Are marine heatwaves killing our oceans?

In a world increasingly disrupted by the ongoing climate crisis, it’s perhaps unsurprising that our oceans are feeling the long-term effects of marine heatwaves.

Right on our doorstep, the Mediterranean is a particular hotspot for this phenomenon.

Some areas of the sea can see temperature anomalies of 5, 6, or even 7 degrees above average.

That is not good news for marine ecosystems, as habitats are disappearing and fish stocks are moving to different areas of the ocean.

Pippa Moore is a professor of Marine Science at Newcastle University in the UK. She is hopeful that changes can be made to mitigate the worst effects of marine heatwaves.

Moore was working as a postdoctoral researcher in Western Australia in 2011 when a significant marine heatwave hit the region.

The incident caused a lot of environmental disruption, like the closures of kelp forests and the widespread loss of scallops as well as significant issues for many fisheries.

She was devastated, but came away determined, too.

“I became part of a marine heatwave working group in 2014 where we actually defined marine heatwaves, basically by stealing it from atmospheric scientists, and then carried on researching it ,” she says.

What is meant by a marine heatwave?

In the last decade, she and her fellow experts have been studying the surges of abnormally warm water, which are reshaping underwater ecosystems with a force that resonates far beyond the surface.

Today, the definition of these underwater heatwaves is fairly clear and distinct from heatwaves on land.

“It takes longer for water to heat up and cool down, so we felt that three days was too short,” Moore explains. In the end, they “defined a marine heatwave as, values above the 90th percentile for five days or more.”

Samantha Burgess, the Deputy Director of the EU's Copernicus Climate Change Service, also began her work on marine heatwaves while in Australia studying for her PhD - but came to the issue in a different way.

She discovered the existence of the heatwaves in relation to previous coral bleaching events, specifically associated with El Niño.

“We all know now that El Niño events lead to high sea surface temperatures, particularly around the Great Barrier Reef and the east coast of Australia - but increasingly other parts of the world as well,” Burgess says, “You know, tracking these signals through the coral skeletons really shows how, recently, our ocean has changed drastically compared with the the archive of hundreds and thousands of years that we have beforehand.”

Marine heatwaves may show up for many of us in regular climate bulletins, yet they reverberate through often delicate underwater ecosystems, triggering a cascade of consequences.

While a casual swimmer might enjoy a warmer sea temperature, these thermal anomalies disrupt the fragile balance of marine life.

In 2016 in California, there was a particularly serious heat wave - now referred to as “the blob”.

The planktonic communities were directly affected, and as they are food for organisms higher up the food chain, the impact was disastrous.

“There were large mortalities of seals and birds, not just because of the direct effects of changes in temperature on those birds or marine mammals, but because of changes in their food, which meant their food wasn't arriving at the same time as they were for their breeding,” Burgess explains.

That event plainly laid bare the interconnectedness of marine ecosystems, and the importance of learning more about these heatwaves and how to prevent them.

Is there any hope for lessening the damage caused by marine heatwaves?

As ever in the oceans, the effects of a particular stress in the ecosystem result in a nuanced tapestry of winners and losers.

Burgess explains that some marine organisms have already managed to work out adaptive strategies for themselves.

“Research plays a really important role of understanding refuges,” she says. "Some particular species in some locations seem better adapted to temperature extremes.”

For scientists, “being able to understand why, in that complex environment, that ecosystem seems to thrive at slightly warmer temperatures,” is crucial, she says.

Overall though, the repercussions of marine heatwaves extend far and wide, casting a shadow on humans and their livelihoods, too.

“A marine heatwave can potentially result in harmful algal blooms or increases in viruses such as Vibrio,” Moore says. For fish stocks, that can be disastrous.

A few years ago, in Chile, millions of Euros worth of fish was lost due to the algal blooms.

After years of research, scientists have built a better picture of how to predict when ocean temperatures will rise dangerously high.

“We're able to provide these forecasts, so that you can actually minimise that impact,” Moore explains.

“Colleagues in Tasmania have developed forecasting tools for marine heatwaves that they feel pretty confident about forecasting out to six months in advance,” Moore says.

This allows fisheries management and other authorities to close sensitive zones and give nature a chance to save itself, free from pressures related to human activity.

Despite this positive step, there is still work to be done, and the journey ahead is fraught with uncertainty.

Research is crucial, with the need to unravel the mysteries of resilient ecosystems and understand the most effective climate refuges is a matter of urgency.

It’s particularly pressing given that the global surface ocean is currently above 21 degrees - and has been for nearly a year - and coral bleaching is so widespread.

Current ocean average temperatures are unprecedented, and the long-term effects of these sustained highs, as well as the spikes in water temperature during heatwaves, are not well modelled or understood.

If you want to learn more about the concerning phenomenon of marine heatwaves, listen to the full episode of Ocean Calls in the player above.

Thanks to our guests, Samantha Burgess, Deputy Director of Europe's Copernicus Climate Change Service and Pippa Moore, professor of Marine Science at Newcastle University in the UK.

At the end of the episode we hear from Chloë McCardel, an Australian swimmer nicknamed 'the queen of the English Channel' after crossing it 44 times - and meeting a bunch of jellyfish along the way.

Ocean Calls is produced in partnership with the European Commission’s Directorate-General for Maritime Affairs and Fisheries.