2nd MARCH 2021: 260 BODIES FOUND WASHED UP ON THE SHORES OF SOUTH AFRICA.
A team of scientists tasked with investigating the mysterious deaths discovered something unexpected, something that is becoming increasingly common but has largely gone unnoticed by mainstream media1. Their findings set off a three-year investigation, culminating in a groundbreaking study published in March 2024. The research centred around a phenomenon called ‘extreme cooling,’ and transformed our understanding of how climate change and ocean currents affect marine species.
CLIMATE CHANGE AND OCEAN TEMPERATURES
As we know, climate change is affecting sea temperatures globally. The ocean has absorbed 90% of the heat generated by human activities to date, causing an increase in global temperatures, as well as more frequent and intense marine heatwaves2. Whilst the effects of these heatwaves (such as coral bleaching and fish die-offs) are widely acknowledged, extreme cooling events, which are also becoming increasingly common with climate change, are far less studied.
To fully understand this phenomenon, we need to understand the conditions that lead to its creation, so read on for a crash-course in oceanography!
HOW OCEAN CURRENTS WORK
The ocean has a global system of currents, often called the ocean conveyor belt, which moves water around the planet. This system is powered by differences in temperature and the saltiness of the water3.
Cold water is denser than warm water, so when ocean water moves toward the poles, it cools down, becomes heavier, and sinks. This cold, dense water then flows deep at the bottom of the ocean and travels toward the equator. Eventually, the cold water returns to the surface in certain areas called ‘upwelling zones’.
Upwelling happens when winds blow across the ocean, pushing the warm surface water away. This creates a “gap,” and cold water from deeper in the ocean rises to fill it. Upwelling can also occur when ocean currents meet or when other forces, like coastal land shapes or currents colliding, push the water upwards. These upwelling zones are crucial for marine life because the rising water brings nutrients from the deep ocean, fuelling the growth of plankton and supporting entire food webs.
However, climate change is wreaking havoc with this organised system, as the upwelling of cold water is increasing in frequency and intensity due to changes in temperature and air pressure. This is known as ‘extreme upwelling’ and can have tragic consequences for marine species that are unable to deal with the unexpected cold.
A recent study by a team of scientists in Australia investigated the effects of extreme upwelling events on marine species. They recorded over 260 carcasses from 81 different species which were killed in a single event off the South-East coast of South Africa, where the Agulhas Current created an unexpected upwelling cell that stretched for 230km, and caused a decrease in sea temperature of 9.2°C in just 24 hours! This extreme decline in temperature would have led to cold shock in many of the animals living in the usually warm water, with manta rays, blacktip sharks and bull sharks being among the fatalities.
Manta ray (left) and black tip reef shark (right); both species effected by extreme upwelling (photos by Canva).
THE CUMULATIVE EFFECTS OF CLIMATE CHANGE
Many marine species are currently undergoing ‘range expansions’ or ‘range shifts’ due to overall changes in ocean temperatures. This means that warm water species may suddenly find themselves able to live at higher latitudes (closer to the poles) where the water was previously too cold for them to survive. However, these warm water species are more likely to be exposed to cold upwelling events in these new areas, resulting in a phenomenon scientists are referring to as the ‘bait-and-switch’ situation. This is where climate change is simultaneously expanding species’ distributions and exposing them to increased risk of mortality at the same time. In the example of the Agulhas Current, this could clearly be seen as 75% of the species killed by the upwelling event were near the southern limit of their range.
The team then went on to track bull shark behaviours as they migrated through a cold upwelling zone, discovering that the sharks were swimming in significantly shallower waters when migrating through these cold patches to keep their bodies closer to the warm temperatures they are used to. This proves that cold upwelling is changing the behaviour of mobile marine species and could affect their range or migration.
HOW TO FIND ANOTHER WAY
The fascinating findings of this study demonstrate the complexity of the effects of climate change on oceanography and its influence on the species that live there, but what can we do to help these animals and prevent further mass death events?
As individuals, we can make an impact by advocating for stronger climate change mitigation measures and policies that protect vulnerable species, such as the establishment of marine protected areas and stricter fishing regulations. While these actions may seem distant from the complex issue of ocean temperature shifts and cold upwelling zones, they are directly linked to the health of marine ecosystems. Climate change is already causing these cold upwelling events, and removing carbon from the atmosphere won’t undo the damage that’s already occurred. However, protecting marine species and reducing additional pressures from overfishing can help bolster the resilience of marine ecosystems.
Additionally, we can raise awareness in our communities, encourage sustainable lifestyles, and hold governments and institutions accountable for enacting policies that address climate change and its impact on marine life.
Article written by Maddie Kirby
Primary Paper
Lubitz, N. et al. (2024) ‘Climate change-driven cooling can kill marine megafauna at their distributional limits’, Nature Climate Change, 14(5), pp. 526–535. doi:10.1038/s41558-024-01966-8.
Other references
- IPCC (2019) Summary for Policymakers. In: Pörtner, H.-O., Roberts, D.C., Masson-Delmotte, V., Zhai, P., Tignor, M., Poloczanska, E., Mintenbeck, K., Alegría, A., Nicolai, M., Okem, A., Petzold, J., Rama, B. and Weyer, N.M. (eds.) IPCC Special Report on the Ocean and Cryosphere in a Changing Climate. In press.
- Carl Wunsch (2002) What is the Thermohaline Circulation?; Science; 298; 1179-1181
