Recent work published in Science suggests that the climate may be a bit less sensitive to carbon dioxide (CO2) than previously thought. The term ‘climate sensitivity‘ refers to how much the planet is expected to warm if the amount of CO2 in the atmosphere doubles. The most commonly cited estimate is 3ËšC.
The recent work, undertaken by Andreas Schmittner, a climate scientist at Oregon State University, suggests that the correct value may be more like 2.4ËšC.
If so, that would be good news for humanity. It would mean that we could burn more fossil fuels while remaining below any particular temperature threshold of warming.
Here’s what you need to know about the study by Schmittner et al in Science (subs. req’d):
* Its key finding is that the so-called “fast-feedbacks sensitivity†of the climate (to a doubling of CO2 levels) is on the low side. This finding is likely wrong, according to many leading climatologists (see below).
* Even if the study’s findings hold up, we are headed toward high warming on our current GHG emissions path. That’s because we are headed toward a tripling or higher of CO2 levels and because the slower feedbacks ain’t so slow (see “NSIDC bombshell: Thawing permafrost feedback will turn Arctic from carbon sink to source in the 2020s, releasing 100 billion tons of carbon by 2100“).
* The study finds that small changes in Earth’s temperature can have huge impacts on the land — that’s why it finds a low sensitivity!
This last, crucial point seems to have escaped the attention of many U.S. reporters on the study — even though it is quite clearly stated in the study’s news release:
“It shows that even very small changes in the ocean’s surface temperature can have an enormous impact elsewhere, particularly over land areas at mid- to high-latitudes,†[Schmittner] added.
Ice age constraints on climate sensitivity
There is a new paper on Science Express that examines the constraints on climate sensitivity from looking at the last glacial maximum (LGM), around 21,000 years ago [Schmittner et al (2011)] (SEA). The headline number (2.3ºC) is a little lower than IPCC’s “best estimate†of 3ºC global warming for a doubling of CO2, but within the likely range (2-4.5ºC) of the last IPCC report. However, there are reasons to think that the result may well be biased low, and stated with rather more confidence than is warranted given the limitations of the study.