‘Killer Cold Events’ on the Rise

“Killer cold events” are becoming more frequent and destroying marine wildlife, a new study has found.

The research published in Nature Climate Change noted that although climate change is causing an increase in dangerous marine heatwaves, extreme cold events are also causing problems too.

Nicolas Lubitz, a researcher at James Cook University in Queensland, Australia, and colleagues found that an extreme cold upwelling was linked to 260 deaths of marine organisms in 81 species, off the coast of South Africa in 2021. They reached these findings by looking at the deaths alongside 41 years of sea surface temperature data and over three decades of wind records.

Upwelling is a process in which deep, cold water rises toward the surface.

They found that cold upwellings such as this have actually increased in the period between 1981 and 2022 across the Indian Ocean Agulhas Current and East Australian Current.

“Our results suggest that climate change driven intensification of upwelling could increase the frequency at which migratory, subtropical marine megafauna experience killer cold events,” Lubitz said in a statement.

These events can be hugely debilitating to migratory species such as bull sharks, the study reports.

Scientists were able to use data from tagged bull sharks to assess how these cold events are affecting animals. They found that the sharks migrated to colder waters only in the warm seasons during this time, as is usual for the species. However, there was a noticeable uptick in the species altering behaviors to reduce the risk of coming into contact with extreme temperature drops, the study reported.

For example, they swam closer to the surface in upwelling zones, and were also observed sheltering in bays to avoid the cold.

Scientists are concerned that cold events could lead to a “bait-and-switch” situation in migratory species, meaning that their range is expanded due to the warming climate, however at the same time, they are also exposed to sudden temperature drops.

The study authors are worried that bull sharks and other species that migrate are already operating inside their long-term thermal limits.

“This species has a similar distribution and lower thermal limit to other subtropical species (for example, whale sharks and manta rays) that died during intense upwelling events,” the authors write in the study.

“In combination, our behavioral and modeling results suggest that if upwelling becomes more frequent and intense, subtropical marine megafauna at poleward distributional limits could more frequently be pushed towards the edge of their thermal limits, or beyond.”

Although scientists are well aware how heatwaves can be fatal, the effects of cold events on species is not well known.

These extreme temperature events are said to be becoming more common due to climate change. The alterations in climate are causing shifts in ocean current and pressure systems. This causes an upwelling of cold water to the surface.

The study authors describe the cold events as “an understudied aspect of
climate change research,” and notes that their findings highlight “the complexities of climate change effects on marine ecosystems.”

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