Abstract
Abstract. Aerosol measurements over the Southern Ocean are used to constrain aerosol–cloud interaction radiative forcing (RFaci) uncertainty in a global climate model. Forcing uncertainty is quantified using 1 million climate model variants that sample the uncertainty in nearly 30 model parameters. Measurements of cloud condensation nuclei and other aerosol properties from an Antarctic circumnavigation expedition strongly constrain natural aerosol emissions: default sea spray emissions need to be increased by around a factor of 3 to be consistent with measurements. Forcing uncertainty is reduced by around 7 % using this set of several hundred measurements, which is comparable to the 8 % reduction achieved using a diverse and extensive set of over 9000 predominantly Northern Hemisphere measurements. When Southern Ocean and Northern Hemisphere measurements are combined, uncertainty in RFaci is reduced by 21 %, and the strongest 20 % of forcing values are ruled out as implausible. In this combined constraint, observationally plausible RFaci is around 0.17 W m−2 weaker (less negative) with 95 % credible values ranging from −2.51 to −1.17 W m−2 (standard deviation of −2.18 to −1.46 W m−2). The Southern Ocean and Northern Hemisphere measurement datasets are complementary because they constrain different processes. These results highlight the value of remote marine aerosol measurements.
Highlights
The uncertainty in the magnitude of the effective radiative forcing caused by aerosol–cloud interactions (ERFaci) due to changing emissions over the industrial period is around twice that for CO2 (Stocker et al, 2013)
Estimates of aerosol forcing over the industrial period rely on models that have been evaluated against measurements made in the present-day atmosphere
It is known that the aerosol forcing depends sensitively on the state of aerosols in the pre-industrial period (Carslaw et al, 2013; Wilcox et al, 2015) when natural aerosols were dominant (Carslaw et al, 2017)
Summary
The uncertainty in the magnitude of the effective radiative forcing caused by aerosol–cloud interactions (ERFaci) due to changing emissions over the industrial period is around twice that for CO2 (Stocker et al, 2013). In this paper we address the following questions: (i) to what extent can measurements of aerosols in pristine (natural) environments help to constrain model simulations and thereby reduce the large uncertainty in aerosol forcing? Previous analysis of HadGEM3 PPEs showed that aerosol measurements in polluted regions help to constrain the uncertainty in aerosol–radiation interaction forcing (RFari) but not the component due to aerosol–cloud interactions (RFaci) (Johnson et al, 2020).
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