The collapse of the main water circulation system in the Earth's oceans is already underway

Today, the Atlantic meridional overturning circulation is the main driver of northward heat transport in the Atlantic Ocean and creates global climate patterns. Whether global warming has affected the strength of this overturning circulation over the past century is still debated: observational studies suggest a persistent weakening since the mid-twentieth century, while climate models systematically simulate a stable circulation. Here, using Earth system and eddy-allowing ocean–sea ice models, we show that freshening of the subarctic Atlantic Ocean and weakening of the overturning circulation increase the temperature and salinity of the South Atlantic on a decadal timescale through Kelvin and Rossby wave propagation. We also show that accounting for the upper meltwater input in the historical simulations significantly improves the model fit to data on past changes in the Atlantic meridional overturning circulation, yielding a slowdown of 0.46 sverdrups per decade since 1950. Including estimates of subarctic meltwater inputs for the coming century suggests that this circulation could be 33 % weaker than its anthropogenically undisturbed state below the 2°C of global warming that can be achieved within the next decade. Such a weakening of the overturning circulation would significantly affect the climate and ecosystems. (Gabriel M. Pontes & Laurie Menviel , more at earth.com)