Reply on RC2

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Ocean temperature and dissolved oxygen shape marine habitats in interplay with species&rsquo; physiological characteristics. Therefore, the observed and projected warming and deoxygenation in the 21st century of the world&rsquo;s oceans may strongly affect species&rsquo; 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 &deg;C of global warming relative to preindustrial, although some individual species are projected to incur losses 2&ndash;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&rsquo; vulnerability to change. Species&rsquo; vulnerability is the most important indicator for large (&gt;5 %) potential habitat losses &ndash; not relative or absolute changes in habitat viability (i.e., AGI^rel or &Delta;AGI), 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 <em>p</em>O₂ 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.