UC Riverside

This dissertation is composed of three parts. In the first part, I investigate the anthropogenic aerosol burden response to future warming perturbations using climate models. Many climate models simulate an increase in anthropogenic aerosol species in response to warming. This is primarily due to a decrease in wet removal associated with reduced precipitation. The enhanced aerosol burden and hydrological changes are related to a robust climate change phenomenon—the land–sea warming contrast. Enhanced land warming is associated with continental reductions in lower-tropospheric humidity that drive decreases in low clouds, which leads to reduced large-scale precipitation and aerosol wet removal. Thus, unless anthropogenic emission reductions occur, a warmer world will be associated with enhanced aerosol pollution.

In the Second part, I explore the mechanisms that drive the Atlantic meridional overturning circulation (AMOC) based on the Coupled Model Intercomparison Project phase 6 (CMIP6) models. The CMIP6 all-forcing simulations show a robust AMOC strengthening during ∼1950−1990 and weakening afterwards (∼1990−2020). These multi-decadal AMOC variations are related to changes in North Atlantic atmospheric circulation, which drive changes in the subpolar North Atlantic surface density flux. CMIP6 anthropogenic aerosol forced simulations yield a similar AMOC including associated atmospheric circulation responses. I conclude that the CMIP6 models yield robust, externally forced AMOC changes, the bulk of which are due to anthropogenic aerosols.

Finally, I quantify the impact of near-term climate forcer (NTCF) mitigationincluding aerosols and chemically reactive gases such as tropospheric ozone and methane-on the AMOC using four chemistry-climate models. Non-methane NTCF mitigation, including aerosols, ozone and precursor gases alone, will amplify greenhouse gas-induced weakening of the AMOC. However, all-NTCF mitigation, which also includes methane reductions, more than offset this weakening. Thus, efforts to improve air quality must also target methane to avoid additional climate change, including weakening of world’s major ocean circulation system.