The study of past climates has demonstrated the occurrence of Heinrich events, during which major ice discharges occurred at the level of the northern polar ice cap. These huge ice break-ups in the North Atlantic, combined with the melting of icebergs reaching as far as the coast of Portugal, all had the characteristic of collapsing the Atlantic Meridional Overturning Circulation (AMOC). These oceanic and then atmospheric reorganizations had an impact on global climate as far away as the Southern Ocean and central China.
We know that human activities such as industry, transport and agriculture release greenhouse gases like CO2, leading to global warming. This global warming can lead to the partial melting of the Antarctic and Greenland ice sheets.
This melting is generally simulated as taking place over centuries (see Fig. 8 of the 6th IPCC report). Many studies consider ice caps to be tipping points with global impacts, the thresholds of which could be crossed as soon as temperatures rise by +1.5°C compared with the pre-industrial period. However, standard climate change scenarios (Representative Concentration Pathways or RCPs) do not take into account accelerated melting of the ice caps, which could produce catastrophic events affecting ocean circulation, such as Heinrich events. RCPs consider only the dynamic evolution of greenhouse gas emissions.
In this study, we aim to assess the impact of a rapid partial melting of the Greenland and West Antarctic ice sheets on climate and monsoon systems, at global and regional scales over the 21st century. We performed simulations with freshwater release using the Institut Pierre Simon Laplace (IPSL-CM5A) low-resolution coupled global climate model to simulate rapid melting of the Greenland and West Antarctic ice sheets, equivalent to +1 and +3 meters of sea-level rise over 50 years (2020-2070). These freshwater inflows have been added to the standard RCP8.5 emissions scenario for the 21st century. Partial melting of the Greenland and West Antarctic ice caps leads to local cooling in areas of water inflow, but has different global impacts.
West Antarctica, located in the polar region and surrounded by the circumpolar ocean current, sees its freshwater supply diluted by this strong current. As a result, its overall impact is moderate. Greenland, on the other hand, is located in the sub-polar region. When it melts, it releases freshwater directly into the North Atlantic, slowing down the AMOC. This slowdown leads to changes in winds, inter-hemispheric temperature and pressure gradients, resulting in a southward shift of the tropical rain belt over the Atlantic region to the eastern Pacific. The American and African monsoons are strongly affected, moving southwards. Precipitation seasonality is affected by the melting of Greenland ice. The North American monsoon starts later, while the South American monsoon starts earlier. The North African monsoon is drier during the boreal summer, while the South African monsoon intensifies during the austral summer. The simulated changes are not significant for the Asian and Australian monsoons.
Autors: Alizée Chemison, Dimitri Defrance, Gilles Ramstein and Cyril Caminade