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sis is likely to blame for these "killer events" on the other end of the temperature spectrum. The world's oceans have been plagued by unprecedented heat over the past year, fueling concerns for marine life. Billions of crabs disappeared in the northern Pacific; sea lions and dolphins are washing up sick; iconic coral reefs are undergoing mass bleaching. But even as ocean temperatures climb, extremely cold upwelling events - when strong winds and ocean currents bring pockets of cold water up to the surface, replacing the warm water that was there - are also becoming more frequent and intense, threatening sea life, according to the study published Monday in the journal Nature Climate Change. "Climate change is actually really complex," said Nicolas Lubitz, lead author of the study and a researcher at James Cook University in Queensland, Australia. "It's not just warming of the globe, but it's really changing the way our oceans function." When Lubitz heard reports of marine animals like sharks, manta rays and squids washing up dead in the southeast coast of South Africa in March 2021, he started investigating. More than 260 marine animals from 81 different species died in that one extreme event. He said seasonal upwelling events are common in that area, with water temperature dropping quickly. But Lubitz said the March 2021 die-off was "quite an extreme event, because we had rather warm water before it happened." "And then the winds changed, and the currents started changing slightly, which is a seasonal thing," he added. "Then all of a sudden, the temperature within 24 hours dropped by 11 degrees." The researchers analyzed killer upwelling events in the Indian Ocean's Agulhas Current and the East Australian Current, using 41 years of sea surface temperature data and 33 years of wind records to see how deadly cold ocean extremes can be. "We're seeing changes in how often the upwelling occurs, how intense it is, which might impact the fishing communities in these areas," he said. "It's really an economic thing as well as the biodiversity thing." According to the study, the lethality of a cold event is likely linked to how fast the temperature drops. If the cold event lasts for multiple days, which has been occurring more frequently, research shows that marine animals including turtles and many fish species could suffer from hypothermia and physiological malfunction or ultimately die. For a different study, Lubitz already had bull sharks electronically tagged with a transmitting locator device, which also recorded the depth and temperature of the part of the ocean they swim to. "That was really the key in this study in that we could see when the sharks migrate," he said. "We could see how the temperature profiles change, and how the sharks were swimming shallower when they were in upwelling areas because they were trying to avoid the colder water from the depths." The findings provide a "very reasonable explanation" to the many unexpected marine mortality events people have been seeing around the world, said Ajit Subramaniam, research professor with Columbia University's Climate School's Lamont-Doherty Earth Observatory. "It's one of those unexpected findings and it's not something we talk about a lot," Subramaniam, who was not involved with the study, told CNN. "And therefore, this is a timely thing to remind us that the climate crisis works in both ways."