The dim and distant past: Constraining aerosol forcing history in the 20th century

Out of the multiple natural and anthropogenic factors affecting climate, greenhouse gases (GHGs) and aerosols from human activity dominate over the industrial era. The warming effect of the accumulating GHGs in the atmosphere is partly compensated by the aerosol cooling effect by reflecting incoming energy from the sun, and the historical disentanglement of these two effects can be used to estimate the future warming potential. This doctoral work aids the disentanglement through investigating atmospheric aerosol concentrations and effects over the industrial era. Initial findings show that a subset of state-of-the-art earth system models do not recreate aerosol effects as we see recorded by ground based instruments. To test whether this discrepancy is caused by errors in the aerosol emission inventories (which are input to these models) we have compared aerosol concentrations in ice cores to that in models and again found discrepancies. Since the discrepancy was most noticeable for Black Carbon (BC), we further investigated the model differences in BC treatment and found a large range in BC atmospheric lifetime. The longer the aerosol lifetime in the atmosphere is, the larger is the effect, meaning some models overestimate BC effects, despite the initial findings showing models in general underestimate aerosol effects, which underlines the complexity of aerosols' role and effects within the climate system