気候変動：AMOC（大西洋子午面循環）は将来の温暖化に耐えられるかもしれない

The Atlantic Meridional Overturning Circulation (AMOC) may be able to withstand future global warming and could avoid collapse, a modelling study presented in Nature suggests. This important ocean current has a role in regulating temperatures around the planet. The findings indicate that although it is likely the circulation will weaken under global warming scenarios, other ocean processes could prevent its collapse. Further research is needed to understand the interactions between these processes.

The AMOC is important for northward heat transport in the Atlantic and has a critical role in heat transfer and carbon uptake, which have implications for global and regional climate change. Projections have suggested that the circulation system could weaken due to an increase in greenhouse gas concentrations and a freshwater influx as a result of meltwater from the Greenland Ice Sheet and precipitation changes linked to climate changes. However, these projections have varied and the future evolution of the AMOC is unclear.

Jonathan Baker and colleagues used 34 models from the IPCC’s Coupled Model Intercomparison Project Phase 6 to assess the AMOC’s response to extreme changes in greenhouse gas concentrations and North Atlantic freshwater levels. Under these conditions the authors found that the AMOC is resilient to climate changes and did not collapse. They suggest that the AMOC continues in a weakened state that levelled off across all the models they considered and propose that upwelling of North Atlantic deep water driven by winds in the Southern Ocean sustains the circulation and prevents its collapse. The authors note that this upwelling must be balanced by downwelling in the Atlantic or Pacific oceans and the AMOC can only collapse if a Pacific overturning circulation (PMOC) develops. While this circulation does develop in all the models they considered, they show that it is too weak to balance the Southern Ocean upwelling, indicating that the collapse of the AMOC is unlikely in this century.

Baker and colleagues conclude that their findings demonstrate the need for further research into the Southern Ocean and Indo-Pacific circulations to understand their implications for maintaining the AMOC to accurately predict its future.