Greenhouse gas levels currently expected by mid-century will produce devastating long-term droughts and a sea-level rise that will persist for 1,000 years regardless of how well the world curbs future emissions of carbon dioxide, an international team of scientists reported yesterday.
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Top climate researchers from the U.S. National Oceanic and Atmospheric Administration, Switzerland and France said their analysis shows that carbon dioxide will remain near peak levels in the atmosphere far longer than other greenhouse gases, which dissipate relatively quickly.
"I think you have to think about this stuff as more like nuclear waste than acid rain: The more we add, the worse off we'll be," NOAA senior scientist Susan Solomon told reporters in a conference call. "The more time that we take to make decisions about carbon dioxide, the more irreversible climate change we'll be locked into."
At the moment, carbon concentrations in the atmosphere stand at 385 parts per million. Many climate scientists and the United Nations' Intergovernmental Panel on Climate Change have set a goal of stabilizing atmospheric carbon at 450 ppm, but current projections put the world on track to hit 550 ppm by 2035, rising after that point by 4.5 percent a year.
The new study, published in the Proceedings of the National Academy of Sciences, projects that if carbon dioxide concentrations peak at 600 ppm, several regions of the world -- including southwestern North America, the Mediterranean and southern Africa -- will face major droughts as bad or worse than the Dust Bowl of the 1930s. Global sea levels will rise by about three feet by the year 3000, a projection that does not factor in melting glaciers and polar ice sheets that would probably result in significant additional sea level rises.
Even if the world managed to halt the carbon dioxide buildup at 450 ppm, the researchers concluded, the subtropics would experience a 10 percent decrease in precipitation, compared with the 15 percent decrease they would see at 600 ppm. That level is still akin to mega-droughts such as the Dust Bowl. The already parched U.S. Southwest would probably see a 5 percent drop in precipitation during its dry season.
Mary-Elena Carr, associate director of the Columbia Climate Center, called the new projections "very sobering." She noted that while societies can try to adapt to reduced precipitation with better farming techniques and other measures, there is a limit to the ability to cope with severe drought.
"When it's drought, that is hard, because we have a finite amount of water and a growing population we need to feed," Carr said, adding that the severe storm surges associated with higher sea levels also pose a dangerous challenge to large populations.
The rising sea levels anticipated under a conservative projection, the authors wrote, would cause "irreversible commitments to future changes in the geography of the Earth, since many coastal and island features would ultimately become submerged."
The scientists noted that the world's oceans are already absorbing an enormous amount of carbon, but over time this will reach a limit and they will no longer absorb as much. As this happens, the atmospheric temperature will remain nearly constant.
Most previous scientific analyses, including the U.N. panel's summary report for policymakers, have assessed climate change impacts on a 100-year time scale. A few researchers, such as Ken Caldeira of the Carnegie Institution's Department of Global Ecology, have argued that it makes more sense to look at a time scale of at least 500 years.
In an e-mail yesterday, Caldeira wrote that he had debated this point with other contributors to the U.N. reports in 2001, adding, "If you took our long term climate commitment seriously, you would not use 100-year [global warming projections] to compare effects of different gases."
Carbon dioxide emissions account only for about half of human-induced global warming, but the several other gases that play a role, including methane, dissipate more quickly. Solomon said policymakers could take this into account when deciding how best to reduce greenhouse gases overall.
"We ought to be extra careful about how much carbon dioxide we put out in the future," she said, adding that politicians often focus on the less certain but potentially disastrous impacts of climate change but would do well to focus on the more predictable consequences. "The parts that we don't know, that are possible but very uncertain, shouldn't get in the way of what we do know."
A separate study in the same journal yesterday suggests that the iconic emperor penguins of the Antarctic could be headed to extinction by 2100 if the sea ice shrinks by the predicted amounts. That paper -- written by scientists from the Woods Hole Oceanographic Institution, National Center for Atmospheric Research, the National Snow and Ice Data Center in Boulder, Colo., and France's Centre National de la Recherche Scientifique -- projects that the number of breeding pairs in a colony in Terre Adelie, Antarctica, will decline from about 6,000 to 400 by the end of the century because the animals depend on sea ice for breeding, foraging and molting habitat.
Emperor penguins would have to migrate or change the timing of their growth stages to avoid extinction, the authors write, but "evolution or migration seem unlikely for such long-lived species at the remote southern end of the Earth."