  Desert dust aerosols affect Earth's global energy balance through direct
interactions with radiation, and through indirect interactions with clouds and
ecosystems. But the magnitudes of these effects are so uncertain that it
remains unclear whether atmospheric dust has a net warming or cooling effect on
global climate. Consequently, it is still uncertain whether large changes in
atmospheric dust loading over the past century have slowed or accelerated
anthropogenic climate change, or what the effects of potential future changes
in dust loading will be. Here we present an analysis of the size and abundance
of dust aerosols to constrain the direct radiative effect of dust. Using
observational data on dust abundance, in situ measurements of dust optical
properties and size distribution, and climate and atmospheric chemical
transport model simulations of dust lifetime, we find that the dust found in
the atmosphere is substantially coarser than represented in current global
climate models. Since coarse dust warms climate, the global dust direct
radiative effect is likely to be less cooling than the ~-0.4 W/m2 estimated by
models in a current global aerosol model ensemble. Instead, we constrain the
dust direct radiative effect to a range between -0.48 and +0.20 W/m2, which
includes the possibility that dust causes a net warming of the planet.
