Southern Ocean Control of 2°C Global Warming in Climate Models

Global warming will soon reach the Paris Agreement targets of 1.5 degrees C/2 degrees C temperature increase above pre-industrial levels. Under a business-as-usual scenario, the time to reach these targets varies widely among climate models. Using Coupled Model Intercomparison Project Phase 5 and 6, we show that a 2 degrees C global warming is determined by Southern Ocean (SO) state closely tied with a low-level cloud (LLC) amount feedback strength during reference (1861-1900) period; climate models with cold SO tend to accompany more low-level cloudiness and Antarctic sea ice concentration due to a strong LLC amount feedback. Consequently, initially cold SO models tend to simulate a fast global warming by absorbing more downward shortwave radiation compared to initially warm SO models because more LLC disappears due to a strong LLC amount feedback during the 2 degrees C rise. Our results demonstrate that climate models that correctly simulate initial SO state can improve 2 degrees C warming projections with reduced uncertainties. Plain Language Summary In December 2015 at Paris, United Nations agreed to hold the increase in the global average temperature to "well below" 2 degrees C above pre-industrial levels and pursuing efforts to limit 1.5 degrees C above pre-industrial levels. It naturally leads to a question as to when these targets will reach. However, under a business-as-usual scenario, the time to reach these targets varies widely among climate models. Using Coupled Model Intercomparison Project Phase 5 and 6, we show that a 2 degrees C global warming is closely related to the late 19th century condition of Southern Ocean (SO) state such that the initially cold SO climate models actually produced fast warming rate and vice versa. This is because these initially cold SO climate models that mostly accompany more low-level cloudiness and Antarctic sea ice concentration, could actually absorb more downward shortwave radiation by reducing cloudiness and sea-ice during a warming progress compared to initially warm SO models. Finally, our results demonstrate that climate models that correctly simulate initial SO state can improve 2 degrees C warming projections with reduced uncertainties.