Simulation of the direct radiative effect of mineral dust aerosol on the climate at the Last Glacial Maximum

Abstract

The climatic responses to the direct radiative effect of dust aerosol at the Last Glacial Maximum (LGM) are examined using a general circulation modelwith online simulation of dust. The predicted global dustemission at the LGMis 2.3 times as large as the present-day value, which is the combined effect of the expansion of dust sources and the favorable meteorological parameters (MPs; e.g., the strong surface wind and the low air humidity) under the LGMclimate. Simulated global dust emission is 1966 Tg yr21 with present-day dust sources andMPs, 2820 Tg yr-1 with LGM dust sources and current MPs, 2599 Tg yr-1 with present-day dust sources and LGM MPs, and 4579 Tg yr-1 with LGM sources and MPs. The simulated percentage increases of dust concentrations are the largest at high latitudes in both hemispheres, which are consistentwith the deposition data fromgeological records. The LGM dust is estimated to exert global annual-mean shortwave (SW) and longwave (LW) radiative forcing (RF) of24.69 and 11.70 W m-2 at the surface, respectively, and 20.58 and 10.68 W m-2 at the top of the atmosphere, respectively. On a global- and annual-mean basis, surface air temperature (SAT) is predicted to be reduced by 0.18 K and precipitation is reduced by 0.06 mm day-1, as a result of the net (SW and LW) radiative effect of dust at the LGM. Two sensitivity studies are performed to identify the uncertainties in simulated climatic effect of LGMdust that arise from the assumed LWand/or SWabsorption by dust: 1) in the absence of dust LW radiative effect, the LGMglobal- and annual-mean SAT is predicted to be further reduced by 0.19 K; and 2) when the single scattering albedo of the Saharan dust at 0.55 mmis increased from 0.89 to 0.98 in the LGMclimate simulation, the LGMdustinduced annual- and global-mean surface cooling increases from 0.18 to 0.63 K even with both SW and LW radiative effects of dust. In these two sensitivity studies, the LGM dust is predicted to induce an average cooling of 0.42 and 0.72 K in SAT, respectively, over the tropical oceans. © 2011 American Meteorological Society.