The longwave direct radiative forcing of desert dust

Historical increases in desert dust aerosols may have substantially affected Earth’s climate by exerting a substantial radiative forcing, which is not accounted for by current climate models and climate assessments. A key yet uncertain component of this dust radiative forcing is due to direct interactions with longwave (LW) radiation, for which model estimates vary five-fold. Here, we constrain the global mean LW dust direct radiative effect (DRE) by using a first-principles analytical model. This model integrates observational estimates of LW direct radiative effects with constraints on dust properties and abundance. Our analysis reveals that dust exerts a global mean LW DRE of +0.26 ± 0.06 Wm^-2 (90% confidence interval) at the top of the atmosphere. Current climate models underestimate the LW DRE by on average a factor of two because they neglect dust scattering of LW radiation, which accounts for approximately half of the LW DRE, and underestimate the abundance of super coarse dust. The approximately 50% increase in dust since pre-industrial times has amplified the dust LW DRE, thereby generating a direct radiative forcing of +0.09 ± 0.05 Wm^-2 that has modestly enhanced greenhouse warming. However, this warming effect has been counteracted by cooling resulting from direct interactions of dust with shortwave radiation, the magnitude of which remains poorly constrained.