Dangerous climate change has not yet arrived, but the tipping point may not be far off. And it may be reached with a smaller temperature rise than recent studies suggest.
Those are among the conclusions from an international team of climate scientists in a study this month, which they say bolsters the case for an alternative strategy to combat climate change. The main idea: focus intensely on cutting greenhouse-gas emissions other than carbon dioxide in the short term, giving the world a little leeway in dealing with the trickier issue of CO2.
Most climate scientists point to rising carbon-dioxide levels from burning coal, oil, and gas as the main driver behind global warming. But the international team says that fighting ozone, soot, and other pollutants, which also can warm the atmosphere, could allow CO2 levels to rise a little higher without reaching the tipping point.
"This is good news," notes Gavin Schmidt, a member of the research team and a scientist at NASA's Goddard Institute for Space Studies (GISS), in an e-mail. "There is scope for effective action, even though it will fall short of stopping human-caused climate change completely."
Yet this more comprehensive approach to curbing emissions is unlikely to remain an option for too long, according to James Hansen, a climate scientist also at GISS and lead author of the study. If global CO2 emissions continue on their current "business as usual" path for another 10 years, he notes, "it becomes impractical to achieve the alternative scenario." The business-as-usual approach allows too many fossil-fuel intensive power plants and factories to be built – investments designed to last for decades, he adds.
While the 10-year window doesn't represent a "drop dead" date, the researchers say, it should be seen as a transition period during which CEOs take emissions-reduction needs into account as they lay out spending plans for new factories and offices, power plants, and lines of cars and trucks.
The report comes at a time of increasing international pressure on the Bush administration to get tougher on carbon dioxide – and firm resistance from the White House to do so. In Europe, German Chancellor Angela Merkel has been fighting for a final statement from next month's Group of Eight (G-8) summit that would call for the world to cut global 1990-level CO2 emissions in half by 2050. The United States has rejected out of hand any such language in the final communiqué. US and Australian officials this week also rejected a proposal that members of the Asia-Pacific Economic Cooperation forum set up a regional emissions-trading scheme.
The study uses information about past climate and the latest modeling techniques to project the effects of emissions that follow the Intergovernmental Panel on Climate Change's "business as usual" emissions path and an alternative path Dr. Hansen and his colleagues set out in 2004. The model incorporates far more factors affecting climate than past models.
The work indicates that ozone and black-carbon soot have played as significant a role in warming the Arctic as CO2. Reducing these pollutants could temporarily slow the rate of warming – and of ice loss – even in the face of rising CO2 levels.
For the tropics, the results indicate that warming from increased greenhouse gases has probably played a substantial role over the past decade in beefing up the strength of hurricanes in the Atlantic.
The notion of "dangerous" climate change is somewhat subjective, the team acknowledges. But looking back at climate patterns since the last warm spell – between ice ages more than 75,000 years ago – the researchers say patterns in the climate's actual behavior suggest that a change in global average temperatures higher than 1 degree Celsius above the level in 2000 would begin to push the climate into the "dangerous" category. That category involves changes, such as sea-level rise, that are outside the local range of experience, the study says.
Holding temperature increases to less than 1 degree over 2000 levels would be likely to hold global average temperatures at a level at least occasionally experienced today.
Such targets represent "advisory speed limits," according to Dr. Schmidt. But they come strongly advised: "What should be the target for mugging old ladies? You want to minimize the number, regardless."