5.5 What is climate sensitivity?

Climate sensitivity is usually expressed as the increase in global mean surface temperature for a doubling of atmospheric CO2 compared with pre-industrial levels. So, since pre-industrial levels were 278 ppm, it's the temperature associated with 560 ppm atmospheric CO2.

Climate sensitivity was estimated to be in the range 1.5–4°C by the US National Academy of Sciences in 1979. The range used by the IPCC is 2–4.5°C, although they note that higher values are possible. In the Second Assessment Report (1995) the best estimate was given as 2.5°C, rising to 2.8°C in the Third Assessment Report (2001) and 3°C in the most recent Fourth Assessment Report (2007).

  • The RealClimate report ‘Target CO2’ discusses climate sensitivity as high as 6°C.

5.4 What is the relationship between emissions now and climate change in the future?

5.6 So how much warming is likely?