Marine cold ‘killer events’ increasing in intensity and frequency

(CN) — The rising intensity and frequency of marine heat waves due to climate change is bleaching coral reefs, killing whole populations of fish, and forcing remaining populations out of their natural habitats. But a study published Monday in Nature Climate Change indicates that climate change is also behind the increasing frequency of extreme cold events across Earth's oceans.

The ocean phenomenon upwelling occurs when colder water from the deep is pushed to the surface after winds blow away the warmer water. As the National Ocean Service notes, upwelling is not entirely negative, since the rise of nutrient-dense water from the sea floor can help grow fish populations. But extreme upwells can lead to "bait and switch" situations, where fish and marine mammals migrate to warmer waters to escape the cold only to end up facing more cold temperatures.

The study authors examined these events from three different angles. First, they dissected an extreme cold upwelling cell off the coast of South Africa in March 2021, Second, the team analyzed data from extreme cold upwelling events from 1981 to 2021 in two major ocean currents. Finally, the team tagged and tracked a population of bull sharks to see how they reacted to upwelling.

The extreme upwelling event near South Africa led to 260 animal deaths across 81 species, the study authors found. Temperature logs indicated the upwell lasted seven days, much longer than the regional average of between one and three days.

Surface temperatures dropped by 14 degrees Fahrenheit, while underwater temperatures dropped by as much as 16.6 degrees Fahrenheit. These rapid temperature declines represent multiple threats to ocean wildlife, and even manta rays and bull sharks — known for their ability to survive in a wide range of water temperatures — died of hypothermia in the March 2021 extreme upwelling event.

Data gathered from the previous three decades indicates the number of extreme upwelling events and their intensity, measured by average temperature drop per day, have both risen dramatically. On both the South African coast and Australian east coast, more of the areas’ winds have been favorable to creating upwells, though the percentage is higher in the Agulhas Current off South Africa. The rise in both intensity and frequency not only helped explain how the March 2021 incident killed so many organisms, but also raises concerns of the increasing mortality of future events.

Finally, the team found that the rise in these upwelling events has already begun to alter wildlife behavior. Tagged bull sharks, examined by researchers through GPS tracking, acoustic tracking, and temperature tracking, have begun to swim closer to the surface in upwelling zones. While this minimizes their risk of exposure to the coldest temperatures from upwelling, many sharks that traveled through upwells still encountered temperatures well below 66.2 degrees Fahrenheit, the lower limit of of habitability for these sharks. The "bait and switch" scenarios like those in March 2021 pose a major threat to these creatures.

These sharks have also begun to stick to shelter estuaries for longer periods of time. South Africa’s Breede River estuary has been a favorite spot for bull sharks due to the year-round presence of prey. But as upwells become more frequent and intense, Breede River has become a major refuge from the cold, with temperatures as much as 22 degrees warmer than the offshore waters.

The bull sharks of Breede River are just one example of a likely trend. As these upwells grow more intense and frequent, more marine life will spread to the shrinking areas that remain habitable for them. Nicolas Lubitz and his team say the "results highlight the complexities of possible climate change effects on marine ecosystems, and emphasize the need to better understand how climate change alters ecological processes around the globe.”

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