How we track marine heatwaves

What the dashboard shows, where the numbers come from, how current they are, and what they can’t tell you.

The short answer

A marine heatwave is a spell when the sea is unusually warm for that place and time of year, long enough to harm life adapted to the normal. This dashboard checks every day which of 214 tropical and subtropical reef regions are in a marine heatwave, how strong each one is, and how often marine heatwaves happen now compared with the 1980s and 1990s.

How we know

The definition. Scientists agreed in 2016 on a common definition (Hobday et al.): a marine heatwave is when the sea surface is warmer than in 90% of years at that place and time of year for at least five days in a row. In 2018 the same group named four categories, by how far above normal the water is in multiples of the gap between normal and that 90% line: Moderate (one to two times), Strong (two to three), Severe (three to four) and Extreme (four or more). Because each place is measured against its own normal, a 1 °C rise can be a heatwave in a calm tropical sea and nothing unusual somewhere more variable.

The data. NOAA Coral Reef Watch publishes satellite sea-surface temperature every day for 214 reef regions, each summarised from its 5-kilometre grid, with a record back to 1985. Every night we download all of them. For each region we work out the normal and the 90% line for every day of the year from 1991–2020, the standard 30-year climate period, then apply the definition and the categories exactly as the two papers describe.

The history. The chart of heatwave days per year uses the same rules over the whole record. Measured against the 1991–2020 climate, most years from 1985 to 2014 had fewer than 20 marine heatwave days per reef region, with peaks in the El Niño years 1987–88, 1998 and 2010. Since 2019 every year has had more than 40, and 2024 had over 170.

The Pacific. NOAA’s Climate Prediction Center publishes monthly sea-surface temperatures for the four Niño regions of the tropical Pacific. We show how far they are from normal, as context: El Niño years bring marine heatwaves to many tropical seas at once.

How current each part is

• Marine heatwaves at each region: NOAA updates its files daily and we download them every night; the latest day is usually one to two days old.
• Heatwave days since 1985: rebuilt every night; the current year is the year so far.
• Niño regions: monthly, updated when NOAA publishes a new month.
• Historic heatwaves and their effects: from the peer-reviewed studies below.

What the dashboard can’t tell you

It covers tropical and subtropical reef seas only. The North Atlantic, the North Pacific, where the “Blob” formed, and polar seas are not included, so the count is not a global total. Satellites measure the surface; heat below it is not seen. Each region is a summary of a larger area, so a bay or lagoon can be warmer or cooler than its region.

It cannot say what a heatwave is doing to sea life in a particular place. The same heat can kill corals on one reef and leave another unharmed, depending on species, depth and currents. And El Niño does not explain every heatwave: winds, currents and clouds decide where the surface heats up.

Further reading

• Hobday et al. (2016), Progress in Oceanography: a hierarchical approach to defining marine heatwaves. doi.org/10.1016/j.pocean.2015.12.014
• Hobday et al. (2018), Oceanography: categorizing and naming marine heatwaves. doi.org/10.5670/oceanog.2018.205
• Oliver et al. (2018), Nature Communications: longer and more frequent marine heatwaves over the past century. doi.org/10.1038/s41467-018-03732-9
• Frölicher, Fischer & Gruber (2018), Nature: marine heatwaves under global warming. doi.org/10.1038/s41586-018-0383-9
• Smale et al. (2019), Nature Climate Change: marine heatwaves threaten global biodiversity and the provision of ecosystem services. doi.org/10.1038/s41558-019-0412-1
• Garrabou et al. (2009), Global Change Biology: mass mortality in Mediterranean rocky seafloor communities after the 2003 heatwave. doi.org/10.1111/j.1365-2486.2008.01823.x
• Wernberg et al. (2016), Science: climate-driven regime shift of a temperate marine ecosystem. doi.org/10.1126/science.aad8745
• Piatt et al. (2020), PLOS ONE: extreme mortality of common murres in the northeast Pacific marine heatwave of 2014–2016. doi.org/10.1371/journal.pone.0226087
• Oliver et al. (2017), Nature Communications: the unprecedented 2015/16 Tasman Sea marine heatwave. doi.org/10.1038/ncomms16101
• Berthou et al. (2024), Communications Earth & Environment: the June 2023 northwest European marine heatwave. doi.org/10.1038/s43247-024-01413-8

Credits

The definition and categories come from Alistair Hobday and the international group of oceanographers and ecologists behind the 2016 and 2018 papers. The temperatures come from NOAA Coral Reef Watch and NOAA’s Climate Prediction Center. We only download their public files and apply the published method.