Comment on “Rethinking the Lower Bound on Aerosol Radiative Forcing”

In an influential and interesting study, Stevens (2015) suggested that the global and also Northern Hemispheric warming during the early industrial period implies that the effective radiative forcing [Formula: see text] by anthropogenic aerosols in the year 2000 compared to 1850 cannot be more negative than −1.0 W m −2 . Here results from phase 5 of the Coupled Model Intercomparison Project are analyzed and it is shown that there is little relationship between [Formula: see text] and the warming trend in the early industrial period in comprehensive climate models. In particular, some models simulate a warming in the early industrial period despite a strong (very negative) [Formula: see text]. The reason for this difference in results is that the global-mean log-linear scaling of [Formula: see text] with anthropogenic sulfur dioxide emissions introduced and used by Stevens tends to produce a substantially larger aerosol forcing compared to climate models in the first half of the twentieth century, when SO 2 emissions were concentrated over smaller regions. In turn, it shows smaller (less negative) [Formula: see text] in the recent period with comparatively more widespread SO 2 emissions.