The uncertainty in radiative forcing due to anthropogenic aerosol has been intractable for the last 20 years, and is the dominant uncertainty in estimates of global radiative forcing. Uncertainty in aerosol forcing has implications for centennial scale climate projections, where it limits our ability to estimate climate sensitivity. It is also important for near-term (20-40 year) projections, where the substantial spread in aerosol emission pathways is compounded by uncertainty in the response to these emissions. In addition to there being large uncertainties in the forcing itself, uncertainties in the dynamical response to forcing contribute to large overall uncertainty in the regional climate response to anthropogenic aerosol changes. This combination of scenario, radiative, and dynamical uncertainty makes anthropogenic aerosol the main factor in the spread in near-term climate projections, and an important consideration for meeting the targets of the Paris Agreement. In this talk I will describe the influence of aerosol forcing uncertainty on simulations of historical climate with HadGEM3-GC3.1, and show our recent attempt to constrain estimates of historical aerosol forcing. I will then turn to the future and examine the role of aerosol forcing uncertainty, and scenario uncertainty, in projections of climate to 2050. Aerosol scenario uncertainty is particularly pronounced over Asia, and I will use the latter stages of the talk to show some of the effects of uncertainty in the parameterisation of aerosol-cloud interactions, uncertainty in dynamical feedbacks, and potential feedbacks between the responses to regional emission changes, on near-term projections in this region.
GFDL Informal Seminar
Tue, Dec 3, 2019, 10:30 am to 11:30 am
Smagorinsky Seminar Room 209