Last glacial maximum radiative forcing from mineral dust aerosols in an Earth system model

The mineral dust cycle in preindustrial (PI) and Last Glacial Maximum (LGM) simulations with the Coupled Model Intercomparison Project Phase 5 model Hadley Centre Global Environment Model 2-Atmosphere (HadGEM2-A) is evaluated. The modeled global dust cycle is enhanced at the LGM, with larger emissions in the Southern Hemisphere, consistent with some previous studies. Two different dust uplift schemes within HadGEM2 both show a similar LGM/PI increase in total emissions (60% and 80%) and global loading (100% and 75%), but there is a factor of 3 difference in the top of the atmosphere net LGM-PI direct radiative forcing (-1.2Wm^-2 and -0.4Wm^-2, respectively). This forcing is dominated by the short-wave effects in both schemes. Recent reconstructions of dust deposition fluxes suggest that the LGM increase is overestimated in the Southern Atlantic and underestimated over east Antarctica. The LGM dust deposition reconstructions do not strongly discern between these two dust schemes because deposition is dominated by larger (2-6 µm diameter) particles for which the two schemes show similar loading in both time periods. The model with larger radiative forcing shows a larger relative emissions increase of smaller particles. This is because of the size-dependent friction velocity emission threshold and different size distribution of the soil source particles compared with the second scheme. Size dependence of the threshold velocity is consistent with the theory of saltation, implying that the model with larger radiative forcing is more realistic. However, the large difference in radiative forcing between the two schemes highlights the size distribution at emission as a major uncertainty in predicting the climatic effects of dust cycle changes.