Abstract/Summary

Aerosols overall have a strong cooling effect on Earth’s climate that masks warming from greenhouse gases, but likely reductions in future aerosol emissions will reduce this cooling. The magnitude of this reduction is uncertain however, partly because of the complex radiative effects of aerosol. To reduce the uncertainty of future climate change requires improved understanding and quantification of the mechanisms and magnitudes of aerosol radiative effects, particularly radiative adjustments. This thesis investigates variations on two existing methods of decomposing aerosol radiative effects, applying them to pre-industrial (1850) to present-day (2014) perturbations of anthropogenic sulphate and black carbon aerosol emissions using the UK Earth System Model (UKESM1). The first method applies nudging to model horizontal winds or horizontal winds and potential temperature to decompose aerosol effects mechanistically into circulation-mediated and atmospheric temperature-mediated adjustments. The second method, a variation of the partial radiative perturbation (PRP) method, decomposes aerosol radiative effects into contributions from individual atmospheric variables while conserving the total radiative effect. Circulation-mediated adjustments are found to be significant for sulphate (0.10±0.08 W m−2 ; 7% of ERF) but not black carbon (-0.04±0.07 W m−2 ; 8% of ERF) perturbation, while atmospheric temperature-mediated adjustments are significant for both sulphate (0.14±0.04 W m−2 ; 10% of ERF) and black carbon (-0.25±0.08 W m−2 ; 47% of ERF) perturbations, with both arising primarily from cloud adjustments. However, significant uncertainties are found with the nudging method that reduce confidence in quantification of temperature-mediated adjustments. The partial radiative perturbation method is difficult to implement but the variation applied here successfully conserves the total radiative effect, offering improvements over the existing method. Both methods offer insights into aerosol radiative effects and address gaps in existing techniques, but the nudging method suffers significant limitations. Isolating aerosol circulation-mediated adjustments does however help fill a gap in the literature by indicating that applying nudging to reduce simulation integration lengths omits significant adjustments due to suppressing circulation responses.