Although extinctions due to climate change are still uncommon, they might surpass those caused by habitat loss or overexploitation over the next few decades. Among marine megafauna, mammals fulfill key and irreplaceable ecological roles in the ocean, and the collapse of their populations may therefore have irreversible consequences for ecosystem functioning and services. Using a trait-based approach, we assessed the vulnerability of all marine mammals to global warming under high and low greenhouse gas emission scenarios for the middle and the end of the 21 st century. We showed that the North Pacific ocean, the Greenland Sea and the Barents Sea host the species that are most vulnerable to global warming. future conservation plans should therefore focus on these regions, where there are long histories of overexploitation and there are high levels of current threats to marine mammals. Among the most vulnerable marine mammals were several threatened species, such as the North Pacific right whale (Eubalaena japonica) and the dugong (Dugong dugon), that displayed unique combinations of functional traits. Beyond species loss, we showed that the potential extinctions of the marine mammals that were most vulnerable to global warming might induce a disproportionate loss of functional diversity, which may have profound impacts on the future functioning of marine ecosystems worldwide. During the past few decades, the Earth has entered a new era of rapid and potentially irreversible climate warming due to the positive radiative imbalances triggered by greenhouse gas emissions from human activities 1,2. The oceans have taken up 93% of the extra energy 1 that has been accumulated in the Earth system in recent decades, and its temperature has increased much faster since 1991 than has been recorded previously 2. In addition, changes in ocean temperature also affect the sea level, sea ice extent and salinity (through changes in precipitation and evaporation). All these changes were found to have negative impacts on marine biota 3,4 and especially on marine mammals (e.g. 5-10). One of the most common responses of marine mammals to temperature changes is shifts in their spatial distributions, which could result in modifications of the ranges of the species (e.g. 11-13). For example, Bryde's whales (Balaenoptera brydei), a widely spread subtropical and tropical species, was increasingly detected in the cooler waters off southern California during the period from 2000-2010 14. In contrast, the white-beaked dolphin (Lagenorhynchus albirostris), a cold-water species, has reduced its range and is declining in abundance 12. As mentioned by Elliott & Simmonds 12 , geography could constrain species with low dispersal ability to a particular area. For example, the distribution of an endemic species of porpoise, the vaquita (Phocoena sinus), is limited to the northern end of the Gulf of California. Changes in water temperatures could alter the life cycles of the prey of marine mammal and provoke mismatches between the abundance of prey and ...
