Determination of a lower bound on Earth’s climate sensitivity

Transient and equilibrium sensitivity of Earth’s climate has 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 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.