Greenhouse Gases Have Not Exceeded "Worst-Case Scenario"

Tim Flannery made international headlines earlier this month when he said "the amount of greenhouse gas in the atmosphere is already above the threshold that could potentially cause dangerous climate change," and that we'd reached that point -- 455 ppm greenhouse gas equivalent -- 10 years ahead of expectations.

A scientist and the author of a book about climate change, The Weather Makers, Flannery is 2007's Australian of the Year. So it's not surprising that his remarks were heard round the world, with reports by the likes of respected news organizations like The Australian Broadcasting Corp. and Reuters appearing on dozens of news Web sites. The Daily Green published this report: Global Warming "Beyond the Worst-Case Scenario."

But the reported interpretation of the data was misleading.

Here's how it really works, according to NASA scientist Gavin Schmidt, and Flannery himself, who we asked to clarify things.

• The level of "carbon dioxide equivalents" indeed reached 455 ppm (parts per million) in 2005. Scientists use the term to describe the concentration of all greenhouse gases, including carbon dioxide, nitrous oxide and others.
• The term is also used, however, to describe the net effect of greenhouse gases, which trap heat in the atmosphere, and other forces that have a cooling effect. Add in these forces, and you get a value of 375 ppm.

• The actual concentration of carbon dioxide in the atmosphere -- and carbon dioxide only -- is about 375 ppm.
• And scientists have suggested 450 ppm carbon equivalent -- by the second "net effect" definition -- as one target the world might aim for to maintain global warming at a manageable level. At that level the Earth might warm up 2 degrees (Celsius).

So the atmosphere has either 375 ppm and 455 ppm of carbon, depending on how you parse the terminology.

But either way, the data does not suggest we've reached a critical threshold, as Flannery was quoted as having said. And we haven't reached any milestone 10 years earlier than expected. (Nor has Flannery seen any data not widely available to other climate scientists; the upcoming Intergovernmental Panel on Climate Change report he referred to is a synthesis of previous reports released earlier this year.)

So what do the numbers really mean?

First off, they mean we've pumped an awful lot of greenhouse gases into the atmosphere. The pre-industrial level of carbon dioxide equivalent (the second, net effect kind) is estimated at 278 ppm -- so we've increased the concentration 35%.

No one knows for sure what concentration represents the danger point, when catastrophic changes will occur. Policy makers have settled on a couple. Among them: 450 ppm, which would result in a 50/50 chance that the world's average temperature would increase 2 degrees; and 550 ppm, which represents a doubling of pre-industrial levels. It could be we've already passed some important threshold.

Whatever the details, Flannery stuck to his basic thesis: That we all ought to be very concerned.

"My main point," Flannery said "is that (carbon dioxide equivalent) is already above the level any of us should be comfortable with. The widespread view that we have a decade to act is not therefore correct. We're already living with an unacceptable risk of dangerous climate change."

Schmidt was committed to clarifying the facts, but in a way he came out in the same general camp -- suggesting that a focus on exact concentrations of greenhouse gases in the atmosphere misses an important point.

"It's not that at 1.999 degrees nothing will happen and at 2.001 degrees all hell will break loose. As warming increases, effects become more severe," he said. "You have to think about it like a speed limit on the freeway. ... 55 mph is picked to try to maintain levels at a point where people aren't killing themselves every time they have a crash."