Open ocean — marine heatwave

Page Last modified 19 Nov 2021

1 min read

Topics: Climate change adaptation

Heatwaves are not confined to land areas. Marine heatwaves are defined as prolonged periods of unusually high sea surface temperatures. They pose many challenges to ocean ecosystems, whereby marine organisms living in the upper ocean layer with limited abilities to move towards colder waters are at greatest risk. Marine heatwaves also jeopardise coastal communities that depend on fishing, aquaculture and ecotourism for their livelihoods.

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Europe's changing climate hazards - an index-based interactive EEA report

Key messages

Marine heatwaves have become more frequent and longer in European regional seas, in particular in the Mediterranean Sea.
Marine heatwaves are projected to further increase in frequency, magnitude and duration in future. The duration of marine heatwaves is projected to increase at least 20-fold along the European coastline under a high-emissions scenario, with much smaller increases under a low-emissions scenario.

Total duration of marine heatwaves

Definition

The total duration of marine heatwaves index represents the number of days in a year within prolonged periods of exceptionally high sea temperature values, defined as a period of at least 5 consecutive days with an SST above the 90th percentile and computed over an 11-day window centred on the same calendar day for a reference period. Variants of the index can adopt other heatwave definitions and different percentiles as thresholds to identify extreme conditions depending on the specific application. If daily data are not available, it is also reasonable to calculate the index using monthly data.

Index factsheet (ETC/CCA Technical Paper): Duration of marine heatwaves

Relevance

The index is mainly relevant for the management of marine ecosystems and fisheries. However, the rather complex definition of the index can make its interpretation difficult.

Past and future changes

As a consequence of mean ocean surface temperature warming, marine heatwaves have become more frequent globally and in European seas over the past century, leading to a considerable increase in the total duration of marine heatwaves, especially in the Mediterranean Sea. Marine heatwaves are projected to further increase in frequency, magnitude and duration. Under a high-emissions scenario (representative concentration pathway (RCP)8.5), the duration of marine heatwaves is projected to increase at least 20-fold along the European coastline; the increase is expected to be much smaller under a low-emissions scenario (IPCC, 2018: Extremes, Abrupt Changes and Managing Risks).

Chapters of the Europe's changing climate hazards report

What will the future bring when it comes to climate hazards? — Overview
Europe’s changing climate hazards — about the report
Explore the hazards and the indices related to the six categories:

National and transnational climate atlases in Europe

Back to main page of the report

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Geographic coverage

Albania Austria Belgium Bosnia and Herzegovina Bulgaria Croatia Cyprus Czechia Denmark Estonia Finland France Germany Greece Hungary Iceland Ireland Italy Kosovo Latvia Liechtenstein Lithuania Luxembourg Malta Montenegro Netherlands North Macedonia Norway Poland Portugal Romania Serbia Slovakia Slovenia Spain Sweden Switzerland Turkey

For references, please go to https://www.eea.europa.eu/publications/europes-changing-climate-hazards-1/open-ocean/open-ocean-marine-heatwave or scan the QR code.

PDF generated on 19 Jul 2025, 07:20 AM