Melbourne, The impacts of ocean warming are profound and well documented. But sometimes due to changes in the patterns of winds and sea currents the sea water suddenly becomes cold.
Surface temperatures can drop rapidly – by 10ºC or more in a day or two. When these conditions persist for several days or weeks, the area experiences a "cold wave", the opposite of the more familiar marine heatwave.
When a "killer cold wave" appeared off the south-east coast of South Africa in March 2021, it killed hundreds of animals across at least 81 species. Most worryingly, these deaths include specimens of vulnerable manta rays and notoriously strong migratory bull sharks.Bull sharks, whale sharks and manta rays have previously died due to such sudden cold events in southern Africa, particularly in the past year.
As we report in Nature Climate Change, over the past four decades the conditions that can drive these deadly cold waves have become increasingly common. Ironically, strengthening winds and currents as a result of climate change also make these deadly local cold waves more likely in these locations. The east coast of South Africa and Australia is also potentially harming highly mobile species such as sharks.
What's going on?Certain wind and current conditions may cool the ocean surface instead of warming it.This occurs when winds and currents force coastal waters away from the shore, which are then replaced by cooler waters of the deep ocean below. This process is known as upwelling.
In some places, such as California on the US west coast, upwelling occurs regularly along hundreds of kilometers of coastline. But the interaction of wind, current and coastline often causes small-scale, localized upwellings at the edges of bays on the eastern coasts of continents.
Previous research had shown that climate change is causing changes in global wind and current patterns.We therefore examined the potential consequences at particular locations, by analyzing long-term wind and temperature data off the south-east coast of South Africa and the Australian east coast. This revealed an increasing trend in the number of annual upwelling events over the past 40 years. We also found that the intensity of such upwelling events increased and the extent to which temperatures dropped on the first day of each event – in other words, how severe and sudden the cooling was.
Investigation into mass deaths is necessary
During an extreme shallowing event off the south-east coast of South Africa in March 2021, at least 260 animals from 81 species died. These included tropica fish, sharks and rays.To examine the impact on marine life, we took a closer look at bull sharks. We tagged the sharks with tracking devices that also recorded depth and temperature.
The bull shark is a highly migratory, tropical species that only travels to upland areas during the warmer months. With the onset of winter, they migrate back to warmer, tropical waters.
Being mobile, they should have been able to survive local, cold temperatures, so why were bull sharks among the dead in this extremely turbulent event? When running and hiding is not enough
Bull sharks survive in environmental conditions that can kill most other marine life.For example, they are often found several hundred kilometers upstream where other marine life cannot thrive.
Our shark tracking data from both South Africa and Australia has shown that bull sharks actively avoid areas of upwelling up and down the coast during their seasonal migration, even when upwelling is not very intense. Some sharks seek shelter in warm, shallow bays until the water becomes warm again. Others stay close to the surface where the water is warmest, and swim as fast as they can to get out to the surface. But if marine cold waves continue to become sudden and intense, hiding or running away may become difficult for these hard-to-reach people. There won't be enough even for the animals.For example, in an incident in South Africa that resulted in the deaths of manta rays and bull sharks, temperatures dropped from 21°C to 11.8°C within 24 hours, while the overall event lasted seven days.
This sudden, severe deterioration over a long period of time made it particularly lethal. If future events continue to become more severe, mass deaths of marine life could become a common sight – especially along the world's mid-latitude eastern coasts.
Still learning how climate change will affect Overall, our oceans are getting warmer. The ranges of tropical and subtropical species are expanding towards the poles.But with some major current systems, sudden short-term cooling can make life difficult for these climate migrants, or even kill them. Particularly if events like those in South Africa become more common, tropical migrants will increasingly find themselves living in these areas at the edge of what they are comfortable with.
Our work emphasizes that climate impacts may be unexpected or pre-intuitive. Even the most resilient life forms can be vulnerable to its effects. While we are seeing overall warming, extreme cold events may also occur due to changes in weather and current patterns.This really reflects the complexity of climate change, as continued overall warming will cause tropical species to expand into higher-latitude areas, leaving them more vulnerable to exposure to sudden extreme cold events. This way, species like bull sharks and whale sharks may well be challenged on their seasonal migrations. The need to limit our impacts on the planet by reducing greenhouse-gas emissions has never been more urgent, nor will it be more urgent in the future. Maybe, there is a need for research on this.(Conversation)
amsams
Surface temperatures can drop rapidly – by 10ºC or more in a day or two. When these conditions persist for several days or weeks, the area experiences a "cold wave", the opposite of the more familiar marine heatwave.
