Impacts and reversibility of meltwater-induced future Atlantic Meridional Overturning Circulation changes

The Atlantic Meridional Overturning Circulation (AMOC) is projected to weaken in the future due to increasing greenhouse gas concentrations, but it is still debated whether anthropogenic climate change can induce an irreversible collapse or “tipping” of the AMOC. Meltwater from the Greenland ice sheet has often been invoked as a key mechanism for a potential AMOC tipping, but it is not explicitly represented in most state-of-the-art (CMIP6) climate models, adding further uncertainty to assessing the likelihood of irreversible AMOC change.

Here, we perform ensemble simulations with the CMIP6 model EC-Earth3 to assess the effects of future Greenland ice sheet melt and to probe AMOC reversibility with and without Greenland meltwater. To this end, we force EC-Earth3 with a strong global warming scenario (SSP5-8.5) and a high-end Greenland meltwater estimate from the coupled climate–ice sheet model CESM2-CISM2 until 2300.

We find that, as expected, the addition of Greenland meltwater significantly exacerbates the greenhouse gas-induced AMOC weakening especially after the 21st century, with differences mostly attributable to the Arctic Ocean. However, we find no indication of an abrupt AMOC weakening. We then branch off idealized reversibility experiments in which the meltwater and/or greenhouse gas forcings are reversed. Although the AMOC recovery is slow (around two centuries), meltwater-driven additional AMOC weakening in EC-Earth3 appears to be reversible. Regardless of the added meltwater, the AMOC also recovers in an idealized CO2 ramp-down experiment, even overshooting its present-day strength. While our modeling results show little support for an irreversible AMOC change due to future Greenland ice sheet melt, they do underline the importance of representing meltwater in future projections, including overshoot pathways.