Marine Heat Waves and Ocean Acidity Extremes Often Strike Together, Study Finds

Marine heat waves and extreme ocean acidification events each cause damage on their own, but new research shows their effects can worsen when both strike at once. The clearest example is "the Blob," a heat wave and acidification event that hit the northeastern Pacific from 2013 to 2015, closing fisheries, stranding marine mammals, killing seabirds, and forcing sea creatures to shift range or die off.

Researchers Luke Gregor and Gruber analyzed 43 years of monthly data, from 1982 to 2024, on surface ocean temperature and acidity to determine when, where and why these "compound" heat-acidity extreme events occur. They defined an extreme event as one where detrended acidity and temperature both exceeded their 95th percentile. The study, published in the journal AGU Advances, found that compound events happen roughly four times more often than expected by chance in low- and mid-latitude regions, mostly in places with permanent stratification, where heat waves also drive surface waters to become more acidic.

Compound events were least common in the eastern equatorial Pacific and near the poles, where deep water normally upwells to the surface. When a marine heat wave hits these regions, a warm surface layer blocks the acidic deep water from rising, producing unusually low acidity instead of a compound extreme.

Most compound events were small and brief: 73% covered less than 500,000 square kilometers, about the size of Spain, and lasted roughly a month. A few, including the 2015 Blob, lasted more than a year, with long-lasting consequences.

El Niño and La Niña are important drivers of compound events, though the events rarely occur right at the equatorial Pacific epicenter of those patterns. During El Niño, warmer waters form a lens that blocks upwelling and lowers the region's typical acidity. During La Niña, cold, more acidic deep water rises, producing an acidification extreme alone. In neighboring regions, though, the knock-on warming and acidity effects trigger compound extremes.