Is there a global solution to global warming?

Friday's release of a much-anticipated report on global warming from the UN's Intergovernmental Panel on Climate Change (IPCC) in effect asks a profound question of humanity: What do you want your global and local environment to look like over the next several centuries – and probably longer?

The report states in unequivocal terms that the climate has warmed globally and that human activity – the burning of fossil fuels and, to a lesser extent, land-use changes – is very likely to be driving the change. Indeed, increased emissions have become the single largest net force for change that the current climate faces.

But besides detailing the attendant effects of warming – including sea-level rise, vanishing alpine glaciers, and increases in severe-weather events – the report hints at the need for conscious control over the environment – and a unity of purpose – that humans have yet to achieve on such an enormous scale.

"When people think about climate impacts, they think about something very narrow – what icky things are going to happen where I live?" says Jerry Mahlman, a senior research associate at the National Center for Atmospheric Research in Boulder, Colo. "They don't say: What's going to happen to the poor Bangladeshi farmers who get hit with a triple whammy" – rising sea levels, more intense tropical cyclones, and reduced supplies of fresh water. "Everyone wants to talk about their particular piece of turf. But this is a problem that is intrinsically and fundamentally global."

The report – the first of four volumes the IPCC will release this year – focuses on the science of climate change. The document the IPCC unveiled Friday is the short form of this science report for policymakers.

Among its conclusions – based on various assumptions of population and economic growth, as well as different rates of technological innovation aimed at reducing emissions:

• Temperatures will rise 2 degrees to 4.5 degrees Celsius by the end of the century, if CO2 concentrations reach twice their preindustrial level and then stabilize. Within that range, the most likely result is 3 degrees C (5.4 degrees Fahrenheit). But that additional warmth will distribute itself unevenly, with the highest increases in the Arctic and progressively smaller increases father south.

• Sea levels will rise by century's end from 28 to 58 centimeters (11 to 23 inches) on a global average. That's a narrower range from the 2001 IPCC study range of of 9 to 88 centimeters. But research published after the deadline for inclusion in this report suggests that the Greenland ice sheet is losing mass faster than expected, leaving open the possibility that sea-level increases will be higher if the melting trend continues to accelerate. If the mass loss continues over the next 1,000 years, the entire Greenland Ice cap would vanish, raising sea levels by seven meters (23 feet).

• It is very likely that precipitation will rise at high latitudes and decrease in most subtropical land areas. Incidents of extreme drought and heat waves are also very likely to multiply.

In the end, because carbon dioxide remains in the atmosphere for hundreds of years, the report holds that between past and future emissions, the climate will warm and sea level will rise for more than 1,000 years.

Policymakers face several challenges in trying to deal with the issue, beyond its longer-than-an-election-cycle time frame. The first is its sheer magnitude. Many climate scientists agree that emissions already are on track to double by century's end, perhaps sooner. Under a "business as usual" approach, which in part assumes today's rate of innovation, the world would have to slash emissions by 70 to 80 percent this century to reach an often talked about CO2 concentration target of 450 parts per million by 2100. Over the long term, stabilizing CO2 concentrations in the atmosphere means reducing emissions close to zero.

While current legislative efforts at the state and local level represent a needed start, current efforts merely "sandpaper the edge off of a catastrophe," Dr. Mahlman says.

A critical step forward would be to agree on a clear target for stabilizing CO2 concentrations, notes Gerald Stokes, a former US Department of Energy scientist and now a vice president with Battelle Laboratories. Merely setting emissions targets isn't enough, he says. With a firm target that means something from the atmosphere's perspective, policymakers can determine the technological path countries will need to follow, as well as put a firm value on carbon-dioxide so the economics of mitigation, and of delay, can be more accurately assessed.

The conclusions the reports draws are based on the latest five years of peer-reviewed studies, which range from field measurements of various processes to climate-change simulations run on powerful computers. Both activities have improved substantially since the last set of reports was published in 2001. Some 75 percent of the scientists taking part did not participate in the 2001 report. And roughly a third of the scientists are relative newcomers, with PhDs that are 10 years old or less.

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