Marine heatwaves are spikes riding on a rising baseline of ocean warming. How scientists separate the two signals — and why the difference matters for reefs.
Every summer the news supplies some version of the same headline: the ocean is the hottest it has ever been. Some of those stories are about a marine heatwave — a discrete, anomalous, weeks-to-months fever in a particular patch of ocean. Others are about ocean warming — the slow, global, decades-long climb of the baseline itself. The two are not synonyms, and confusing them leads to real mistakes: in how we read the news, how we attribute damage, and how a reef manager responds when the thermometer spikes. This is the distinction, made carefully. (For what heatwaves do to kelp specifically, start with Marine Heatwaves 101; this piece is about the difference between the spike and the slope.)
Picture a temperature record from a kelp-coast mooring stretching back forty years. If you could see it at a glance, you would see two superimposed shapes:
Take the slope away and the spikes still happen — the ocean has always had weather. But raise the slope, and the spikes get taller, longer, and more frequent without any change in the weather machine at all. Most of the marine-heatwave statistics people find alarming in the last decade are, mathematically, the second picture: natural variability surfing on a rising floor.
A heatwave is a relative event — defined against what is normal for that place and that time of year. The widely adopted research framework, standardised by a 2016 synthesis of the field, sets two thresholds:
Everything about that definition is local. Two degrees above normal is a brutal heatwave off Tasmania and ordinary variability elsewhere. The choice of baseline also matters enormously: run the definition against a fixed early-industrial climatology and the modern ocean falls into "heatwave" conditions a startling fraction of the time, because the whole baseline has moved. Run it against a shifting, recent-period climatology and you isolate just the weather-scale anomalies. Both views are legitimate; they answer different questions. Tracking the biological damage — kelp die-back, range-shifting species, coral bleaching — mostly wants the fixed-baseline view, because organisms experience absolute temperature, not percentiles.
The field also grades events by intensity (Category I through IV, multiples of the gap between mean and 90th-percentile temperatures), and duration and cumulative intensity are tracked as separate diagnostics from peak temperature. A modest six-month event can do more ecological work than a violent six-day one.
Ocean warming is a different beast. It is not defined against a baseline; it is the movement of the baseline. Its best measures are not surface temperature at all but ocean heat content — the integrated heat in the top hundreds to thousands of metres, tracked by the Argo float array and ship surveys (see Ocean Heat Content and Argo Floats Explained). Several features distinguish it:
Where warming is planetary bookkeeping, a marine heatwave is mechanistic and local. The usual suspects:
Attribution science can now quantify, for a specific event, how much more likely or intense the underlying climate trend made it — the same logic as land heatwave attribution, applied to water.
Kelp does not read percentiles; it experiences absolute temperature, day length of exposure, and the nutrient starvation that warmth usually brings with it. Warming sets the shrinking envelope of where a species can live at all. Heatwaves deliver the acute blows within that envelope — and because they are relative events on a rising baseline, what used to be a one-in-thirty-year shock becomes a once-in-five-year one without the weather changing its character at all. A restoration program, ours included, plans for both: the trend decides which seedstock genetics the Holdfast hatchery grows; the spikes decide when field teams must be ready to survey, reassess, and sometimes reseed.
No. The research definition requires the anomaly to exceed the local 90th-percentile threshold for at least five consecutive days — brief warm spells and ordinary seasonal peaks don't qualify. Relative, multi-day persistence is the point.
Researchers do both, deliberately. A moving baseline isolates weather-scale anomalies for studying the machinery of events. A fixed, historical baseline tracks what ecosystems actually experience. Confusion between the two explains many apparently contradictory heatwave statistics in the media.
It was a heatwave — a multi-year event with identifiable atmospheric and oceanic drivers — riding on and amplified by the long-term trend. The northeast Pacific cooled after it ended; the trend did not stop.
Yes — "bottom marine heatwaves" along continental shelves are an active research area, sometimes more persistent than surface events because the deep ocean changes slowly in both directions. Bottom events matter enormously for kelp, seafloor ecosystems and groundfish.
Our monitoring layers track both signals separately: long baselines against which recovery is judged, and near-real-time anomaly alerts that trigger field response during events. All numbers in our impact dashboard are illustrative, but the two-signal logic — spike and slope, tracked apart — reflects how the published science frames the problem.
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