A paper in press for the journal Tellus finds the "best estimate" of climate sensitivity to CO2 is about 33% less than estimated by the IPCC. According to the authors, the best estimate of transient climate sensitivity is 1.5C for a doubling of CO2 levels, and equilibrium climate sensitivity [after hundreds of years] about 2C. The paper finds a lower bound on equilibrium climate sensitivity of 1.16C per doubling of CO2. The paper adds to many recent peer-reviewed papers finding climate sensitivity to CO2 is significantly less than claimed by the IPCC.
Determination of a lower bound on Earth’s climate sensitivity
Lennart Bengtsson 1, Stephen E. Schwartz 2
1 Environmental Systems Science Centre, University of Reading, UK;
2 Atmospheric Sciences Division, Brookhaven National Laboratory, Upton NY 11973, USA
Submitted to Tellus, October 23, 2012
Accepted, August 14, 2013 Revised, May 20, 2013; July 27, 2013; August 15, 2013
ABSTRACT: Transient and equilibrium sensitivity or Earth's climate have been calculated using global temperature, forcing, and heating rate data for the period 1970-2010. We have assumed increased long wave radiative forcing in the period due to the increase of the long-lived greenhouse gases. By assuming that the change in aerosol forcing in the period to be zero we calculate what we consider to be lower bounds to these sensitivities, as the magnitude of the negative aerosol forcing is unlikely to have diminished in this period. The radiation imbalance necessary to calculate equilibrium sensitivity is estimated from the rate of ocean heat accumulation as 0.37 ± 0.03 W m-2 (all uncertainty estimates are 1-σ). With these data we obtain best estimates for transient climate sensitivity 0.39 ± 0.07 K (W m-2)-1 and equilibrium climate sensitivity 0.54 ± 0.14 K (W m-2)-1, equivalent to 1.5 ± 0.3 and 2.0 ± 0.5 K (3.7 W m-2)-1, respectively. The latter quantity is equal to the lower bound of the "likely" range for this quantity given by the 2007 IPCC Assessment report. The uncertainty attached to the lower-bound equilibrium sensitivity permits us to state, within the assumptions of this analysis, that the equilibrium sensitivity is greater than 0.31 K (W m-2)-1, equivalent to 1.16 K (3.7 W m-2)-1, at the 95% confidence level.
Related: Why climate sensitivity is even less than these new estimates