From Envirowiki

Climate sensitivity is the change in temperature in the atmosphere due to a doubling of greenhouse gas levels. The change is measured from the temperature equilibrium before the doubling to the equilibrium after the doubling. Such an equilibrium may take centuries to stabilise.

Climate sensitivity, unless otherwise stated, usually refers to a doubling of CO₂ from pre-industrial values, ie. from 275ppm to 550ppm.

[edit] Predictions

The IPCC's Fourth Assessment Report (4AR) estimates that "equilibrium climate sensitivity is likely to be in the range 2°C to 4.5°C, with a best estimate value of about 3°C. It is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded, but agreement with observations is not as good for those values. Probability density functions derived from different information and approaches generally tend to have a long tail towards high values exceeding 4.5°C. Analysis of climate and forcing evolution over previous centuries and model ensemble studies do not rule out climate sensitivity being as high as 6°C or more."