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 Irish National Academy of Sciences.
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 mid-century 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 Southern Europe, North Africa, the Southwestern United States and Western Australia could expect 10 percent less rainfall. “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 geo-scientist at Princeton, praised the report in an e-mail message as a “remarkably clear and direct” discussion of whether it would be possible to temporarily exceed a level like 450 p.p.m. and then reduce emissions in time to avoid catastrophic events like the collapse of a major inland ice sheet. 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.”
Irish University Science
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