Marine heatwaves: Why is the ocean getting hotter?
A recent report explained the growing frequency and intensity of marine heatwaves, periods of abnormally high sea-surface temperature, and their link to global warming.
It cited past events such as a 2023 marine heatwave off Saudi Arabia's Red Sea coast, associated with the deaths of large numbers of fish washed ashore over a 60-kilometre stretch.
The report noted that oceans absorb the vast majority of the extra heat trapped by greenhouse gases, making marine heatwaves an increasingly visible signal of the broader warming trend.
It highlighted impacts on corals, fish stocks, kelp forests, and seagrass meadows, and flagged a scientifically documented rising trend in the frequency and severity of these events since the 1980s.
Marine Heatwave (MHW) — Definition and Classification
A marine heatwave is a discrete, prolonged period of anomalously warm sea-surface temperature relative to the local seasonal climatology, formally defined (Hobday et al., 2016) as an event where sea-surface temperature exceeds the local 90th-percentile threshold for at least five consecutive days. Events are ranked using a four-tier severity scale based on how many multiples of the threshold anomaly are exceeded.
Key Details
- Category I (Moderate): 1-2x the local threshold anomaly.
- Category II (Strong): 2-3x; Category III (Severe): 3-4x; Category IV (Extreme): greater than 4x the threshold anomaly.
- Drivers include short-term atmospheric weather systems, ocean current advection and air-sea heat flux, seasonal-to-decadal climate modes such as El Nino, and, as the dominant long-term driver, anthropogenic climate change.
The Red Sea fish-kill event cited in the report is an example of localized marine ecosystem stress attributable to a marine heatwave crossing into the higher-severity categories of this classification system.
Ocean Heat Uptake and the IPCC Findings
The ocean acts as the planet's principal heat sink: it has absorbed roughly 90% of the additional heat energy trapped by greenhouse gas emissions since the industrial era, moderating atmospheric warming at the cost of steadily rising ocean heat content. The IPCC's Sixth Assessment Report (AR6, 2021-2022) concluded that human influence is the main driver of the observed ocean warming since at least the 1970s, and that marine heatwaves have become more frequent, more intense, and longer-lasting since the 1980s, very likely attributable to human-caused climate change since 2006.
Key Details
- IPCC AR6 projects marine heatwaves could become roughly four times more frequent by 2081-2100 relative to 1995-2014 under a low-emissions scenario, and about eight times more frequent under a high-emissions scenario.
- Frequency and severity of mass coral bleaching events have increased sharply worldwide since the 1980s, per IPCC findings, largely driven by marine heatwave exposure.
- Estimated global fisheries losses attributable to marine heatwaves between 1985 and 2022 run into billions of US dollars, a category of loss often absent from conventional disaster damage assessments.
The report's framing of "why the ocean is getting hotter" maps directly onto the IPCC's attribution finding that anthropogenic warming, mediated by the ocean's outsized heat absorption, is the principal cause of the rising trend in marine heatwave frequency described in the piece.
Marine Heatwaves in the North Indian Ocean
The Arabian Sea and Bay of Bengal, both part of the North Indian Ocean, have shown a measurable rising trend in marine heatwave frequency and duration in recent decades, a trend of direct relevance to India's coastal ecology, fisheries, and monsoon dynamics. The Indian National Centre for Ocean Information Services (INCOIS), an autonomous body under the Ministry of Earth Sciences, monitors and issues advisories on marine heatwave conditions across Indian Ocean regions.
Key Details
- Studies covering 1982-2019 found the northern and southeastern Arabian Sea (off India's west coast) experienced an increase of roughly 20 marine-heatwave days per decade.
- Research on the Bay of Bengal (1982-2021) found an average of about three marine heatwave events per year, with a rising trend of roughly one additional event per decade.
- INCOIS issues periodic marine heatwave advisories covering major Indian Ocean regions, including the Arabian Sea and Bay of Bengal, flagging areas under heatwave "watch," "warning," or more severe stress categories.
The global warming trend and impacts on coral, fish, and coastal ecosystems described in the report are mirrored regionally in Indian waters, where rising sea-surface temperatures threaten coral reefs (such as those in the Gulf of Mannar and Lakshadweep), fisheries livelihoods, and are being linked by ongoing research to changes in South Asian monsoon rainfall patterns.
- Marine heatwave (Hobday 2016 definition): sea-surface temperature above the local 90th percentile for 5 or more consecutive days.
- Severity scale: Category I (Moderate, 1-2x threshold) to Category IV (Extreme, >4x threshold).
- Oceans have absorbed approximately 90% of the extra heat generated by global warming (IPCC).
- IPCC AR6 (2021-2022): marine heatwaves projected to be ~4x (low emissions) to ~8x (high emissions) more frequent by 2081-2100 versus 1995-2014.
- Arabian Sea marine heatwave days rose by roughly 20 days per decade during 1982-2019 in its northern and southeastern parts.
- Bay of Bengal averages about 3 marine heatwave events per year, rising by roughly 1 event per decade (1982-2021 study period).
- August 2023 Red Sea marine heatwave: nearly 1,000 dead fish washed up along a 60-km stretch of Saudi Arabia's coastline.