Abstract/Summary

Stevens (2015, hereinafter S15) used energy balance arguments to estimate a lower limit on real-world aerosol forcings. The essence of this argument is that we expect any externally forced component of the warming between preindustrial and 1950 to have been positive. Therefore we would expect the sign of the corresponding net external forcing to also be positive. S15 uses simple global forcing–emission relationships and historical emission changes to show that large-magnitude present-day aerosol forcing would not be consistent with a 1950 positive net forcing. This analysis predicts that negative present-day aerosol forcings exceeding −1.3 or −1.0 W m−2 can be ruled out based on either 1950 global or Northern Hemispheric (NH) net energy balance, respectively. However, this argument is inconsistent with the warming in available CMIP5 simulations, which brings into question whether such an analysis does indeed imply a constraint on the real world. Out of the 10 CMIP5 simulations for which present-day aerosol forcing estimates are available, six simulate aerosol forcing equal to or larger in magnitude than −1.0 W m−2 and three simulate it equal to or greater than −1.3 W m−2, yet all reproduce a global warming trend, and almost all predict a positive NH trend (see Table 1). Understanding why S15’s energy balance analysis is not a good guide of the CMIP5 response is not straightforward. However, we have identified several factors in the S15 analysis that would provide partial explanations. These are 1) the degree of linearity of global aerosol forcing and 2) limitations of the regional energy budget analysis. We also identify two other aspects of the analysis where plausible alternative choices would lead to different constraints on the lower limit of real-world aerosol forcing: 3) past aerosol emissions and 4) choice of analysis period. The impact of adopting these alternative assumptions, in the S15 methodology, suggests that any real-world aerosol forcing constraint is likely to be considerably weaker than the S15 headline results.