The largest contributor to the planetary energy 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. Over the duration of the historical experiment (1861-2005) excess heat uptake at the top of the atmosphere and ocean surface occurs almost exclusively in the Southern Hemisphere, with AAs canceling the influence of GHGs in the Northern Hemisphere. This 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 towards the Northern Hemisphere oceans.
Plain Language Summary
Climate change is fundamentally an energy balance problem. Due to the influence of greenhouse gas (GHG) emissions, the amount of solar energy absorbed by Earth is currently greater than the amount of energy radiated to space. This energy imbalance is partially offset by particulate matter released into the atmosphere from burning fossil fuels (anthropogenic aerosols; AAs), which is most concentrated in the northern mid-latitudes. In this study, model simulations of the historical and future climate are compared to single forcing simulations that apply only GHG or AA emissions. We find that the historical uptake of excess heat is strongly skewed towards the Southern Hemisphere because AAs cancel the influence of GHGs in the Northern Hemisphere. The oceanic storage of that heat shows little difference between the hemispheres due to a strong northward transport of excess heat. In future, the models suggest excess heat storage will skew towards the Northern Hemisphere.
Key Points
Single forcing simulations of the historical (1861-2005) climate suggest anthropogenic aerosols cause most excess heat uptake to occur in the Southern Hemisphere
This interhemispheric asymmetry in heat uptake is eliminated (after accounting for ocean volume) by a northward oceanic transport of excess heat
In future, the storage of excess heat will be skewed towards the Northern Hemisphere