Human Activity is Increasing Severity and Frequency of Major Marine Heatwaves

A marine heatwave in the northern Pacific Ocean in September 2019.

A marine heatwave in the northern Pacific Ocean in September 2019. NASA Earth Observatory

Marine heatwaves have become more than 20 times more frequent over the past 40 years due to human activity and the burning of greenhouse gases, according to a new study published in the journal Science. The research is the first to analyze the anthropogenic impacts on marine heatwaves, and points to the need for ambitious climate action.

The research, led by marine scientists at the University of Bern in Switzerland, examined satellite measurements of sea surface temperatures from 1981 to 2017. It found that marine heatwaves have become much longer, hotter, and more frequent. In the 1980s, satellites recorded 27 major marine heatwaves, each of which lasted about a month with water temperatures reaching a maximum 4.8 degrees Celsius (8.6 degrees Fahrenheit) above average. In the past decade, however, there were 172 major marine heatwaves across the globe, lasting 48 days on average with peak temperatures of 5.5 Celsius above average.

A widespread heatwave in the Pacific Ocean, for example, often referred to as “the blob,” increased sea surface temperatures in that region by more than 2.5 degrees C between 2013 and 2015.

Heatwaves can have lasting impacts on marine species, ecosystems, and economies. The high temperatures can trigger algal blooms, impact nutrient availability, cause mass coral bleaching, and alter fish migration patterns. Oceans “need a long time to recover afterwards — if they ever fully recover,” Charlotte Laufkötter, a marine scientist at the University of Bern and lead author of the new study, said in a statement.

In pre-industrial times, extreme marine heatwaves similar to those seen in the last decade would have only occurred once every few hundred to thousands of years. But Laufkötter and her colleagues found that if global temperatures rise 1.5 degrees Celsius — the goal of the Paris Climate Agreement, requiring aggressive and immediate climate action — such marine heatwaves could happen once a decade or century. If temperatures increase by 3 degrees C, marine heat waves could become as frequent as once a year or decade.

“Ambitious climate goals are an absolute necessity for reducing the risk of unprecedented marine heatwaves,” said Laufkötter. “They are the only way to prevent the irreversible loss of some of the most valuable marine ecosystems.”