Abstract
The Mediterranean Sea, one of the regions warming fastest under climate change(1,2), harbours lush seagrass (Posidonia oceanica) meadows that form the basis for a key ecosystem in the region(3). Recent field results have shown that increased maximum annual seawater temperature in the Mediterranean has already led to increased seagrass mortality(4). Here we project the trajectory of P. oceanica meadows under the warming expected in the western Mediterranean through the twenty-first century to conclude that warming will lead to the functional extinction of P. oceanica meadows by the middle of this century (year 2049 +/- 10) even under a relatively mild greenhouse-gas emissions scenario. Efforts to alleviate local stresses adding to the loss of P. oceanica meadows will have a limited effect in conserving the meadows under climate change. Efforts to mitigate climate change are urgently needed to preserve this key ecosystem.