Estimating climate sensitivity from 3 million years ago
A new paper Earth system sensitivity inferred from Pliocene modelling and data (Lunt 2010) looks further into climate sensitivity as determined from the past. It examines the mid-Pliocene warm period,about 3.3 to 3 million years ago. This period is useful because CO2 levels and temperature were higher than pre-industrial conditions, giving us an insight into how climate responds when it’s already warm. At this time, the main external forcing driving climate was tectonic changes in mountain ranges which led to changes in atmospheric CO2 (driven by both tectonic-related emissions and weathering).
What they find is the temperature response to changes in CO2 is 30 to 50% greater than the response based on fast feedbacks. This is due to other feedbacks operating over greater timescales (there is still uncertainty over the timescales involved, from hundreds to thousands of years). These slow feedbacks include changes in dust and other aerosols, vegetation, ice sheets and ocean circulation. This is confirmed by the geological record which records changes to ice sheets and vegetation over this period.
This means climate may be more sensitive to carbon dioxide than previously thought.