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A key threshold crossed

An Intergovernmental Panel on Climate Change report to be released next month will show that the limit on greenhouse-gases scientists hoped to avert has already been surpassed.

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In Ray Bradbury's science fiction novel "Fahrenheit 451," that number represented the temperature at which books would burn, a symbol of a disturbing future under a totalitarian government.

For climate scientists, a similar number, 450 parts per million (ppm), holds its own ominous meaning. It represents a dangerous concentration of greenhouse gases in the atmosphere; a total that they were not expecting to be passed for at least another decade.

But a new UN-sponsored report, to be released next month, will show that as of 2005 the concentration of greenhouse gases in the atmosphere had already reached 455 ppm, according to Tim Flannery, a prominent Australian climate scientist who says he's seen the raw data that go into the document.

In an interview on Australian television this week, Dr. Flannery said that an Intergovernmental Panel on Climate Change (IPCC) report will show that carbon dioxide (CO2), nitrous oxide, methane, hydrofluorocarbons (HFCs), and other greenhouse gasses are at much higher concentrations than previously thought. Reuters quotes him:

"We thought we'd be at that threshold within about a decade.... We thought we had that much time. But the new data indicates that in about mid-2005 we crossed that threshold.... What the report establishes is that the amount of greenhouse gas in the atmosphere is already above the threshold that could potentially cause dangerous climate change."

About 75 percent of the total ppm represents carbon dioxide, associated with burning fossil fuels. The rest is a combination of the other gasses, he said.

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