Little net clear-sky radiative forcing from recent regional redistribution of aerosols

摘要

Aerosols both scatter and absorb incoming solar radiation, with consequences for the energy balance of the atmosphere. Unlike greenhouse gases, atmospheric aerosols are distributed non-uniformly around the Earth. Therefore, regional shifts in aerosol abundance could alter radiative forcing of the climate. Here, I use multi-angle imaging spectroradiometer (MISR) satellite data and the Atmospheric and Environmental Research radiative transfer model to assess the radiative effect of the spatial redistribution of aerosols over the past decade. Unexpectedly, the radiative transfer model shows that the movement of aerosols from high latitudes towards the Equator, as might happen if pollution shifts from Europe to southeast Asia, has little effect on clear-sky radiative forcing. Shorter slant paths and smaller upscatter fractions near the Equator compensate for more total sunlight there. Overall, there has been an almost exact cancellation in the clear-sky radiative forcing from aerosol increases and decreases in different parts of the world, whereas MISR should have been able to easily detect a change of 0.1 W m-2 per decade due to changing patterns. Long-term changes in global mean aerosol optical depth or indirect aerosol forcing of clouds are difficult to measure from satellites. However, the satellite data show that the regional redistribution of aerosols had little direct net effect on global average clear-sky radiative forcing from 2000 to 2012.