After Tim Flannery told Australian Broadcasting Corp. last week that greenhouse gases had passed a threshold not expected for another decade, a climate modeler at the NASA Goddard Institute for Space Studies called that assessment "incorrect" on www.realclimate.org, a Web site run by climate scientists.

Flannery said the Intergovernmental Panel on Climate Change's upcoming Synthesis Report — a summary of the previous three reports the U.N. panel issued this year — includes a figure called the carbon-dioxide equivalent. The figure groups other greenhouses gases (such as methane and nitrous oxide) with carbon dioxide to measure all the gases' warming potential in terms of carbon dioxide. So while there are currently about 378 parts per million of carbon dioxide in the atmosphere, adding other greenhouse gases makes the warming potential equivalent to about 455 parts per million of carbon dioxide.

Scientists have cited 450 parts per million of carbon dioxide as a limit for avoiding severe consequences of global warming, hence Flannery's warning that we've passed the threshold for change.

The 455 figure Flannery referred to measures the gases regulated by the Kyoto Protocol. But if you include particulates such as sulfate aerosols, many of which act to cool (and pollute) the planet, the result is a carbon-dioxide equivalent of 375 parts per million, "with quite a wide error bar," the Goddard Institute's Gavin Schmidt says in his Real Climate post.

"What the planet cares about is the sum total," Schmidt says, while the first equation is a convenience for comparison and analysis.

But Flannery says the cooling pollutants are only a temporary mask.

"The problem for me is the short-lived nature (hours or days) of the cooling agents. As we clean up our air quality, the long-lived greenhouse gases are unleashed to do their worst," he wrote in an e-mail responding to questions about the post.

Computational ocean chemist David Archer, another Real Climate contributor, agreed with Flannery that the comfort level of the lower figure can, indeed, be deceiving, in part because some of the cooling particulates are released by the same power plants that emit the carbon dioxide. The U.S. and some European countries have reduced sulfate emissions, which cause acid rain. If more nations act to reduce pollution, the cooling pollutants will disappear quickly from the atmosphere, while the carbon dioxide stays for decades.

But even with their short life span, the particulates make a difference to the planet as long as they continue to be emitted, Schmidt said.

He also said the level of carbon dioxide is not 10 years ahead of predictions: "For 30 years it's been going up at the same track we've expected."

Flannery said his comments remain unchanged. "We're at the danger point for carbon-dioxide equivalent, but the effect is being masked in part, short-term, by other pollutants."

All three scientists agree, however, that the time to act to reduce emissions is now. "I have no problem with Tim pushing that as a problem and making clear that it's an urgent issue," Schmidt said. "When people have been saying you've got 10 years to act, it means you need to act as soon as possible."