The New York Times,
Many people who worry about global warming hope that once emissions of heat-trapping gases decline, the problems they cause will quickly begin to abate.
Now researchers are saying that such hope is ill-founded, at least with regard to carbon dioxide.
Because of the way carbon dioxide persists in the atmosphere and in the oceans, and the way the atmosphere and the oceans interact, patterns that are established at peak levels will produce problems like "inexorable sea level rise" and Dust-Bowl-like droughts for at least a thousand years, the researchers are reporting in the Proceedings of the National Academy of Sciences.
"That peak would be the minimum you would be locking yourself into," said Susan Solomon, a senior scientist at the National Oceanic and Atmospheric Administration, who led the work.
The researchers describe what will happen if the atmospheric concentration of carbon dioxide -- the principal heat-trapping gas emission -- reaches 450 to 600 parts per million, up from about 385 p.p.m. today. Most climate researchers consider 450 p.p.m. virtually inevitable and 600 p.p.m. difficult to avoid by midcentury if the use of fossil fuels continues at anything like its present rate.
At 450 p.p.m., the researchers say, rising seas will threaten many coastal areas, and
"Ten percent may not seem like a high number," Dr. Solomon said Monday in a telephone news conference, "but it is the kind of number that has been seen in major droughts in the past, like the Dust Bowl."
At 600 p.p.m., there might be perhaps 15 percent less rain, she said.
In 1850, atmospheric carbon dioxide was roughly 280 p.p.m., a level scientists say had not been exceeded in at least the previous 800,000 years.
In their paper, Dr. Solomon and her colleagues say they confined their estimates to known data and effects. For example, they based their sea level estimates largely on the expansion of seawater as it warms, a relatively straightforward calculation, rather than including the contributions of glacial runoff or melting inland ice sheets more difficult to predict but potentially far greater contributors to sea level rise.
The new work dealt only with the effects of carbon dioxide, which is responsible for about half of greenhouse warming. Gases like chlorofluorocarbons and methane, along with soot and other pollutants, contribute to the rest. These substances are far less persistent in the atmosphere; if these emissions drop, their effects will decline relatively fast.
Michael Oppenheimer, a geoscientist at
Dr. Oppenheimer said the new analysis showed that "some dangerous consequences could be triggered and persist for a long, long time, even if emissions were cut radically."
"Policy makers need to understand," he continued, "that in some ways once we are over the cliff, there's nothing to stop the fall."
Dr. Solomon said it would be wrong to view the report as evidence that it was already too late to do much good by reducing carbon emissions. "You have to think of this stuff as being more like nuclear waste than acid rain," she said.
Acid rain began to abate when pollution contributing to it was limited. But just as nuclear waste remains radioactive for a long time, the effects of carbon dioxide persist.
"So if we slow it down," she said, "we have more time to find solutions."
For example, engineers may one day discover ways to remove the gas from the atmosphere. But "those solutions are not now in hand," Dr. Solomon said. "They are quite speculative."
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.
Top climate researchers from the U.S. National Oceanic and Atmospheric Administration,
"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
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
"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."