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

Abstract. Ocean temperature and dissolved oxygen shape marine habitats in an interplay with species' physiological characteristics. Therefore, the observed and projected warming and deoxygenation of the world's oceans in the 21st century 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, 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, the impact of this decrease is strongly species dependent. Most species lose less than 5 % of their contemporary habitat volume at 2 ∘C of global warming relative to preindustrial levels, although some individual species are projected to incur losses 2–3 times greater than that. We find that the in-habitat spatiotemporal variability of O2 and temperature (and hence AGI) provides a quantifiable measure of a species' vulnerability to change. In the event of potential large habitat losses (over 5 %), species vulnerability is the most important indicator. Vulnerability is more critical than changes in habitat viability, temperature, or pO2 levels. Loss of contemporary habitat is for most epipelagic species driven by the warming of ocean water and is therefore elevated with increased levels of global warming. In the mesopelagic 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; and by high uncertainty in model O2 projections in equatorial regions. A focus on these topics in future research will strengthen our confidence in assessing climate-change-driven losses of contemporary habitats across the global oceans.