The Separate Influence of Anthropogenic Aerosols and Greenhouse Gases on Forced Changes in the Global Energy and Water Cycles

Human activities have substantially altered the radiative properties of the atmosphere, giving rise to a planetary energy imbalance. The largest contributor to this imbalance is well-mixed greenhouse gases (GHGs), which are partially offset by poorly-mixed (and thus northern mid-latitude dominated) anthropogenic aerosols (AAs). To isolate the effects of GHGs and AAs, we analyze data from the CMIP5 historical (i.e. all natural and anthropogenic forcing) and single forcing (GHG-only and AA-only) experiments. We find that over the duration of the historical experiment (1861-2005), global excess heat uptake at the top of the atmosphere and ocean surface occurs almost exclusively in the Southern Hemisphere (SH). The influence of GHGs is similar for both hemispheres, with AAs completely offsetting that influence in the Northern Hemisphere (NH) but having little impact in the SH. The interplay between GHG and AA forcing also explains key features within each hemisphere, with AAs offsetting GHG-forced local maxima in ocean heat uptake in the NH but not SH. The AA-induced interhemispheric asymmetry in surface heat uptake is eliminated by a northward oceanic transport of excess heat, as there is little hemispheric difference in historical ocean heat storage after accounting for ocean volume. Data from the 1pctCO2 and RCP 8.5 experiments suggests that the future storage of excess heat will be skewed toward the NH oceans. To compliment this analysis of changes to the global energy budget, initial results from a similar analysis of water cycle changes in the CMIP5 (and CMIP6 as they become available) single forcing experiments will be presented. Together, these single forcing analyses provide potential new insights into the fundamental mechanisms behind many aspects of both simulated and observed regional climate change.