Abstract
AbstractThe uncertainty in present‐day anthropogenic forcing is dominated by uncertainty in the strength of the contribution from aerosol. Much of the uncertainty in the direct aerosol forcing can be attributed to uncertainty in the anthropogenic fraction of aerosol in the present‐day atmosphere, due to a lack of historical observations. Here, we present a robust relationship between total present‐day aerosol optical depth and the anthropogenic contribution across three multimodel ensembles and a large single‐model perturbed parameter ensemble. Using observations of aerosol optical depth, we determine a reduced likely range of the anthropogenic component and hence a reduced uncertainty in the direct forcing of aerosol.
Highlights
Aerosols affect the climate both directly by scattering and absorbing incoming solar radiation and indirectly by providing the nuclei on which cloud droplets form
We present a robust relationship between total present‐day aerosol optical depth and the anthropogenic contribution across three multimodel ensembles and a large single‐model perturbed parameter ensemble
Anthropogenic perturbations to the natural background aerosol population can change the balance in radiation at the top of the atmosphere and provide a forcing, which offsets some of the forcing due to anthropogenic greenhouse gasses
Summary
Aerosols affect the climate both directly by scattering and absorbing incoming solar radiation and indirectly by providing the nuclei on which cloud droplets form. A number of cases have been found recently where relatively simple relationships between observable and unobservable quantities can be discerned, belying the apparent complexity of the underlying system (Allen & Ingram, 2002; Hall & Qu, 2006) These relationships can be exploited using observations to constrain model ensemble values of the unobservable quantity. Perturbed parameter ensembles (PPEs) enable exploration of the parametric uncertainty, and with sufficient observations, a constrained estimate of a quantity from a given model. This method can say little of the structural deficiencies of a model and large intermodel differences may be unaccounted for. We use satellite‐based observations of τPD to constrain the uncertainty in τant in a large PPE, and in turn the clear‐sky aerosol forcing (RFari)
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