Impact of deoxygenation and warming on global marine species in the 21st century

Abstract. Ocean temperature and dissolved oxygen shape marine habitats in interplay with species’ physiological characteristics. Therefore, the observed and projected warming and deoxygenation in the 21st century of the world’s oceans may strongly affect species’ habitats. Here, we implement an extended version of the Aerobic Growth Index (AGI), which quantifies whether a viable population of a species can be sustained in a particular location. We assess the impact of projected deoxygenation and warming on the contemporary habitat of 47 representative marine species covering the epipelagic, mesopelagic/bathypelagic, and demersal realms. AGI is calculated for these species for the historical period and into the 21st century using bias-corrected environmental data from six comprehensive Earth System Models. While habitat viability decreases nearly everywhere with global warming, impact of this decrease is strongly species-dependent. Most species lose less than 5 % of their contemporary habitat volume over the 21st century even at 3 °C of global warming relative to preindustrial, although some individual species are projected to incur losses 2–3 times greater than that. We find that the contemporary spatiotemporal variability of O₂ and temperature (and hence AGI) provides a quantifiable measure of a species’ vulnerability to change. Species’ vulnerability is the most important indicator for large (>5 %) potential habitat losses – not relative or absolute changes in habitat viability (i.e., AGI^rel or ΔAGI), temperature or O₂. Loss of contemporary habitat is for most epipelagic species driven by warming of ocean water and is therefore expanded with increased levels of global warming. In the mesopelagic/bathypelagic and demersal realms habitat loss is also affected by pO₂ decrease for some species. Our analysis is constrained by the uncertainties involved in species-specific critical thresholds, which we quantify, by data limitations on 3D species distributions as well as by high uncertainty in model O₂ projections in equatorial regions. Focus on these topics in future research will strengthen our confidence in assessing climate-change driven losses of contemporary habitat across the global oceans.