Abstract
AbstractTropospheric aerosol radiative forcing has persisted for many years as one of the major causes of uncertainty in global climate model simulations. To sample the range of plausible aerosol and atmospheric states and perform robust statistical analyses of the radiative forcing, it is important to account for the combined effects of many sources of model uncertainty, which is rarely done due to the high computational cost. This paper describes the designs of two ensembles of the Met Office Hadley Centre Global Environment Model‐U.K. Chemistry and Aerosol global climate model and provides the first analyses of the uncertainties in aerosol radiative forcing and their causes. The first ensemble was designed to comprehensively sample uncertainty in the aerosol state, while the other samples additional uncertainties in the physical model related to clouds, humidity, and radiation, thereby allowing an analysis of uncertainty in the aerosol effective radiative forcing. Each ensemble consists of around 200 simulations of the preindustrial and present‐day atmospheres. The uncertainty in aerosol radiative forcing in our ensembles is comparable to the range of estimates from multimodel intercomparison projects. The mean aerosol effective radiative forcing is −1.45 W/m2 (credible interval of −2.07 to −0.81 W/m2), which encompasses but is more negative than the −1.17 W/m2 in the 2013 Atmospheric Chemistry and Climate Model Intercomparison Project and −0.90 W/m2 in the Intergovernmental Panel on Climate Change Fifth Assessment Report. The ensembles can be used to reduce aerosol radiative forcing uncertainty by challenging them with multiple measurements as well as to isolate potential causes of multimodel differences.
Highlights
Earth's radiative energy balance is strongly affected by atmospheric aerosol particles that directly scatter and absorb solar and terrestrial radiation and indirectly modulate the radiative properties of clouds
The comparable magnitude of aerosol RF and ERF uncertainty in the two samples suggests that in the absence of rapid atmospheric adjustments the aerosol parameters included only in AER (Table S3) cause as much additional uncertainty as the physical atmosphere parameters included in AER‐ATM (Table S4)
We have presented two perturbed parameter ensemble (PPE) of the Hadley Centre Global Environment Model (HadGEM) atmosphere‐only global climate model coupled to the UKCA chemistry and aerosol model
Summary
Earth's radiative energy balance is strongly affected by atmospheric aerosol particles that directly scatter and absorb solar and terrestrial radiation and indirectly modulate the radiative properties of clouds. Despite major improvements in global observing systems and climate models representing aerosols more realistically, the estimated net aerosol RF over the industrial period ranges from near zero to around −2 W/m2 (Boucher et al, 2013). This uncertainty has persisted through all intergovernmental panel assessment reports since 1996 and significantly limits our ability to understand the causes of historical climate change and our confidence in climate change projections (Andreae et al, 2005; Collins et al, 2013; Schwartz, 2018; Seinfeld et al, 2016). While our knowledge about aerosols and the driving processes has improved, this has not yet been translated into more reliable climate model simulations
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