Successful efforts to stabilize the level of greenhouse gasses in the atmosphere could still result in serious ecological damage if the cutbacks do not begin soon enough, according to a new analysis.
Scientists Michael Oppenheimer of Princeton University and Brian O'Neill of Brown University studied various strategies for reducing greenhouse gas emissions, comparing plans that begin reductions right away and others that delay action and make a more intensive effort later. Even if they ultimately reach the same target level of greenhouse gasses, the plans would have dramatically different environmental consequences, the scientists found.
"Delay inevitably means more warming and faster warming," said Oppenheimer. "And that could be detrimental not just to ecosystems, but to major elements of the climate system."
In one scenario, for example, two different paths to the same goal could mean the difference between widespread loss of the world's coral reefs and more limited damage. Other scenarios could involve the difference between the disintegration or the stability of major ice sheets in Antarctica or Greenland, which would dramatically affect sea level.
The researchers reported their findings in a paper to be published in the Proceedings of the National Academy of Sciences. Oppenheimer is Princeton's Albert Milbank Professor of Geosciences and International Affairs. O'Neill holds appointments at Brown and the International Institute for Applied Systems Analysis in Austria.
A dominant assumption in climate change research has been that there is an economic advantage to delaying action to curb greenhouse gas emissions, said Oppenheimer. One benefit to waiting is that new clean-energy technologies are likely to have been developed, so they could be deployed more intensively at less cost than today's technologies. Also, a general economic principle dictates that expenses delayed to the future are preferable to expenses made today, said Oppenheimer.
Previous studies, however, had not looked at the ecological costs of delay. "There are large uncertainties, but given what we know today, we can be confident there are substantial ecological costs associated with delays in reducing greenhouse emissions," Oppenheimer said.
Using simplified computer models that simulate global temperature to 2200 and beyond, the scientists looked at three possible goals: stabilizing all greenhouse gasses at levels equivalent to 500, 600 or 700 parts per million of carbon dioxide, which is the chief human-caused greenhouse gas. The current level of carbon dioxide is 380 parts per million; before the industrial revolution it was 280. For each of the targets, the researchers looked at three paths to reaching them: a slow, controlled increase; a fast, unchecked increase followed by a rapid leveling; or a fast, unchecked increase that overshoots the target followed by a reduction down to the target.
The researchers found that in the fast-rising and overshooting scenarios, some ecosystems would likely be stretched beyond their ability to adapt to rising temperatures. For example, a controlled increase in carbon dioxide to 500 parts per million would likely keep the average rise in global temperature to 1 degree Celsius, but a scenario that overshoots that target and returns to it by 2200 would cause warming of 1.5 degrees -- enough to severely damage many coral reefs, which appear to be sensitive to small temperature increases. Higher targets could cause warming beyond 2.5 degrees, which could affect many more ecosystems as well as the global ocean currents and ice sheets.
"The bottom line is that people and their governments ought to develop a sense of how much warming they think is too much," said Oppenheimer. "Then appropriate limits on carbon dioxide should be set and countries ought to move promptly to achieve those limits. Not acting promptly or not sticking to the target could prove highly disruptive, if not devastating."
Oppenheimer and O'Neill are following up their work with efforts to improve the estimates of how coral reefs, ice sheets and other major systems are likely to react to different warming scenarios.