Science http://dx.doi.org/10.1126/science.1203513 (2011)

How much will our planet warm if carbon dioxide levels double from pre-industrial levels? In other words, what is the planet's 'sensitivity' to carbon dioxide?

This has proven a hard question to answer, mainly because of uncertainty about how aerosols and the ocean alter heating effects. The 2007 Intergovernmental Panel on Climate Change report set out a best estimate of 3 °C, with a 66% chance that the true answer lies between 2 °C and 4.5 °C, and a slight but real possibility of more than 10 °C of warming.

Now, Andreas Schmittner of Oregon State University, and colleagues, have produced a more precise answer by using a more complete temperature reconstruction of the last ice age — 21,000 years ago — than was previously available. Their warming estimate is 2.3 °C, with a 66% chance the answer lies between 1.7 °C and 2.6 °C. More than 6 °C of warming, the researchers conclude, would be implausible.

The latest estimate doesn't fully cover the impact of ice sheets, vegetation or clouds on climate sensitivity, so the numbers shouldn't be taken as definitive, the researchers caution.