Global warming causes many marine animals to die en masse because of... cold

The effects of ocean warming are profound and well documented. But sometimes changes in wind patterns and ocean currents have counterintuitive consequences.

It is very difficult to believe that while the Earth is warming every day, the opposite effect appears in some seas : Surface temperatures can drop rapidly - from 10°C or more in a day or two. When these conditions persist for several days or weeks, the region will experience a 'cold wave', as opposed to the more familiar marine heat waves.

When a ' cold death wave' appeared along the southeast coast of South Africa in March 2021, it killed hundreds of animals from at least 81 species. Even more worrying is that these deaths include vulnerable species. In southern Africa, bull sharks (or white-eyed bull sharks), whale sharks and rays have died en masse after such sudden cold spells , especially in the past 15 years.

In the journal Nature Climate Change, two scientists Nicolas Benjamin Lubitz - Marine Ecologist, James Cook University and David Schoeman - Professor of Global Change Ecology, University of the Sunshine Coast have confirmed the conditions that exist. These deadly cold snaps have become increasingly common over the past four decades.

Its consequences can put even highly mobile species such as sharks in danger.

Bull sharks die en masse because of the cold.

What's happening?

Certain wind conditions and ocean currents can make the ocean surface cooler instead of warmer. This occurs when winds and ocean currents push coastal waters offshore, then cold water masses from the deep ocean rise to replace them. This process is called convection.

In some places, such as on the California coast in the western United States, cold water rises regularly in convection cycles that occur along hundreds of kilometers of coastline. But localized surges can also occur seasonally on a smaller scale, often at the edges of bays on the continent's east coast due to the interaction of winds, ocean currents and coastlines.

Previous research has shown that climate change causes changes in global wind and ocean current patterns. So the two scientists studied the potential consequences at specific locations by analyzing long-term temperature and wind data along the southeast coast of South Africa and the east coast of Australia.

This shows an increasing trend in the number of annual cold water surge events over the past 40 years. They also found an increase in the intensity of such cold surges and a decrease in temperature on the first day of each flooding event. In other words, these coastal cooling waves are becoming increasingly severe and changing suddenly.

Investigation of mass deaths

During an extreme cold surge along the southeast coast of South Africa in March 2021, at least 260 animals of 81 species died. To investigate the taxonomy of marine fauna, two scientists took a closer look at bull sharks. They attached tracking devices to spawning sharks that recorded depth and temperature.

Bull sharks are a tropical, highly migratory species, only tending to move to cooler waters during warmer months. When winter begins, they migrate back to warm tropical waters. As animals with high activity areas, they should be able to escape local cold temperatures. So why are bull sharks among the species in this extreme cold water surge event?

When running away is not enough

Bull sharks can survive in environmental conditions that most other marine creatures cannot adapt to. For example, they are often found in rivers hundreds of kilometers from the sea, where other marine creatures do not dare to venture.

Shark tracking data from two scientists from both South Africa and Australia shows that bull sharks actively avoid areas of cold water surge during seasonal migration, even when the surge is not too great. Some shark species take refuge in warm, shallow bays until the ocean surface warms again. Others stick close to the surface where the water is warmest and swim as fast as they can to escape the rising cold water.

But if the cold waves at sea continue to rise suddenly and become more intense, running away or hiding may no longer be safe enough for them. For example, during the event in South Africa that killed manta rays and bull sharks, water temperatures dropped from 21°C to 11.8°C in less than 24 hours while the weather event lasted seven days.

This sudden, severe drop combined with its prolonged duration makes this localized cold weather event especially dangerous. If future weather events continue to become more severe, marine die-offs could become a more common phenomenon, especially along the east coast in mid-latitudes. jar.

Still learning how climate change will play out

Overall, the ocean is warming. The ranges of tropical and subtropical species are expanding toward the poles. But due to the influence of convection, sudden cooling in a short period of time can cause difficulties for migratory species. Even the most resilient forms of life can be vulnerable to the effects of localized cold.

Although we see an overall warming trend for the Earth, changes in current weather and morphology can also cause localized extreme cold events. This really shows the complexity of climate change, as tropical species will expand into higher latitudes as warming continues, putting them at risk of losing their lives as face sudden local cold. Because of this, species like bull sharks and whale sharks are most likely having difficulty during their seasonal migrations.

The need to limit our human impact on the planet by reducing greenhouse gas emissions is urgent, as is the need to research what the future might hold.