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Earth's Transient Climate Sensitivity Evaluated From AR6 Estimates of Total Forcing and Observed Time Series of Global Temperature Change
  • Stephen Schwartz
Stephen Schwartz
Brookhaven National Laboratory

Corresponding Author:[email protected]

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Earth’s transient climate sensitivity Str is the rapid change, plateauing at ~5 yr, in global mean surface temperature GMST per change in forcing (e.g., Held et al., JGR, 2010). Str is readily evaluated from time series of total forcing F and temperature anomaly ΔT as the slope of a regression of ΔT vs F, with ΔT from model or observations and F generally modeled based on change in atmospheric composition. Prior estimates of Str have varied quite widely, mainly because of uncertainty in aerosol forcing. F is evaluated as total non-aerosol forcing, dominated by positive GHG forcing, plus negative aerosol forcing; large magnitude aerosol forcing results in small F and in turn high Str, and vice versa. Forcing time series derived from the Fifth IPCC Assessment Report (AR5, 2013) resulted in best estimate Str 0.35 K (W m-2)-1; 5% to 95% uncertainty range 0. 27 to 0.55 (Schwartz, JGR, 2018). New time series of total forcing from the (2021) Sixth IPCC Assessment report (AR6) permit similar evaluation of Str, Figure 1, as 0.46 (0.36 to 0.50) K (W m-2)-1. The increase in best-estimate Str is due to increased magnitude of best-estimate aerosol forcing in AR6 vs. AR5. Poor long-term correlation of forcing time series and observed ΔT for the 5% forcing estimate (large negative aerosol forcing added to GHG forcing, yielding low total forcing) suggests that that the corresponding bound on aerosol forcing magnitude may be an over-estimate, with the correlation substantially improved for best estimate and even more so for lowest estimate of aerosol forcing magnitude, thus more consistent with lower values of Str. A somewhat higher range of Str, 0.42 to 0.75 K (W m-2)-1, is obtained using time series of forcings obtained with individual models (Smith et al., ACP, 2020). Figure 1. Time series of total forcing F and as convolved with 5-year decaying exponential Fc (left); correlations of observed temperature anomaly ΔT (GISS) vs Fc (center); slope denotes transient sensitivity Str; and time series of ΔT (right; left axis) and Fc (right axis, scaled to ΔT by Str). Top row, lower 5% bound on forcing time series; middle row, best estimate forcing; bottom row, 95% bound. Forcing data from draft AR6 report, expected release August 9, 2021, potentially subject to change.