Observed and projected global warming pressure on coastal hypoxia

Abstract

Coastal hypoxia is a major environmental problem of increasing severity. A global 40-year observational gridded climate data record and 21st-century projections from the Community Earth System Model (CESM) under RCP8.5 (Representative Concentration Pathway) forcing are analyzed for long-term linear trends in summer-month conditions, with a focus on warming-related pressures on coastal oxygen levels. Projected surface temperature and oxygen conditions are compared to global observations over the 16-year overlapping period (2006–2021). Median linear trends for 2006–2100 along the global coast are 0.32 ∘C, −1.6, and −1.2 mmol m−3 per decade for sea-surface temperature (SST), oxygen saturation concentration at the surface (surface oxygen capacity), and vertical-minimum oxygen concentration, respectively. These trends point to more rapid deterioration in coastal conditions than experienced over recent decades; the projected median coastal trends for SST and oxygen capacity are 148 % and 118 % of the corresponding observed rates. Companion analysis of other models and climate scenarios indicates projected coastal oxygen trends for the more moderate RCP4.5 and updated SSP5–8.5 (Shared Socioeconomic Pathway) scenarios, respectively, are 37 %–77 % and 103 %–196 % of the CESM RCP8.5 projections. Median rates for the coast and documented hypoxic areas are higher than in the global ocean. Warming and oxygen declines tend to be fastest at high latitudes, one region where new hypoxic areas may emerge as oxygen conditions deteriorate. There is considerable pressure on current hypoxic areas, since future oxygen declines of any magnitude will make hypoxia more severe. The projections can inform coastal environmental management strategies to protect future water quality and ecosystem services.