Hakai Magazine

Coastal science and societies

ocean temperature illustration
Like a heatwave on land, a marine heatwave is a prolonged period of above average temperatures that can cause a host of environmental issues. Illustration by Mark Garrison

Marine Heatwaves, Ranked

Scientists now have a better way to categorize the Blob and other marine heatwaves.

Authored by

by Alex Dropkin

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In the past few years, marine heatwaves have been responsible for cataclysmic environmental destruction. One off northern Australia in 2016 contributed to one of the worst mangrove die-offs in history. A year earlier, a heatwave in the northeast Pacific, ominously referred to as the Blob, played a role in mass strandings of marine mammals, the deaths of seabirds, and a toxic algal bloom. And in the Gulf of Maine in 2012, a marine heatwave decimated shellfish populations.

The string of disasters sent scientists scrambling to better understand them. Yet the term “marine heatwave” was only coined in 2011, and research groups have their own ways of describing the events, which make studying trends, and understanding why the biological impacts vary so widely from event to event, extremely difficult.

Now, after a few years of research, a team led by biological oceanographer Alistair Hobday at Australia’s Commonwealth Scientific and Industrial Research Organisation has designed a framework to rank marine heatwaves. Introduced at a recent scientific meeting in Portland, Oregon, the system is expected to help the scientific community better understand past marine heatwaves, and compare and analyze events as they occur.

Like the Saffir-Simpson Hurricane Wind Scale and the Fujita Tornado Damage Scale, the new classification system separates marine heatwaves into categories of intensity. The system ranks the events at one of four levels depending on how much the sea temperature has deviated from the local average. A Category 1 heatwave is the lowest intensity, but still consists of a period of above average ocean temperatures—with a sustained temperature in the 90th percentile of recorded temperatures for that section of ocean—that lasts for at least five days. In a Category 4 heatwave, the temperature spike is more than four times the difference between the 90th percentile temperature and the average.

Under the new system, a heatwave that hit the Mediterranean Sea in 1999 with temperatures 1.92 degrees above the local average would be considered a Category 1 heatwave. The Blob is a Category 3, while a 66-day heatwave that exterminated kelp forests in Western Australia in 2011 with temperatures 4.89 degrees above average would be a Category 4.

Though higher category heatwaves cause the most damage, even a more moderate event can cause significant problems. But without this framework, the nuances between these heatwaves are not easy to convey.

The inconsistency “made it very hard to determine if they were increasing in frequency or becoming longer or occurring in new locations,” Hobday says.