In general, it denotes the surface warming that will occur if CO2
More precisely, it denotes global-mean-temperature response to a forcing once all the fast feedbacks have occurred (e.g., atmospheric temperatures, clouds, water vapour, snow, and sea ice), but before any of the slow feedbacks have kicked in (e.g., ice sheets, vegetation, and the carbon cycle).
Note that its inverse is called gamma.
The politics of climate sensitivity
It's politically important because if climate sensitivity is low for CO2, there's no need to take strong immediate actions to reduce our emissions. Here's Roy Spencer presenting that line of argument at a Senate Environment and Public Works Committee hearing in July of 2008.
1. For a more detailed explanation, click here to read a Real Climate post that gives a good overview of the state of the science.
2. The standard modern estimate of climate sensitivity - 3°C ± 1.5°C - originates with a committee on anthropogenic global warming convened in 1979 by the National Academy of Sciences and chaired by Jule Charney. Only two sets of models were available:
(1) Syukuro Manabe's showed a climate sensitivity of 2°C and
(2) James Hansen's showed a climate sensitivity of 4°C.
According to Manabe, "Charney chose 0.5°C as a not-unreasonable margin of error, subtracted it from Manabe's number, and added it to Hansen's. Thus was born the 1.5°C-to-4.5°C range of likely climate sensitivity that has appeared in every greenhouse assessment since..." (Source)
3. Click here to read Skeptical Science's overview the state of the science on this (as of 2010).
4. But wait there's more: A 2011 paper in Science claims that it might be lower than 3°. Click here to read a summary of the findings (and click here for the full paper).
Excerpt of Summary: "Existing studies of climate sensitivity mostly rely on data gathered from weather stations, which go back to roughly 1850. Dr Schmittner takes a different approach. His data come from the peak of the most recent ice age, between 19,000 and 23,000 years ago. His group is not the first to use such data (ice cores, fossils, marine sediments and the like) to probe the climate’s sensitivity to carbon dioxide. But their paper is the most thorough. Previous attempts had considered only small regions of the globe. He has compiled enough information to make a credible stab at recreating the climate of the entire planet.
The result offers that rarest of things in climate science—a bit of good news. The group’s most likely figure for climate sensitivity is 2.3°C, which is more than half a degree lower than the consensus figure, with a 66% probability that it lies between 1.7° and 2.6°C. More importantly, these results suggest an upper limit for climate sensitivity of around 3.2°C."
Click here for Real Climates initial critique of the results. Excerpt:
"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."
Example Usages -
1. “There is word circulating that a paper soon to appear in Science magazine concludes that the climate sensitivity — how much the earth’s average temperature will rise as a result of a doubling of the atmospheric concentration of carbon dioxide—likely (that is, with a 66% probability) — lies in the range 1.7°C to 2.6°C, with a median value of 2.3°C.
This is a sizeable contraction and reduction from the estimates of the climate sensitivity given by the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), in which the likely range is given as 2.0°C to 4.5°C, with a best estimate of 3.0°C.”
(Source: Patrick Michaels, World Climate Report )
2. "Remember that climate sensitivity is generally defined as the equilibrium globally-averaged surface temperature rise for a doubled concentration of atmospheric CO2 - so it's a simple benchmark to describe the sensitivity of the global climate to the sort of perturbation we are imposing."
(Source: James' Empty Blog: Climate sensitivity is 3C )