Estimating the Transient Climate Response

The transient climate response (TCR) is the global average temperature response to doubled CO₂ in an idealized 1% per year CO₂ increase climate model experiment. The TCR is a useful metric for policy makers because it characterizes the influence of anthropogenic forcing on decadal-to-centennial time scale climate change. The Intergovernmental Panel on Climate Change (IPCC) 5th report states that the TCR is likely (>66%) 1 to 2.5 K. The large range is mainly driven by uncertainty in the magnitude of aerosol radiative forcing. The attempt to constrain the TCR range uses temperature observations and historical greenhouse gas and aerosol radiative forcing. The method is tested using historical and four future radiative forcing scenarios from the 1950 to 2050 time period. According to the International Institute for Applied Systems Analysis estimates, the magnitude of forcing due to aerosols is approximately equal in 1950 and 2050; therefore the net effect of aerosols is small and radiative forcing over this time period is almost entirely due to greenhouse gasses. Scaled temperature changes from 1950 to 2050 were found to have strong linear correlation with TCRs in the IPCC's AR5 model experiments. Slight, scenario dependent, high biases are consistent with the lower rate of forcing increase in the projections relative to 1% per year CO₂ increase. A matrix equation estimate of the TCR was subsequently used to obtain earlier estimates. The earliest TCR estimate, made with observations available in 2035, was estimated to likely range 0.47 K (four scenario average). This estimate is 1/3 of the likely range cited in the IPCC 5th report.