Comment on acp-2022-228

Abstract

Mineral dust plays an important role in earth system models, being linked to many components: atmospheric wind-speed, precipitation and radiation, surface vegetation cover and soil properties, and oceanic biogeochemical systems. In this paper the dust scheme in the first configuration of the United Kingdom Earth System Model UKESM1 is described, and simulations of dust and its radiative effects are presented and compared with results from the parallel coupled atmosphere-ocean general circulation model (GCM) HadGEM3-GC3.1. Not only changes in the driving model fields but also changes in the dust size distribution are shown to lead to considerable differences to the present-day dust simulations and to projected future changes. UKESM1 simulations produce a present-day top of the atmosphere (ToA) dust direct radiative effect (DRE – defined as the change in downward net flux directly due to the presence of dust) of 0.086 Wm-2 from a dust load of 19.5 Tg. Under climate change pathways these values decrease considerably: in the 2081–2100 mean of Shared Socioeconomic Pathway SSP5-8.45 ToA DRE reaches 0.048 W m-2 from a load of 15.1 Tg. In contrast, in HadGEM3-GC3.1 the present-day values of -0.296 W m-2 and 15.0 Tg are almost unchanged, at 0.289 W m-2 and 14.5 Tg in the 2081–2100 mean. The primary mechanism causing the differences in future dust projections is shown to be the vegetation response, which dominates over the direct effects of warming in our models. Though there are considerable uncertainties associated with any such estimates, the results presented demonstrate both the importance of the size distribution for dust modeling, and also the necessity of including earth system processes such as interactive vegetation in dust simulations for climate change studies.