Total months spent in a compound (OAX ∩ MHW) extreme at each location.
During marine heatwaves, warming raises [H⁺], while changes in mixing and sDIC can oppose it. Drag the control to explore that balance, or choose The Blob for manuscript-derived event magnitudes.
Recent history of surface ocean acidification extremes that compound marine heatwaves
Marine heatwaves and ocean‑acidification extremes increasingly strike the same waters at the same time — compound extremes that stress marine life far more than either event alone. Here is their recent history across the global ocean, 1982–2024.
Sea-surface temperature in the top 5% of its historical record.
Hydrogen-ion concentration [H⁺] in the top 5% of its record.
Both extremes in the same place and month.
Compound area peaks at the end of — or just after — El Niño events, often during the following La Niña. Hover a bar to trace that year in the table & map.
Hover a row to locate it on the map and highlight its year in the chart.
| Event↕ | Year↕ | ↕Area (Mkm²) | ↕Duration |
|---|---|---|---|
| The Blob (NE Pacific) | 2015 | 12.6 | 12 mo |
| Central Atlantic | 2023 | 12.5 | 14 mo |
| Southeast Asia | 1998 | 7.5 | 20 mo |
| North Atlantic | 2023 | 6.9 | 11 mo |
| South Pacific | 2016 | 4.0 | 8 mo |
| Great Barrier Reef | 2022 | 3.3 | 9 mo |
| Mediterranean Sea | 2003 | 1.3 | 4 mo |
| Madagascar | 1987 | 1.0 | 8 mo |
| Western Australia ★ most intense | 2011 | 0.7 | 6 mo |
The two hazards do not overlap randomly: circulation and stratification make their coincidence strongly latitude-dependent.
Most common size ~500,000 km² — roughly the area of Spain — lasting a single month.
The largest 20% of events contribute more than
of the total compound-event area in the global ocean.
Compound extremes occur nearly twice as often in summer. Their latitude range expands poleward in summer and contracts toward the tropics in winter.
An extreme is unusual for that location and time of year—not simply the warmest or most acidic value on the map.
quadratic trend
1982–2024
January values
= extreme
At every ocean pixel, we first remove a quadratic trend from the monthly SST and [H⁺] series. This keeps the focus on unusually strong departures while preserving changes in variability.
From each detrended 1° time series, we collect matching calendar months into local records: all Januaries together, all Februaries together, and so on.
January is compared with Januaries, July with Julys, and so on. A month is extreme when it exceeds its month-of-year 95th percentile.
A compound extreme is recorded only when SST and [H⁺] both cross their local thresholds in the same pixel and month.
Why not a fixed baseline? With rapid warming and acidification, once-rare conditions quickly become common. Detrending lets us examine the processes that still create exceptional months within a changing ocean.