Marine ‘Coldwaves’: The Unseen Climate Change Threat Decimating Global Marine Wildlife

Climate Change Triggers Marine ‘Coldwaves’ with Deadly Consequences for Wildlife

While the escalating issue of ocean warming due to climate change captures global attention, an equally alarming but less-known phenomenon is taking place beneath the waves. Oceanographic anomalies, significantly affected by the fluctuating patterns of winds and ocean currents, are precipitating sudden drops in sea surface temperatures, known as marine ‘coldwaves’. These coldwaves, characterized by sharp decreases in temperature sometimes exceeding 10ºC within a few days, are wreaking havoc on marine ecosystems, particularly along the southeast coasts of continents like Africa and Australia.

In March 2021, a severe marine coldwave occurred off South Africa’s southeast coast, leading to a catastrophic death toll among marine wildlife. The casualties spanned over 81 different species, including vulnerable manta rays and the typically resilient migratory bull sharks, underscoring the indiscriminate nature of this threat. This event is not isolated; similar occurrences have been reported over the past fifteen years, manifesting a disturbing trend.

Research into these phenomena has revealed that climate change is exacerbating the frequency and intensity of marine coldwaves. Alterations in global wind and current patterns, a consequence of a warming planet, are believed to be key drivers of these chilling events. Particularly affected are the east coasts of continents, where winds and currents can promote upwelling – a process that brings cold, deep ocean water to the surface.

California’s coast is famously known for its extensive upwelling zones, but more localized and seasonal upwelling events also pose significant threats, especially in the southeast regions of Africa and Australia. Investigations into temperature and wind data collected over the past four decades indicate not only an uptick in the number of upwelling events but also in their severity and the abruptness with which temperatures plummet at their onset.

The March 2021 coldwave stands as a stark example of how severe these events can be. The rapid decline in water temperature from 21°C to nearly 11.8°C in less than 24 hours, coupled with the coldwave’s duration, resulted in the extensive loss of marine life. Tropical and migratory species, such as bull sharks, which are known for their resilience and adaptive behaviors, were among the most affected, challenging the notion that mobility alone can safeguard against these extreme environmental changes.

Bull sharks, in particular, have demonstrated remarkable environmental adaptability, venturing into freshwaters far from typical marine habitats. However, tracking data revealed these sharks actively avoid upwelling zones, seeking refuge in warmer waters or staying near the surface to escape the cold. The intensity and suddenness of recent coldwaves, however, suggest that such strategies may no longer suffice, placing even the most robust species at risk.

The broader implications of these findings are profound. As the globe continues to warm, the migration patterns of tropical and subtropical species are extending polewards. Yet, these climate-driven migrants may find themselves battling against an increased likelihood of lethal cold events triggered by the very changes that are expanding their ranges.

This paradox highlights the complexity and unpredictability of climate change impacts on marine ecosystems. The occurrence of marine coldwaves not only threatens the survival of specific species but also underscores the vulnerability of marine biodiversity to climate anomalies. As these events become more frequent and severe, their potential to induce mass mortality events in marine wildlife could escalate, posing unprecedented challenges for conservation efforts.

The emerging reality of marine coldwaves as a significant consequence of climate change emphasizes the urgent need for continued research and proactive measures to mitigate their impacts. Reducing greenhouse gas emissions remains a crucial step in addressing the root causes of climate change. However, understanding the multifaceted nature of its effects on marine life is equally important in developing strategies to protect vulnerable species and preserve biodiversity in an era of ecological uncertainty.