Earth nears tipping point on climate change

A rise of 1 degree Celsius could be enough to trigger 'dangerous' warming, scientists warn.

from the May 30, 2007 edition

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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."

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