UC San Diego

Marine species redistribution projections typically rely on long-term climatological means, potentially overlooking the role of thermal extremes in shaping habitat. We investigated how four marine megafauna species respond to long-term warming and interannual thermal extremes in the historical (1991-2020) and future (2071-2100) periods, asking whether the thermal extremes drive habitat redistribution comparable to or exceeding the effects of long-term mean warming. Focused on the California Current Ecosystem (CCE; 116-134°W, 30-48°N), we assessed habitat suitability for California sea lion (Zalophus californianus), humpback whale (Megaptera novaeangliae), leatherback sea turtle (Dermochelys coriacea), swordfish (Xiphias gladius), derived from species distribution models forced by three regionally-downscaled Earth System Models. We identified thermal extremes as the 10% warmest and coolest months within each 30-year period and quantified biogeographic responses to long-term warming and historical and future thermal extremes using displacement distance, displacement direction, and change in core habitat area. We found that future thermal extremes drive range centroid displacements comparable to or exceeding long-term warming for three of four species and produce onshore and equator-ward redistribution of California sea lion and humpback whale habitat—a directional pattern absent from historical extremes and, for humpback whales, from long-term warming. Leatherback sea turtle and swordfish habitat show consistent poleward and offshore displacement across all scenarios. Changes in core habitat area vary considerably across species and warming drivers, with no driver consistently dominant. Interannual thermal extremes represent an under-appreciated driver of marine megafauna habitat redistribution that may not be fully anticipated from long-term mean projections alone.