The world still has a realistic chance of avoiding some, although not all, of the more disruptive effects of global warming, according to a new analysis.
Doing so, however, will require substantial reductions in greenhouse gas emissions by 2010, consistent with those required by the Kyoto Protocol, scientists from Princeton and Brown universities reported in the June 14 issue of Science.
The researchers focused on three possible consequences of global warming, reflecting a range of likelihoods and potential for disruption: the destruction of coral reefs, the potential rise of sea levels caused by melting of an Antarctic ice sheet and the shutdown of large-scale ocean currents. They found that if aggressive measures are taken by 2010, maintaining the ocean currents is "likely," but saving the coral reefs is "probably not feasible" and preventing the ice-sheet melting is "plausible, but by no means certain."
"The Bush Administration regards large climate changes as inevitable and proposes adaptation as the main response," said Princeton geoscientist Michael Oppenheimer who co-wrote the report with Brian O'Neill of Brown University. "But some climate changes are so disruptive that avoiding them through emissions reduction is the only sensible alternative.
"Some have argued that the Kyoto Protocol would have little effect on long-term climate change," Oppenheimer added. "But we find that the total emissions reductions it envisions appear to be an important first step toward avoiding dangerous warming."
Oppenheimer and O'Neill wrote that they conducted their analysis in response to a growing demand for well-defined long-term objectives in dealing with climate change. A central question has been to define what would constitute a "dangerous interference" with the natural climate system. They selected coral reefs because they represent a unique ecosystem that could be decimated. They addressed the ice sheet melting and ocean circulation shutdown because they are examples of "large-scale discontinuities" that could occur in the climate system.
In looking at these three systems, the researchers focused on what would happen if the nations of the world were successful in stabilizing the concentration of carbon dioxide in the atmosphere at 450 parts per million, with a corresponding increase in average global temperature of 1.2 to 2.3 degrees Celsius in this century. Carbon dioxide is a greenhouse gas that comes from burning fossil fuels and destroying forests. The current level is 370 parts per million. Before the industrial revolution, it was about 280 parts per million.
The researchers drew these conclusions:
- Coral reefs are likely to undergo annual "bleaching" and eventually experience severe damage if the average global temperature increases more than 1 degree Celsius.
- The potential for the Western Antarctic Ice Sheet to disintegrate is highly uncertain, but is likely to require an increase of 2 degrees Celsius or more. Complete disintegration, which could take hundreds of years or longer, would result in an increase in sea level of 13 to 20 feet. That would submerge much of the world's coastlines, including large sections of Manhattan and southern Florida.
- A shutdown of the "thermohaline" ocean current would probably require a 3-degree Celsius increase during this century. The extent of the disruption that would result for societies and ecosystems is uncertain, but "it would be unwise to find out by continuing to pump up levels of greenhouse gases," said Oppenheimer.
A key finding in the paper is that if the industrialized world delays in curbing its carbon dioxide emissions until 2020, the actions needed to stay within the 450 parts-per-million limit would become dramatically more difficult, if not impossible. Currently, a large amount of the carbon dioxide pumped into the atmosphere is reabsorbed and rendered harmless by forests and other natural systems. Rapid cutbacks in emissions beyond 2020 would only be possible if this reabsorption remains strong, the researchers found. It is not clear how strong this effect will be in coming decades. "It makes a big difference," said Oppenheimer. "If the biosphere is taking up carbon fast, it gives us more time. If we delay and the biosphere takes up carbon slowly, we could be out of luck." The uncertainties associated with this effect play much less of a role if emissions are reduced sooner, he said.
The researchers conclude by comparing their analysis to the cutbacks outlined in the Kyoto Protocol, the international agreement -- rejected by the United States, but recently ratified by the European Union and Japan -- for reducing greenhouse gas emissions. They found that the Kyoto reductions would be an adequate first step for staying with the 450 parts-per-million limit. Further steps, however, would be required after 2010.
"There is a lot of uncertainty," said Oppenheimer. "But we are asking, 'What policies would be consistent with the science as we know it today?' Here is a framework for decision-making, and it points in the direction of action now. If you don't act now, you risk letting one of the important options, the 450 parts-per-million limit, drop off the table. It's really about keeping options on the table even in the face of uncertainty. How do we keep ourselves in a position where, as uncertainty is resolved, we can still avoid dangerous levels of warming?"
Oppenheimer holds joint appointments in Princeton's Department of Geosciences and the Woodrow Wilson School of Public and International Affairs.
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