Uncertainty in observational estimates of the aerosol direct radiative effect and forcing

Abstract:

Aerosols continue to be responsible for the largest uncertainty in
determining the anthropogenic radiative forcing of the climate. To both
reconcile the large range in satellite-based estimates of the aerosol direct
radiative effect (DRE, the direct interaction with solar radiation by all
aerosols) and to optimize the design of future observing systems, we build a
framework for assessing uncertainty in aerosol DRE and the aerosol direct
radiative forcing (DRF, the radiative effect of just anthropogenic aerosols,
RF_ari). Shortwave aerosol radiative kernels (Jacobians) were derived using
the MERRA-2 reanalysis data. These radiative kernels are used to compute a
lower-bound on the systematic uncertainty in observational estimates of the
aerosol DRE/DRF by making the optimistic assumption that global aerosol
observations can be made with the accuracy found in the Aerosol Robotic
Network (AERONET) sun photometer retrievals. The total uncertainty is shown
to be dominated by contributions from the aerosol single scattering albedo
uncertainty. These uncertainty estimates were compared to a literature survey
of mostly satellite-based aerosol DRE/DRF values. Comparisons to previous
studies reveal that most have significantly underestimated the aerosol DRE
uncertainty. Past estimates of the aerosol DRF uncertainty are smaller (on
average) than our optimistic observational estimates, including the aerosol
DRF uncertainty given in the Intergovernmental Panel on Climate Change (IPCC)
fifth assessment report (AR5).