A dangerous synergy of warming and hypoxia: the past and present for the future

Abstract

Anthropogenic increases in atmospheric CO2 levels are raising seawater temperatures, lowering seawater oxygen concentrations and pH (‘climate-related stressors’, CRS), among other changes. Under such change, some marine groups, such as stony corals, may have less ability to tolerate stress than others, potentially leading to population loss and thereby extinction. However, whether these vulnerabilities directly scale up to extinction risk, or tend to be somehow compensated e.g. by rapid range shifts, is unclear because few modern, global extinctions are unequivocally caused by CRS. We use meta-analysis to attempt a quantitative cross-scale evaluation of the role of CRS as extinction triggers. We do this by comparing individual responses to CRS in laboratory experiments and the range shift responses to global warming over recent decades, to extinction selectivity at ancient hyperthermal events and across the sum of metazoan evolutionary history, the Phanerozoic aeon. Clades and traits with strong negative responses to experimental CRS are most likely to go extinct at hyperthermal events, their genera also having the shortest geological durations. Among CRS, the synergistic combination of seawater warming and hypoxia has the most severe impact on marine organism performance. Response details suggest chiefly metabolic constraints in setting organism performance responses to CRS, which appear to scale up to extinction probabilities of past (and potentially future?) hyperthermal events. Ecological experiments are the only way to test underlying physiological mechanisms. Meanwhile, the fossil record documents myriad extinctions, often with strong selectivity patterns, that amount to mass proportions under hyperthermal conditions. Uniting the strengths of the two approaches may enrich our abilities to predict the future.