The Climate Sensitivity and Its Components Diagnosed from Earth Radiation Budget Data

Piers Mde F Forster,Jonathan M Gregory

doi:10.1175/jcli3611.1

Piers Mde F Forster, Jonathan M Gregory

Open Access

https://doi.org/10.1175/jcli3611.1

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Abstract

Abstract One of the major uncertainties in the ability to predict future climate change, and hence its impacts, is the lack of knowledge of the earth’s climate sensitivity. Here, data are combined from the 1985–96 Earth Radiation Budget Experiment (ERBE) with surface temperature change information and estimates of radiative forcing to diagnose the climate sensitivity. Importantly, the estimate is completely independent of climate model results. A climate feedback parameter of 2.3 ± 1.4 W m−2 K−1 is found. This corresponds to a 1.0–4.1-K range for the equilibrium warming due to a doubling of carbon dioxide (assuming Gaussian errors in observable parameters, which is approximately equivalent to a uniform “prior” in feedback parameter). The uncertainty range is due to a combination of the short time period for the analysis as well as uncertainties in the surface temperature time series and radiative forcing time series, mostly the former. Radiative forcings may not all be fully accounted for; however, an argument is presented that the estimate of climate sensitivity is still likely to be representative of longer-term climate change. The methodology can be used to 1) retrieve shortwave and longwave components of climate feedback and 2) suggest clear-sky and cloud feedback terms. There is preliminary evidence of a neutral or even negative longwave feedback in the observations, suggesting that current climate models may not be representing some processes correctly if they give a net positive longwave feedback.

Highlights

One of the major uncertainties in the ability to predict future climate change, and its impacts, is the lack of knowledge of the earth’s climate sensitivity
The methodology is applicable to: 1) an equilibrium nonforced situation, where Q would be constant and N is responding to internal variability in ⌬Ts; 2) transient forced climate change situations, where N and ⌬Ts are responding to a radiative forcing (Q); and 3) a combination of internal variability and forced climate change
We found that volcanic and wellmixed greenhouse gas radiative forcings played the most important role in determining YNET; changes in total solar irradiance had some effect, but these values could be derived directly from the Earth Radiation Budget Satellite (ERBS) instruments, which largely eliminated them as a source of uncertainty

Summary

Introduction

One of the major uncertainties in the ability to predict future climate change, and its impacts, is the lack of knowledge of the earth’s climate sensitivity. Data are combined from the 1985–96 Earth Radiation Budget Experiment (ERBE) with surface temperature change information and estimates of radiative forcing to diagnose the climate sensitivity. In 1990 the Intergovernmental Panel on Climate Change (IPCC) suggested a range of 1.5–4.5 K for the global surface equilibrium temperature increase associated with a doubling of CO2 (Houghton et al 1990). There have been several efforts to try and narrow this range using a combination of observations and climate model data; the large uncertainties in climate sensitivity have, if anything, increased (Forest et al 2002; Gregory et al 2002; Harvey and Kaufmann 2002; Knutti et al 2003). Where F is a surrogate parameter for external radiative forcing mechanisms and X represents all the other internally driven mechanisms that might affect N, which are not directly or indirectly related to surface temperature

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