A weakened AMOC may prolong greenhouse gas–induced Mediterranean drying even with significant and rapid climate change mitigation

Significance

The Mediterranean is a projected hot spot for climate change, with significant warming and rainfall reductions. We use climate model ensembles to explore whether these Mediterranean rainfall declines could be reversed in response to greenhouse gas reductions. While the summer Mediterranean rainfall decline is reversed, winter rainfall continues to decline. The continued decline results from prolonged weakening of Atlantic Ocean poleward heat transport that combines with greenhouse gas reductions to cool the subpolar North Atlantic, inducing atmospheric circulation changes that favor continued Mediterranean drying. This is a potential “surprise” in the climate system, whereby changes in one component (Atlantic Ocean circulation) alter how another component (Mediterranean rainfall) responds to greenhouse gas reductions. Such surprises could complicate climate change mitigation efforts.

Abstract

The Mediterranean region has been identified as a climate hot spot, with models projecting a robust warming and rainfall decline in response to increasing greenhouse gases. The projected rainfall decline would have impacts on agriculture and water resources. Can such changes be reversed with significant reductions in greenhouse gases? To explore this, we examine large ensembles of a high-resolution climate model with various future radiative forcing scenarios, including a scenario with substantial reductions in greenhouse gas concentrations beginning in the mid-21st century. In response to greenhouse gas reductions, the Mediterranean summer rainfall decline is reversed, but the winter rainfall decline continues. This continued winter rainfall decline results from a persistent atmospheric anticyclone over the western Mediterranean. Using additional numerical experiments, we show that the anticyclone and continued winter rainfall decline are attributable to greenhouse gas–induced weakening of the Atlantic Meridional Overturning Circulation (AMOC) that continues throughout the 21st century. The persistently weak AMOC, in concert with greenhouse gas reductions, leads to rapid cooling and sea ice growth in the subpolar North Atlantic. This cooling leads to a strong cyclonic atmospheric circulation anomaly over the North Atlantic subpolar gyre and, via atmospheric teleconnections, to the anticyclonic circulation anomaly over the Mediterranean. The failure to reverse the winter rainfall decline, despite substantial climate change mitigation, is an example of a “surprise” in the climate system. In this case, a persistent AMOC change unexpectedly impedes the reversibility of Mediterranean climate change. Such surprises could complicate pathways toward full climate recovery.