Climate change will last a millenium news
28 January 2009

No matter how well the world manages its ecology from here on, the climate changes already triggered will take at least another 1,000 years to reverse, says new reseaech by National Academy of Sciences. By Jagdeep Worah

No matter how well the world manages its ecology from here on, the climate changes already triggered will take at least another 1,000 years to reverse, according to an authoritative new study. The gases already present in the atmosphere, and the heat that has been absorbed by the ocean, will exert their effects for centuries, says an analysis published in the Proceedings of the National Academy of Sciences.

In the long haul, the warming will melt the polar icecaps more than had previously been estimated, raising ocean levels substantially. At the same time, changes in rainfall patterns will bring droughts to the American Southwest, southern Europe, northern Africa and Western Australia comparable to those that caused the 1930s 'Dust Bowl' drought in the US, warns the study.

"People have imagined that if we stop emitting carbon dioxide, the climate would go back to normal in 100 years, 200 years," lead author Susan Solomon, a senior scientist at the National Oceanic and Atmospheric Administration, said in a telephone news conference. "That's not true." The changes will persist until at least the year 3000, according to Solomon, who conducted the study with colleagues in Switzerland and France.

The authors of the report, along with scientists familiar with its contents, warned that efforts to curtail carbon dioxide emissions and stop environmental destructions must be stepped up.

The slowness with which ocean water circulates is central to the new findings. Carbon dioxide is primarily removed from the atmosphere through absorption into seawater, an incredibly slow process because of the time it takes for surface water saturated with the gas to be replaced by deeper water that can further absorb carbon dioxide.

That gas accounts for about half of the global warming caused by greenhouse gases. The other polluting gases are removed from the atmosphere more quickly. Thus, the long-term influence of carbon dioxide will have the greatest effect on climate change, the report said. Moreover, heat absorbed by the ocean is released slowly, and will continue to contribute to global warming even if the concentration of greenhouse gases should decline.

Solomon said in her press statement that the absorption of carbon dioxide and release of heat - one process acting to cool the Earth and the other to warm it - would "work against each other to keep temperatures almost constant for more than 1,000 years."

The study looked particularly at ocean levels and rainfall. The team found that by thermal expansion of ocean water alone, sea levels would rise from 1.3 to 3.2 ft if carbon dioxide climbs from the current level of 385 parts per million to 600 parts per million, and twice that if CO2 levels peak at 1,000 parts per million.

Melting of the icecaps would increase sea levels even more, inundating low-lying islands and continental shorelines, but the effects are too uncertain to quantify. Reductions in rainfall would also last centuries, the report said, decreasing drinking water supplies, increasing fire frequency and devastating dry-season farming of wheat and maize.

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," Solomon said. "The more time that we take to make decisions about carbon dioxide, the more irreversible climate change we'll be locked into."

At present, carbon concentration in the atmosphere is 385 parts per million. Many climate scientists, as well as the United Nations' intergovernmental panel on climate change, have set the goal of stabilizing atmospheric carbon at 450 ppm, but the study puts the world on track to hit 550 ppm by 2035, rising after that point by 4.5 per cent a year.

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 which would probably result in significant additional sea level rises.

Even if the world manages to halt the carbon dioxide buildup at 450 ppm, the researchers concluded, the subtropics would experience a 10 per cent decrease in rainfall, compared with the 15 per cent decrease they would see at 600 ppm. The already parched US Southwest would probably see a five per cent drop in precipitation during its dry season.

"We ought to be extra careful about how much carbon dioxide we put out in the future," Solomon 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."

In their paper, 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.

Scientists praise study: "As a climate scientist, this was my intuition," said geoscientist Jonathan T Overpeck of the University of Arizona. "But they have done a really good job of working through the details and . . . make a case that the situation is more dire than we thought if we don't act quickly and aggressively to curb carbon dioxide emissions."

Kevin Trenberth, head of climate analysis at the National Center for Atmospheric Research in Boulder, Colorado said the persistence of climate change caused by global warming was "poorly appreciated by policymakers and the public, and it is real."

"The policy relevance is clear: We need to act sooner, even if there is some doubt about exactly what will happen, because by the time people and policymakers realize that the changes are here, it is far too late to do anything about it," Trenberth added.

Geoscientist Jorge L. Sarmiento of Princeton University said, "This is really a wake-up call about the seriousness of this issue."

Most previous scientific analyses, including the UN 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.

After the new study, Caldeira wrote that he had debated this point with other contributors to the UN reports in 2001, adding, "If you took our long term climate commitment seriously, you would not use 100-year projections to compare effects of different gases."

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.

The report came as US President Barak Obama ordered the Environmental Protection Agency (EPA) to consider allowing states the right to enact auto emission standards stricter than federal rules. Secretary of State Hillary Clinton also is expected to appoint a new envoy for climate change to bolster the administration's credentials in environmental policy.

Emperor penguins in hot water: 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, also written by an international team of scientists, 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. This is because the animals depend on sea ice for breeding, foraging and molting habitat.


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Climate change will last a millenium