where the climate is, on average, 3 degrees Celsius (5.4 degrees Fahrenheit) warmer than today.

That projection – more specific than any previous one – is just one element expected to emerge this week as some 500 scientists from around the world gather to put the finishing touches on a major report on the Earth's climate and what the future may hold for it as humans continue to pump heat-trapping greenhouse gases into the atmosphere.

It's the first of three volumes set for release this year by the UN-sponsored Intergovernmental Panel on Climate Change (IPCC). Details in the document, which focuses on climate change, remain closely held until its release Friday morning. Leaks to the press based on earlier drafts, however, suggest that the researchers are projecting temperature increases of between 2 and 4.5 degrees C (3.6 and 8.1 degrees F.) by century's end if carbon-dioxide concentrations in the atmosphere reach twice their preindustrial levels. Their "most likely" increase is expected to be about 3 degrees C.

"Three degrees is very significant warming," acknowledges Thomas Delworth, a researcher at the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory in Princeton, N.J.

Of course, projecting future climate is a dicey proposition. High-powered computers are loaded with mind-numbing programs whose math represents a range of key processes in the oceans, atmosphere, and land. Scientists enter a few key numbers at start-up, such as the sun's radiation level and levels of greenhouse gases at a beginning time, then press "enter."

Still, "we're not completely sure of a lot of the physics, and it's hard to build a model for something you don't understand," says William Collins, a modeler at the National Center for Atmospheric Research in Boulder, Colo. "We don't know the trajectory for man-made greenhouse gases over the next century."

To finesse that issue, the IPCC has developed a range of emissions profiles, based on different assumptions about population and economic growth and the pace of adoption of new technologies to reduce greenhouse-gas emissions. The emissions profile that yields the 3-degree C warming "is fairly optimistic," Dr. Collins says. It assumes rapid economic growth, a rapid influx of new, more efficient technologies, and a world population that peaks mid century, then starts to decline. Based on past and current emissions, many climate scientists say that a doubling of atmospheric CO2 by century's end is a done deal.

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