Official Reminder: A Hotter Millennium Is a Bad Thing

A committee of the National Research Council (NRC) weighed in today with the latest and most quantitative estimates yet of how the coming global warming could affect the world. It still isn't pretty.

The kinds of impacts the NRC committee discusses may sound familiar from the 2007 report of the Intergovernmental Panel on Climate Change. But the numbers lend a more threatening tone to the NRC report, which warns that "emissions reductions choices made today matter in determining impacts experienced not just over the next few decades, but in the coming centuries and millennia."

For example, if society managed to stabilize the atmospheric concentration of greenhouse gases at a level equivalent to 670 parts per million of carbon dioxide—shy of the doubling or tripling that unrestrained emissions might produce by century's end—the world would warm within a few decades by 2°C, the committee estimates. That kind of warming would reduce rainfall in southwest North America, the Mediterranean, and southern Africa by 10% to 20%, the committee finds. It would reduce crops of corn in the United States and Africa and wheat crops in India by 20% to 30%.

Even if the stabilization of greenhouse gases could be maintained, the report says, another 2°C of delayed warming would follow over many decades. That would double the cited impacts to devastating 20% to 40% reductions in rainfall in those areas and 40% to 60% reductions in the cited crops. And there would be no relief for a very long time.

Carbon dioxide lingers so long in the atmosphere, "it could effectively lock Earth into warming for thousands of years," says the committee's chair, atmospheric scientist Susan Solomon of the National Oceanic and Atmospheric Administration in Boulder, Colorado. On those time scales, the very existence of the Greenland and West Antarctic glaciers would be in doubt.

The closest the committee comes to a policy recommendation is to point out the magnitude of the challenge. "Emissions reductions larger than about 80%, relative to whatever peak global emissions rate may be reached, are required to approximately stabilize carbon dioxide concentrations," it observes.

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