Climate Change-Driven Coldwaves Strike Marine Habitats

Surface temperatures can plunge quickly by 10ºC or more over a day or two. When these conditions persist for a few days or weeks, the region experiences a "coldwave", which is something contrary to more familiar marine heatwaves.

When a "killer coldwave" appeared along South Africa's southeast coast in Walk 2021, it killed many creatures across at least 81 species. Seriously stressing still was the fact these deaths included vulnerable manta rays and even specimens of notoriously robust migratory bull sharks. In southern Africa, bull sharks, whale sharks, and manta rays have previously cleaned up dead following such unexpected cold events, especially over the past 15 years.

As we report in Nature Climate Change, the circumstances that can drive these killer cold waves have developed progressively more common over the past four decades. Unexpectedly, strengthing winds and currents as a result of environmental change can also make these destructive localized cold waves more likely in places like the east coast of South Africa and Australia, possibly putting even highly mobile species like sharks at risk.

Mass deaths warrant investigation

During the extreme upwelling occasion along the southeast coast of South Africa in March 2021, somewhere around 260 animals from 81 species died. These included tropical fish, sharks, and rays.

To investigate the ramifications for marine fauna, it's necessary to have a closer look at bull sharks. and label sharks with GPS beacons additionally record depth and temperature.

Bull sharks are a highly migratory, tropical species that tend to travel to upwelling regions during the warmer months. With the beginning of winter, they relocate back to warm, tropical waters.

Being mobile, they should have had the option to stay away from the local, cool temperatures.

Bull sharks survive environmental circumstances that would kill most other marine life. For instance, they're often found several hundred hundred kilometers up waterways, where other marine life wouldn't venture.

The shark tracking data from both South Africa and Australia showed bull sharks effectively avoid areas of upwelling during their migrations up and down the coast, even, when upwelling isn't excessively extraordinary. A few sharks take shelter in warm, shallow bays until the water warms again. Others stick close to the surface where the water is warmest and swim as fast as possible to escape the upwelling.

In any case, if marine coldwaves continue to become more sudden and intense, escaping or hiding may no longer be enough for these tough beasts. For instance, in the event in South Africa that caused the death of manta rays and bull sharks water temperatures decreased from 21°C to 11.8°C in less than 24 hours while the general occasion endured seven days.

This sudden, serious drop matched with the long span made this event particularly deadly. If future events continue to become more severe, mass deaths of marine life could become a more common sight - particularly along the world's mid-latitude east coasts.

Still figuring out how environmental change will work out

Overall, our oceans are warming. The range of tropical and subtropical species are extending towards the poles. But along some major current systems, sudden short-term cooling can cause major problems for these environmental migrants, or even kill them. Particularly assuming occasions like the one in South Africa become more common. Tropical migrants would progressively be living on the edge of what they are comfortable with in these areas.

Environmental effects can be unexpected or even counterintuitive. Even the most resilient life forms can be vulnerable to its effects. While we do see an overall warming, changes in climate and current patterns can cause extreme cold events also.

This truly shows the complexity of environmental change, as tropical species would expand into higher-latitude regions as overall warming proceeds, which then puts them at risk of exposure to unexpected extreme cold events. In this way, species, such as bull sharks and whale sharks might just be running the gauntlet on their seasonal migrations.

The need to restrict our effects on the planet by reducing greenhouse gas emissions has never been more urgent, nor has been the need for research into what our future might hold.