How much the globe will warm depends crucially on how sensitive the climate is to additional atmospheric concentrations of greenhouse gases, chiefly carbon dioxide emitted from burning fossil fuels. As I explained elsewhere:
The term climate sensitivity conventionally refers to how much warming can be expected from a doubling of carbon dioxide concentrations in the atmosphere…
With regard to climate sensitivity, in 2007 the Fourth Assessment Report of the United Nation's Intergovernmental Panel on Climate Change (IPCC) estimated that "climate sensitivity is likely to be in the range of 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values." In IPCC parlance, likely means that there is a 66 percent probability that climate sensitivity falls between 2 and 4.5°C (3.6 to 8.1°F), with 3°C (5.4°F) as the best estimate.
Some researchers suggest that temperatures could in fact rise well beyond the IPCC's best estimates by more than 6 degrees Celsius (13 degrees Fahrenheit).
Over at the New York Times' excellent DotEarth blog, science reporter Andrew Revkin rounds up a bunch of recent studies that suggest that climate sensitivity is somewhat less than the IPCC's old estimate. In his post, "A Closer Look at Moderating Views on Climate Sensitivity," Revkin reports that for this …
…one critically important metric — how hot the planet will get from a doubling of the pre-industrial concentration of greenhouse gases, a k a "climate sensitivity" — some climate researchers with substantial publication records are shifting toward the lower end of the warming spectrum….
But while plenty of other climate scientists hold firm to the idea that the full range of possible outcomes, including a disruptively dangerous warming of more than 4.5 degrees C. (8 degrees F.), remain in play, it's getting harder to see why the high-end projections are given much weight.
This is also not a "single-study syndrome" situation, where one outlier research paper is used to cast doubt on a bigger body of work — as Skeptical Science asserted over the weekend. That post focused on the as-yet-unpublished paper finding lower sensitivity that was inadvisedly promoted recently by the Research Council of Norway.
In fact, there is an accumulating body of reviewed, published research shaving away the high end of the range of possible warming estimates from doubled carbon dioxide levels. Chief among climate scientists critical of the high-sensitivity holdouts is James Annan, an experienced climate modeler based in Japan who contributed to the 2007 science report from the Intergovernmental Panel on Climate Change.
Apparently, the best estimate of climate sensitivity is converging on a value a bit less than 2 degrees Celsius.
Revkin makes it clear that he nevertheless believes that even if truly catastrophic future warming can be ruled out that humanity should still make the transition to a low-carbon and no-carbon energy future to mitigate warming consequences like sea level rise, shifting weather patterns, and ocean acidification. He ends by observing:
I can understand why some climate campaigners, writers and scientists don't want to focus on any science hinting that there might be a bit more time to make this profound energy transition. (There's also reluctance, I'm sure, because the recent work is trending toward the published low sensitivity findings from a decade ago from climate scientists best known for their relationships with libertarian groups.)
Nonetheless, the science is what the science is.
Yes, it is.
For more background see my column, "The Sky is Falling Less?" and my post, "Man-Made Warming Likely Benign, Reports WSJ Columnist Matt Ridley."
Update: Revkin asks me to alert H&R readers to go click over to his post for an addendum response from the RealClimate folks.