In the year 2021, marine heatwaves had devastating consequences, leading to the demise of more than a billion sea creatures. Scientists are now gaining insights into the havoc they wreak on the seafloor.
Marine heatwaves, defined as prolonged periods of abnormally high ocean temperatures lasting for five days or longer, are becoming increasingly common and severe due to the effects of global warming. Although satellites have revolutionized oceanography by providing daily measurements of sea surface temperatures, their view is limited to the surface. Recent research, however, has uncovered that marine heatwaves can penetrate deep beneath the ocean's surface, sometimes reaching all the way to the seafloor.
These subsea heatwaves can manifest at any time of the year but are most destructive during the summer when the ocean is already at its warmest. With more than 90% of the heat trapped by greenhouse gases being absorbed by our oceans, the average global ocean temperature has reached unprecedented levels, particularly from April onward this year.
The impact of these profound marine heatwaves is substantial, with the continental shelf being particularly vulnerable. This region is home to a diverse range of marine life, including fish, kelp forests, sponges, cold-water corals, shellfish, and crustaceans, all of which are sensitive to extreme temperature changes. For example, a temperature increase from 12 degrees Celsius to 15 degrees Celsius can have devastating consequences.
Marine heatwaves can trigger massive die-offs among sea creatures. In 2021, a single heatwave off the western coasts of the United States and Canada resulted in the deaths of over a billion marine organisms. While mobile species like fish can migrate to cooler waters, immobile species must endure the heat or face perishing. These migrations can disrupt ecosystems as new species arrive in search of refuge.
Despite their significance, our comprehension of deeper marine heat waves remains limited. The waters covering the continental shelf are relatively shallow compared to the deep ocean, making it impossible to monitor subsea events using satellites or high-frequency radar.
Australia stands out as one of the few places where long-term data on sub-surface temperatures is being collected, thanks to oceanographic moorings located off its southeastern coast. Equipped with sensors, these moorings have been recording daily temperature readings from the sea surface down to the seafloor since 1993.
This valuable data has illuminated the fact that marine heatwaves at depth can be more intense and persistent than those at the surface. They also exhibit different characteristics and have various triggers, some of which are seasonal in nature. For instance, winter marine heatwaves often extend from the sea surface to the seafloor and are driven by the warm East Australian Current. In contrast, summer heat waves may be limited to the surface or occur primarily near the seafloor.
Understanding the various types of marine heatwaves is essential for predicting when and where they may occur and assessing their impact on different parts of the water column and the ecosystems they affect. While the ultimate solution to prevent marine heatwaves lies in reducing carbon emissions, this knowledge could aid in the development of strategies to mitigate their effects on marine life and industries such as tourism and fishing.
Â
