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New Study Finds Climate-Driven Cooling Event Linked to the Death of Organisms From 81 Ocean Species

A bull shark swims near the surface of the water.
Source: Dennis Hipp/Wikimedia

While a lot of debate occurs among scientists and the public about the effects of rising average global temperatures and ocean heat, an often not-talked-about consequence of rising temperatures is a paradoxical increase in the number and intensity of extreme cold events in Earth’s oceans.

A study published Monday in the journal Nature Climate Change aimed to examine the impacts of extreme cold events on marine life, which the study asserts is “poorly misunderstood” and an “understudied aspect of climate change research.”

“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.”

Upwelling is a process that occurs in the open ocean and places where water meets the coastline. As ocean temperatures climb winds push warm water away, causing deep cold water to rapidly rise to the surface to fill the vacancy. Strong winds and ocean currents are responsible for bringing pockets of cold water up to the surface which can cause a dramatic, sudden change in temperature.

The study links the death of organisms from 81 species to a ‘killer’ upwelling event in the Agulhas Current in 2021. Researchers were able to find a correlation between an increase in the frequency and intensity of the upwelling event and its effect on various species.

One method they were able to do this was through electronic tagging. Researchers were able to show how upwelling events coincided with changes in behavior in bull sharks, which caused them to change migratory patterns resulting in a higher risk of mortality.

Sharks, squids, manta rays, and other marine fauna changing their movement patterns end up moving closer to the surface, swimming in shallower water, and potentially washing up dead on beaches and coasts across the planet.

“The attempted avoidance of temperatures <19 °C is demonstrated by the dive profiles of bull sharks in southern Africa and Australia. In both WBCs, sharks swam consistently closer to the surface while migrating through upwelling zones, especially when encountering upwelling cells (verified by corresponding records from nearby tem-perature loggers) compared with when in subtropical and tropical areas,” said the study.

In March 2021, 260 marine animals from 81 different species of ocean fauna washed up dead on the coast of South Africa after a single upwelling event.

“Satellite data show that surface temperatures dropped by up to 7.3 °C within 48 hours. A temperature logger deployed 3 km off Port Alfred at 30 m depth recorded a decrease of 9.2 °C (from 21 °C to 11.8 °C) within 24 hours,” the study said.

The study authors assert that the lethality of any cold event is “probably related” to how fast the temperature is dropping. 

A rapid onset of a cold event shocks the local marine life, and if the event lasts for several days, it could cause “hypothermia, physiological malfunction and death in a variety of teleosts, marine turtles, elasmobranchs and marine mammals, often at their poleward distributional limit.”

Ajit Subramaniam, a research professor with Columbia University’s Climate School’s Lamont-Doherty Earth Observatory thinks the findings are important to remind people of how climate change works in ways people might not expect.

“It’s one of those unexpected findings and it’s not something we talk about a lot,” Subramaniam told CNN. “And therefore, this is a timely thing to remind us that the climate crisis works in both ways.”


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