A hint that things may not be as bad as we thought

I would be remiss if I didn't direct your attention to a new paper in Science that concludes, however tentatively, that the global climate may not be as sensitive to rising atmosheric CO₂ levels as everyone has assumed. It is, after all, a rare dose of optimism in a field that has been characterized by "it's worse than expected" findings for pretty much its entire history.

"Climate Sensitivity Estimated From Temperature Reconstructions of the Last Glacial Maximum" appeared last week on American Thanksgiving, thus managing to avoid much in way of media coverage. Science has put it behind a firewall, but you can find a full version here thanks to the coauthor Nathan Urban of Princeton University. In the paper, Urban, lead author Andreas Schmittner of Oregon State University and six others try to nail down climate sensitivity to a doubling of carbon dioxide levels by looking at paleoclimatic data from what went on 20,000 years ago during the last glacial maximum.

While most estimate of 2xCO₂ effects come up with a range of between 2 and 4.5 °C, with a median (sort of like a "most likely") figure of 3 °C, Schmittner's team narrow it down to between 1.7 and 2.6 degrees, with a median of just 2.3. They also conclude that anything beyond 6 degrees is "implausible." This latter part is important because the previous models assign a non-zero likelihood to increases of up to 10 degrees.

Chopping the media sensitivity from 3 to 2.3 degrees doesn't sound like much. But we are already at 0.8 degrees above pre-industrial levels, with another 0.5 or more in the pipeline due to climate inertia, so just how much wiggle room we have left before committing the planet to what is called, for lack of a better word, "dangerous" climate change, is a critical question. And the lower estimate implies we have one or two more decades than previously thought to play with before hitting the 2 °C mark, which is where most estimates say the bad stuff kicks in. Those few decades could give us the time we need to develop and deploy clean, renewable alternatives to the fossil fuels that are the source of the problem.

So how much rejoicing should there be with the advent of this study? Not too much. First, it is just one paper. Many others, which also use paleoclimatic data, come up with higher sensivities. James Hansen and his team, for example, say what happened millions of years ago suggest the sensitivity estimate of 3 degrees is probably about right, in the short run, with a long-term sensitivity due to positive feedback in the system of about 6 degrees.

Second, the new study uses what was going on during a cold spell, instead of a warm one. And there are those who suspect that climate sensitivity during ice ages isn't necessarily the same as during interglacial periods, like the one we're in now. Here's some useful context supplied by the BBC's Jennifer Carpenter

Climatologist Andrey Ganopolski, from Potsdam Institute for Climate Impact Research, Germany, went further and said that he would not make such a strong conclusion based on this data.

"The results of this paper are the result of the analysis of [a] cold climate during the glacial maximum (the most recent ice age)," he told BBC News.

"There is evidence the relationship between CO2 and surface temperatures is likely to be different [during] very cold periods than warmer."

Scientists, he said, would therefore prefer to analyse periods of the Earth's history that are much warmer than now when making their projections about future temperatures.

However, although good data exists for the last million years, temperatures during this time have been either similar to present, or colder.

"One should be very careful about using cold climates to [construct] the future," he added.

Finally, the authors aren't exactly shouting about their findings from the rooftops. The paper's penultimate paragraph is:

Our uncertainty analysis is not complete and does not explicitly consider uncertainties in radiative forcing due to ice sheet extent or different vegetation distributions. Our limited model ensemble does not scan the full parameter range, neglecting, for example, possible variations in shortwave radiation due to clouds. Non-linear cloud feedbacks in different complex models make the relation between LGM and 2ÃCO2 derived climate sensitivity more ambiguous than apparent in our simplified model ensemble (27). More work, in which these and other uncertainties are considered, will be required for a more complete assessment.

And just in case anyone wants to add the authors to the list of those who doubt global warming is something worry about, the BBC's Carpenter made a point of asking them about the implications of their work:

The authors stress the results do not mean threat from human-induced climate change should be treated any less seriously, explained palaeoclimatologist Antoni Rosell-Mele from the Autonomous University of Barcelona, who is a member of the team that came up with the new estimates.

See also this interview with Urban.
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Schmittner, A., Urban, N., Shakun, J., Mahowald, N., Clark, P., Bartlein, P., Mix, A., & Rosell-Mele, A. (2011). Climate Sensitivity Estimated from Temperature Reconstructions of the Last Glacial Maximum Science DOI: 10.1126/science.1203513