Marine heatwaves are expanding their reach across the globe

You might be surprised to learn that marine heatwaves represent one of the most pressing threats to our oceans.

With 2023 etching itself into history as the first year that ocean temperatures exceeded 1°C over pre-industrial levels, we find ourselves facing a significant challenge.

Peculiar nature of marine heatwaves

Marine heatwaves, quite simply, are episodes of unusually high sea temperatures that either inhabit local hotspots or stretch across more substantial portions of ocean basins. These heatwaves last at least five days, but some persist for months or even years in extreme cases.

The impacts of marine heatwaves are far-reaching and often devastating for ecosystems, the economy, and coastal communities.

Coral bleaching in the tropics and the subsequent losses to the tourism industry are perhaps the most vivid representation of this. Effects also ripple through to our fisheries and natural capital, both of which undergo significant changes due to the wave.

Cooler waters of the UK

Interestingly, prior to 2023, major marine heat waves had largely bypassed the UK. However, June of that year saw a brief yet intense marine heat wave around UK and Ireland shores.

This event in UK waters revealed a critical gap in our understanding of the characteristics and potential future effects of marine heatwaves.

In an effort to close this gap, researchers set out to characterize both marine heatwaves and their counterparts, known as marine cold spells, around the UK.

Regional heatwave trends

Despite not standing out as a major hotspot in the wider North Atlantic, UK waters display marked regional diversity, particularly in terms of marine heatwaves.

The southern North Sea and the English Channel tend to witness longer and moderately intense heatwaves, while episodes in the eastern North Sea are typically shorter but considerably intense.

Marine heatwaves occur throughout the year. The location, intensity, duration, and occurrence time show substantial variation. This might be why chlorophyll-a, a proxy for phytoplankton abundance, shows an inconsistent response to these events.

Emergence of greenwaves and bluewaves

Within the wider North Atlantic, “greenwaves” (highly productive events) are more prevalent in the southern North Sea and closer to the coast.

On the other hand, marine heatwaves are more likely to coincide with “bluewaves” (low chlorophyll-a extremes) in the tropics and subtropics. Meanwhile, cold spells are usually associated with greenwaves at higher latitudes.

Impacts of marine heatwaves

The timing and location of marine heatwaves can dramatically affect marine ecosystems. When these events occur in summer, they cause stress for species like seaweed and seagrass.

On the flip side, if they occur in winter or spring, the warm temperatures may trigger phytoplankton blooms, which could either enhance productivity or result in harmful algal blooms.

Given these complexities, targeted research is critical to understand the future impact of marine heatwaves on marine ecosystems and society. 

There is an urgent need to identify at-risk regions so that the resilience of species and coastal communities can be assessed and properly managed.

Future heatwaves in the UK

The June 2023 marine heat wave should serve as a wake-up call. Even though these events do not seem as long-lasting or intense as other heat waves around the world, they are projected to increase.

With the silent crisis growing beneath the surface of the seas, the UK is in a unique position to use this head start and learn from the experiences of other nations.

As we continue to understand and discover more about our oceans, the role we play in shaping its future becomes increasingly evident.

“It is also worth noting that summer terrestrial heatwaves in the UK have recently caused excess deaths, among other adverse health impacts,” wrote the study authors.

“Given the link between the June 2023 marine heat wave and the coincident terrestrial heatwave, understanding interactions and feedbacks between marine and terrestrial heatwaves is another important research direction.”

The study is published in the journal Frontiers in Marine Science.

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