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.

One rule of thumb: Wet areas will get wetter and dry areas will get drier. The most rapid warming is expected over the continents. In essence, climate bands move north, giving Wisconsin the kind of summers once limited to places like southern Mississippi. Warming in northern North America and north-central Asia would be largest in winter. Already, disappearing sea ice, melting permafrost, shifts in vegetation, and melting Greenland ice are signaling the changes under way in the far north.

One likely hot spot: coastal areas around the Mediterranean Sea.

"Every model projects strong drying over the Mediterranean, from Spain through the Middle East," Dr. Delworth says. The region could range on average from 4 to 8 degrees F. warmer in the summer than today, sending any remaining beachgoers on the Costa del Sol scrambling for cooler climes. "That's a very profound impact" on a geopolitically important part of the world, he adds.

Another region of concern is Africa's Sahel, which has seen a series of severe droughts over the past 30 years. Yet Delworth acknowledges that the models fail to agree on whether the drying trend of the past 30 years will continue.

For the US, global warming will squeeze more moisture out of the already dry Southwest. "But the consensus among models is not as high," he explains. One reason: Models are still having a hard time capturing the wind patterns that bring seasonal monsoons to the region.

Mountains in the US West will still get precipitation in winter, but it's more likely to be rain than snow. Throughout the country, when it rains, it will pour, as extreme-weather events become more common – raising the likelihood of floods and giving fits to Western water managers.

In one study published last year, researchers from the US and Australia compared projections from several models and found that climate extremes – ranging from more frequent and intense heat waves and fewer frost days to longer dry spells and heavier rainfall – appear around the globe, although consensus among the models begins to evaporate when they tried to look at regional patterns.

One broad area that may receive more scrutiny: the portions of Kansas, Nebraska, and Colorado that host vast expanses of sand dunes. A recent study by researchers at the University of Nebraska and the University of Wisconsin notes that many of these dune systems are on the knife's edge of mobilization, and could begin to wander across the landscape if moisture becomes much more scarce.

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