Spatial patterns of radiative forcing and surface temperature response

Abstract

Examination of effective radiative forcing (ERF), a measure of changes in Earth’s energy balance, facilitates understanding the role of various drivers of climate change. For short-lived compounds, ERF can be highly inhomogeneous geographically. The relationship between the spatial patterns of ERF and surface temperature response is poorly characterized, however. We examine that relationship in the latest generation of global climate models. We find that the uneven distribution of historical aerosol, ozone, and land use forcing leads to substantial differences compared to the well-mixed greenhouse gases (WMGHG). There is a stronger response per unit global mean forcing to historical inhomogeneous forcing than to WMGHG both globally and in much of the Northern Hemisphere (NH) extratropics, in fairly good agreement with results inferred from observations. Our results indicate that the enhanced global mean response is attributable to the concentration of inhomogeneous forcing in the NH extratropics, where there is strongest sensitivity to forcing, rather than to processes specific to the inhomogeneous forcers. In many regions, inclusion of inhomogeneous forcing greatly increases the spread in historical temperature changes simulated by the models, suggesting that better forcing characterization could play an important role in improving modeling of decadal-scale regional climate change. Finally, incorporating observed temperatures, the results provide estimates of global historical aerosol forcing (-1.0 ± 0.4Wm-2) consistent with other studies (though with narrower uncertainties) and also provide constraints on NH and NH extratropical historical aerosol forcing (-1.4 ± 0.6 and -1.2 ± 0.6Wm-2, respectively) and aerosol + ozone forcing.