What is a marine heatwave?
We know that heatwaves occur in the atmosphere. We are all familiar with these extended periods of excessively hot weather. However, periods of extreme heat also occur in the ocean and these are known as marine heatwaves, or MHWs. These extreme warming events can have significant impacts on marine ecosystems as well as human wellbeing and regional economies.​ MHWs can occur in any season - they are defined based on differences with expected sea temperatures for the location and time of year.

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What causes marine heatwaves?

MHWs are caused by a wide range of processes, which vary in their importance between regions and events. The most common drivers of MHWs include ocean currents which can build up areas of warm water and air-sea heat flux, or warming through the ocean surface from the atmosphere. Winds can enhance or suppress the warming in a MHW, and climate modes like El Niño can change the likelihood of events occurring in certain regions.

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How have marine heatwaves changed?

The oceans are warming at an unprecedented rate. Global sea surface temperatures have warmed by nearly 1°C since 1900 (IPCC AR6). This warming in turn increases the likelihood of MHWs occurring. While ecosystems have evolved within a certain coping range, and can adapt to conditions slightly outside that range, MHWs manifest as extreme events that lead to ecosystem vulnerabilities. In a warmer climate we are more likely to experience these vulnerability-causing extremes.

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What are the impacts of marine heatwaves?

Major MHWs have had significant impacts on a wide range of species, from plankton to fish to sea birds, by affecting biological processes such as growth, reproduction and survival. MHWs can affect ecosystem structure, by favouring certain species and suppressing others. MHWs can cause major economic losses through impacts on fisheries and aquaculture, and changes to carbon and nutrient cycles.

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What about the future?

The Intergovernmental Panel on Climate Change (IPCC) 6th assessment report projects that the global ocean will continue to warm well into the 21st century. The warming in the upper ocean is projected to be between 0.5°C and 2.5°C by 2100. We can therefore expect a continuation, and possibly acceleration, of the historic warming-driven changes in MHWs. Ultimately, without improved global action to tackle climate change, the oceans will continue to warm leading to more intense and frequent MHWs, with consequences for marine life and human societies. 
 

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