Marine heatwaves, still little known to the general public, have been on the rise since the 1980s. This phenomenon, now closely monitored by oceanographers, has a lasting impact on marine ecosystems and human activities. It is characterized by an abnormal and persistent rise in sea surface temperature lasting from several days to several months.
What Is a Marine Heatwave?
A marine heatwave occurs when sea surface temperatures remain unusually high for several consecutive days. Scientists define a heatwave when these temperatures exceed normal seasonal levels, based on a precise statistical threshold. To qualify, the temperature must remain above seasonal averages for at least five straight days. These events typically occur under stable weather conditions: little wind, clear skies, and high atmospheric pressure.
Causes and Aggravating Factors
Marine heatwaves result from complex interactions between the ocean and the atmosphere. Blocking high-pressure systems can cause excessive surface warming. At the same time, reduced vertical water mixing prevents heat from dissipating.
Global climate change plays a key role in the increased frequency of these events. According to IFREMER, the number of marine heatwaves doubled between 1982 and 2016. Forecasts suggest they could become up to ten times more frequent by 2100 if greenhouse gas emissions are not significantly reduced.
The Mediterranean Case
The Mediterranean Sea, semi-enclosed and highly responsive to climate shifts, serves as an open-air laboratory for studying marine heatwaves. In 2022, it experienced one of the longest marine heatwaves ever recorded, with temperatures exceeding 28°C for several weeks.
This prolonged warming caused mass die-offs among invertebrates and fixed species. One of the hardest-hit was the purple gorgonian (Paramuricea clavata), a coral species endemic to the western Mediterranean. These soft corals, which play a key role as ecosystem engineers, saw population declines in some coastal areas, notably in Provence and Catalonia.
Did You Know?
The “Blob” in the North Pacific lasted over three years, disrupting marine life all the way to Alaska.
How Does It Compare Worldwide?
While the Mediterranean is a key area of observation, marine heatwaves are affecting other parts of the world as well. The North Atlantic has recorded several intense episodes, especially near Newfoundland. In the North Pacific, the “Blob” detected between 2013 and 2016 disrupted the food chain, affecting salmon and sea lion populations in Alaska. The Indian Ocean, though harder to monitor, is also impacted, with noticeable effects on coral biodiversity.
Ecological and Human Impacts
Marine heatwaves weaken entire food chains. Coral bleaching, die-offs of benthic species, and shifts in fish migrations have already been observed. Coastal economic activities like fishing, tourism, and aquaculture are being directly affected.
In some regions, marine heatwaves have forced temporary fishery closures, reduced aquaculture output, and caused mass mortality among mussels, oysters, and fish. These losses have severe economic consequences for local communities dependent on marine resources.
Maritime insurance providers are starting to factor in these risks, as the frequency and intensity of marine heatwaves continue to rise. Coastal zone management will have to adapt to these new climatic realities.
How Nature and Science Are Responding
In response to these disruptions, some marine species are shifting their behavior or migrating to cooler waters. Populations of pelagic fish, such as mackerel and sardines, are changing their spawning grounds and distribution ranges. Scientists have also observed physiological adaptations in some invertebrates, though these are often not enough to offset extreme thermal stress.
Research is intensifying to assess ecosystem resilience. Pilot projects in the Mediterranean and the Caribbean aim to restore coral habitats using heat-resistant strains. Adaptive marine protected areas are also being tested, to limit human pressures during periods of elevated temperature.
Monitoring and Solutions
Agencies like IFREMER and the Copernicus programme now monitor these events in real time. Forecast models increasingly incorporate marine heatwaves into climate scenarios. Protecting vulnerable zones, reducing local pollution, and supporting ecological restoration are critical.
Some strategies include early warning systems for maritime professionals, the creation of natural thermal refuges, and adapting aquaculture by selecting more heat-tolerant species. International cooperation is vital for coordinating scientific and political responses to this global threat.
Did You Know?
In 2022, over 90% of the Mediterranean Sea experienced above-average temperatures for more than 40 consecutive days.
What Should We Remember About Marine Heatwaves?
Marine heatwaves are not a future issue they are already having concrete effects on marine ecosystems. Their increasing frequency highlights the urgency of climate action and the need to rethink how we manage coastal areas. Understanding, anticipating, and responding to underwater heatwaves is becoming a priority for the ocean and for humankind.
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This article may be of interest to you: The Purple Gorgon: An Endemic Mediterranean Jewel