When a "killer cold wave" appeared off the south-east coast of South Africa in March 2021, it killed hundreds of animals across at least 81 species. Most worryingly, these deaths include specimens of vulnerable manta rays and notoriously strong migratory bull sharks.Bull sharks, whale sharks and manta rays have previously died due to such sudden cold events in southern Africa, particularly in the past year.
As we report in Nature Climate Change, over the past four decades the conditions that can drive these deadly cold waves have become increasingly common. Ironically, strengthening winds and currents as a result of climate change also make these deadly local cold waves more likely in these locations. The east coast of South Africa and Australia is also potentially harming highly mobile species such as sharks.
What's going on?Certain wind and current conditions may cool the ocean surface instead of warming it.This occurs when winds and currents force coastal waters away from the shore, which are then replaced by cooler waters of the deep ocean below. This process is known as upwelling.
In some places, such as California on the US west coast, upwelling occurs regularly along hundreds of kilometers of coastline. But the interaction of wind, current and coastline often causes small-scale, localized upwellings at the edges of bays on the eastern coasts of continents.
Previous research had shown that climate change is causing changes in global wind and current patterns.We therefore examined the potential consequences at particular locations, by analyzing long-term wind and temperature data off the south-east coast of South Africa and the Australian east coast. This revealed an increasing trend in the number of annual upwelling events over the past 40 years. We also found that the intensity of such upwelling events increased and the extent to which temperatures dropped on the first day of each event – in other words, how severe and sudden the cooling was.
Investigation into mass deaths is necessary
During an extreme shallowing event off the south-east coast of South Africa in March 2021, at least 260 animals from 81 species died. These included tropica fish, sharks and rays.To examine the impact on marine life, we took a closer look at bull sharks. We tagged the sharks with tracking devices that also recorded depth and temperature.
The bull shark is a highly migratory, tropical species that only travels to upland areas during the warmer months. With the onset of winter, they migrate back to warmer, tropical waters.
Being mobile, they should have been able to survive local, cold temperatures, so why were bull sharks among the dead in this extremely turbulent event? When running and hiding is not enough
Bull sharks survive in environmental conditions that can kill most other marine life.For example, they are often found several hundred kilometers upstream where other marine life cannot thrive.
Our shark tracking data from both South Africa and Australia has shown that bull sharks actively avoid areas of upwelling up and down the coast during their seasonal migration, even when upwelling is not very intense. Some sharks seek shelter in warm, shallow bays until the water becomes warm again. Others stay close to the surface where the water is warmest, and swim as fast as they can to get out to the surface. But if marine cold waves continue to become sudden and intense, hiding or running away may become difficult for these hard-to-reach people. There won't be enough even for the animals.For example, in an incident in South Africa that resulted in the deaths of manta rays and bull sharks, temperatures dropped from 21°C to 11.8°C within 24 hours, while the overall event lasted seven days.
This sudden, severe deterioration over a long period of time made it particularly lethal. If future events continue to become more severe, mass deaths of marine life could become a common sight – especially along the world's mid-latitude eastern coasts.
Still learning how climate change will affect Overall, our oceans are getting warmer. The ranges of tropical and subtropical species are expanding towards the poles.But with some major current systems, sudden short-term cooling can make life difficult for these climate migrants, or even kill them. Particularly if events like those in South Africa become more common, tropical migrants will increasingly find themselves living in these areas at the edge of what they are comfortable with.
Our work emphasizes that climate impacts may be unexpected or pre-intuitive. Even the most resilient life forms can be vulnerable to its effects. While we are seeing overall warming, extreme cold events may also occur due to changes in weather and current patterns.This really reflects the complexity of climate change, as continued overall warming will cause tropical species to expand into higher-latitude areas, leaving them more vulnerable to exposure to sudden extreme cold events. This way, species like bull sharks and whale sharks may well be challenged on their seasonal migrations. The need to limit our impacts on the planet by reducing greenhouse-gas emissions has never been more urgent, nor will it be more urgent in the future. Maybe, there is a need for research on this.(Conversation)
amsams
