Why marine heat waves and acidification strike together

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Add as preferred source Marine heat waves and extreme ocean acidification events are each damaging on their own. But when both stressors hit simultaneously, their individual effects can be exacerbated.

That threat may be best exemplified by "the Blob" of 2013–2015, during which both a marine heat wave and an extreme ocean acidification event hit the northeastern Pacific. Fisheries were closed, marine mammals were stranded, seabirds died, and sea creatures either changed their distribution or perished.

But our understanding of these compound heat-acidity ocean extreme events is limited, with more research focused on the marine heat wave component than on the acidification aspect.

Gregor and Gruber used 43 years of monthly data (1982–2024) on surface ocean temperatures and acidity to determine when, where and why compound heat-acidity extreme events have occurred. They defined extreme events as situations in which detrended acidity and temperature exceeded their 95th percentiles. The findings are published in the journal AGU Advances .

Compound ocean heat-acidity events happen more often than would be expected by chance, the authors found. In the low to midlatitudes, they occur roughly four times more often than expected by chance, mostly in places with permanent stratification. In these places, heat waves drive waters to be more acidic.

The compound events were least common in the eastern equatorial Pacific and around the poles, where deep waters upwell to the surface. When marine heat waves strike in these upwelling regions, a warm lens (top layer of water) prevents the surfacing of the deep acidic waters, leading to unusually low acidity for the region.

A majority (73%) of compound heat-acidity extreme events in the study period were smaller than 500,000 square kilometers (193,000 square miles, roughly the size of Spain) and lasted about a month. But a few events , like the Blob in 2015, lasted more than a year, sometimes with long-lasting consequences.

El Niño and La Niña events are important drivers of these compound events, but the events seldom happen at the weather events' epicenter in the equatorial Pacific. During El Niño, warmer waters create a lens that prevents upwelling, thus reducing the region's typical acidity. During La Niña, cold, deep waters with higher acidity upwell, resulting in only an acidification extreme. However, in neighboring regions, the knock-on effects of this warming or acidity cause compound extremes to occur.

The new findings are in line with several key facets of earlier work and offer more insights into temporal patterns and drivers.

Luke Gregor et al, Recent History of Surface Ocean Acidification Extremes That Compound Marine Heatwaves, AGU Advances (2026). DOI: 10.1029/2025av002112

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