From the Blob to the record-breaking 2023–24 ocean, the last decade of marine heatwave records shows extremes growing — and kelp forests in the firing line.
The short answer: the records are unambiguous. Marine heatwaves — prolonged periods of anomalously warm ocean water — have become roughly twice as frequent as they were in the 1980s, and they are lasting longer and reaching higher peak temperatures. The past decade contains most of the most extreme events ever observed, culminating in the record-shattering global ocean temperatures of 2023 and 2024, when average sea-surface temperatures sat above every previous daily record for months on end. This article reviews what the last decade of records actually shows, how scientists measure these events, and why kelp forests are among the ecosystems absorbing the hardest hits. For the basics first, start with our Marine Heatwaves 101 explainer.
Before the records, the ruler. The standard definition (Hobday and colleagues, 2016) says a marine heatwave is any period of five or more consecutive days in which sea temperature exceeds the 90th percentile of the local historical average for that time of year. Events are ranked by intensity into categories I through IV, based on how many multiples of the local "normal range" the anomaly reaches. A Category IV event — "extreme" — means water more than four times beyond the typical envelope.
This percentile approach matters: it makes a heatwave in Norway and a heatwave off Chile comparable, because each is measured against its own regional baseline. The thermometer network behind these records is the global satellite sea-surface-temperature record (continuous since 1981–82), thousands of moorings and coastal stations, and the Argo fleet of profiling floats that tracks heat content in the upper 2,000 metres.
Across that record, three findings repeat in every major synthesis, including the IPCC's Sixth Assessment Report:
In late 2013 a dome of warm water formed in the northeast Pacific under a stubborn high-pressure ridge that suppressed winter storms and mixing. By 2014 "the Blob" spread from the Gulf of Alaska to Baja California, with anomalies locally approaching 3°C. It merged with the 2015–16 El Niño (see El Niño and kelp), and its ecological bill is still being tallied: the northeast Pacific's largest seabird die-offs, shifts in fish stocks, the sea-star wasting epidemic's worst years, and — along northern California — the collapse of more than 90% of bull kelp canopy measured by aerial and satellite surveys within a few seasons.
Off southeast Australia and Tasmania, an intensifying East Australian Current drove a heatwave in the 2015/16 austral summer that ran for roughly eight months — among the longest events on record anywhere at the time — with anomalies around 2–3°C. It hit during a summer of disease outbreaks in farmed salmon and shellfish and compounded decades of stress on Tasmania's giant kelp, whose story we tell in Tasmania's Vanishing Giant Kelp.
In 2019, warm anomalies re-formed across the northeast Pacific in a pattern so similar to the Blob that scientists called it, informally, "Blob 2.0." It was a reminder that the 2014–16 event was not a freak accident but a pattern the warming Pacific can reproduce.
Then the records stopped behaving like records. In March 2023, global mean sea-surface temperature began exceeding previous daily records — and kept doing so, day after day, for more than a year, well into 2024. Within that global signal:
Researchers spent much of 2024 debating how much of the surge was predictable from El Niño and background warming versus genuine surprise — a live scientific question. What was not debated: the ocean was absorbing the excess heat of the climate system, as it has absorbed more than 90% of it, and the margin for temperate ecosystems was shrinking.
Kelp is exquisitely productive precisely because it lives near its thermal limits in many regions, drawing nutrients from cold, well-mixed water (see upwelling explained). Heatwaves attack on two fronts at once. Warmer water crosses physiological thresholds directly — giant kelp declines when temperatures sit above roughly 20°C for sustained periods — and, just as dangerously, warm surface water stratifies the ocean, starving kelp of the nitrate that cold depths would normally supply. It is usually the nutrient starvation, run in parallel with heat stress, that kills.
The documented record from this decade of extremes:
The pattern is consistent: heatwaves do not need to kill every plant. They need only push recruitment below replacement — then grazers and time finish the job.
Under every emissions scenario assessed by the IPCC, marine heatwaves keep increasing in frequency, duration and intensity through mid-century; the scenarios diverge after that. Under high emissions, modelling work projects that parts of the ocean could approach a near-permanent heatwave state by late century — water that rarely returns to what was once the 90th-percentile threshold. Under strong mitigation, the curves bend meaningfully. The physics commits us to a hotter ocean for decades; it does not commit us to a worst-case one.
For a restoration practitioner, the record changes the design brief. Seedstock chosen for the climate of the 1990s will fail in the ocean of the 2030s. That is why Tidewright's fictional Holdfast nursery program screens kelp strains for thermal tolerance before a single line reaches the water — the method is outlined on our platform page — and why our fictional impact methodology reports canopy persistence through heatwave years separately from raw area planted (see /impact/methodology). Restoration that ignores the trend line is gardening for a garden that no longer exists.
No. El Niño is a coupled ocean-atmosphere cycle in the tropical Pacific; a marine heatwave is a local or regional temperature extreme defined against that region's own history. El Niño can trigger or amplify heatwaves around the Pacific rim, but heatwaves also occur with no El Niño at all — the Blob formed under a persistent high-pressure ridge, and the North Atlantic 2023 event had its own regional drivers.
Yes. Because the definition is relative to local norms, the Arctic and Southern Oceans experience marine heatwaves too — and polar and subpolar species, adapted to a narrow cold band, can be more sensitive to a 2°C anomaly than temperate species to the same shift.
Sometimes, quickly, if the heatwave killed adults but left spores, juvenile stages and grazer predators intact — many forests flushed back within two or three seasons after the Blob faded. Recovery fails when heatwaves are paired with predator loss and urchin outbreaks, which is why the northern California system has been so slow to return.
The global backbone is the satellite sea-surface-temperature archive (continuous since 1981–82), maintained by agencies such as NOAA and the Copernicus programme, plus the Argo float array for subsurface heat. Event-specific intensities cited here — the Blob, Ningaloo Niño, Tasman Sea 2015/16, North Atlantic 2023 — are documented in the peer-reviewed literature and agency assessments; Tidewright's own illustrative monitoring data appears in our impact dashboard.
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