Coupled Climate Stressors along the West Coast of North America: Drought, Marine Heat Waves, HABs, and Hypoxia

There is growing concern that climate change is increasing variability in earth systems, resulting in more extreme weather and ecosystem events. Marine heat waves, severe drought, and short-term bouts of hypoxia and harmful algal blooms (HABs) in the coastal marine environment have gripped the western United States in recent years, causing a variety of biological impacts and socio-economic hardships. While each of these extreme events might be a chance occurrence on its own under natural climate forcing, their current spatiotemporal coincidence may indicate that the recent events are attributable to anthropogenic climate change. The simultaneity of the stressors in these coupled terrestrial and marine ecosystems may stimulate combined ecological catastrophes in western states that are unlike those experienced previously. We hypothesize that the synchrony of fires and drought (and hence, poor air quality) in the terrestrial realm are mechanistically coupled to episodes of hypoxia, harmful algal blooms, heat waves, and acidification in the coastal marine environment through large-scale atmospheric forcing. To test this hypothesis, we use the ensemble of Earth System Model output produced as part of the CMIP6 effort and explore three metrics of change in ecosystem conditions: 1) change in mean state; 2) change in the interannual variability of extremes (both frequency and intensity); and 3) shifts in the seasonal cycle. Each of these characteristics can have important (and distinct) implications for the marine and terrestrial events considered here. Anthropogenic changes in the likelihood of extreme events and anomalies in the seasonal cycle are assessed using “fraction of attributable risk” analyses. Improved understanding of the nature of these events under future climate forcing—particularly the chances that they will occur simultaneously in marine, freshwater, and terrestrial systems—can inform efforts to adapt and prepare for such catastrophes.