For more than 70 days this summer, a marine heatwave cooked the waters of the western Mediterranean. "We've been witnessing marine heatwaves during the last 20 years," said Garrabou, who's also coordinator of the T-MEDNet marine monitoring network. A 2016 marine heatwave along Chile's southern coast caused huge algae blooms that wiped out fish farms and cost the aquaculture industry some $800 million, said scientist Kathryn Smith with the Marine Biological Association of the United Kingdom. As the world warms, marine heatwaves are expected to become more frequent, according to the U.N. Intergovernmental Panel on Climate Change. Though economists have yet to account fully for the impacts of marine heatwaves, recent experience has many concerned.

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