World - Marine heatwaves - Climate change and El Niño driving increased frequency, intensity and duration

Towards the end of August each year, the Quinault tribe digs for razor clams on the Olympic Peninsula in Washington state. The commercial-clamming day—known as the school-clothes dig—is a family event that helps Quinault members raise extra cash for supplies before the start of the school year. In 2015-16 however, those clams contained a toxin that could lead to brain damage or even death.

That event was triggered by a record-breaking marine heatwave off the coast of the United States and parts of Canada, with corals bleached and toxic phytoplanktons driven to shore. In 2023, records are again being broken.

Dr Vera Trainer is a specialist in phytoplankton blooms of the sort that caused havoc across the Pacific seaboard in 2015—and, because of the way toxins are retained in shellfish or crab for example—into the following year too.

“We saw the toxin domoic acid, which is produced by the algae, become concentrated in the flesh of filter feeders,” explains Dr Trainer, a research lead at the University of Washington’s Olympic Natural Resources Centre. Those filter feeders include many species that people eat, such as sardines, anchovies—a food of whales too—shellfish and crab. One of the most lucrative fisheries on the eastern Pacific was shut down at short notice and for many months.

A cascading effect

Dr Trainer describes the effects of such heatwaves as a “cascade”, affecting biodiversity, the economy and coastal communities alike.

Defined in 2015 as a period of at least five consecutive days when water temperatures exceed the 90th percentile of a baseline threshold, the impact of marine heatwaves is widespread and potentially long-lasting: as well as the prospect of toxic algal blooms, there is increased mortality of economically important species, aquaculture is affected, and there is biodiversity loss and the knock-on effects that come with that. Warm water increases the likelihood of extreme weather, as well as adding to ocean stressors through stratification, acidification and deoxygenation.

They are also on the rise: according to the Marine Heatwave International Working Group, there has been a 50% increase in these events over the past ten years, while marine heatwaves are now ten times stronger than in pre-industrial times.

Even with that in mind, the scope and intensity of the 2023 heatwaves have taken the scientific community by surprise, says Professor Michael Burrows, a marine ecologist at the Scottish Association for Marine Science and a member of the working group. He says the word “unprecedented” is overused, but that is exactly how he describes the situation the world finds itself in today. “The increase [in marine heatwaves] over the last two years has been massive relative to previous annual increases,” he says. “It just has gone off the scale in terms of what we might expect.”

Because marine heatwaves are about warmer oceans rather than about warm water, they can occur anywhere in the world. As scientists in Florida were preparing to move coral from the ocean and into labs this summer, in a bid to save them from temperatures above the bleaching threshold of 30.63 degrees, the United Kingdom and Ireland were experiencing their own record marine heatwave. “The water off the west coast of Ireland should have been around 13 degrees: it was over 17 degrees. That is a drastic difference,” says Dr Peter Miller, a marine-earth observation scientist at the Plymouth Marine Laboratory and part of a team that has developed satellite algorithms to warn salmon farmers about harmful algal blooms.

A new El Niño cycle

While climate change and overall warmer oceans are driving an uptick in marine heatwaves, these events remain hard to predict. As Dr Miller points out, ocean models are increasingly sophisticated but what they can’t do is pinpoint when an event like the June heatwave will happen. “That depended on so many factors happening at one time,” he explains. “It might have been to do with atmospheric currents of warm air together with the timing of El Niño,  together with maybe a deep current in the ocean that brought warmer water within range—all kinds of things conspiring together.”

Professor Burrows also talks about El Niño as a factor. “Ocean circulation events like El Niño do produce conditions where more extreme heatwaves are more likely,” he says. As we enter a new El Niño cycle, marine heatwaves are expected to increase further. “There are other systems—oceanographic phenomena—that make certain places and certain periods more prone to heatwaves,” he continues. “There are ocean-current changes and a connection to weather systems too.”

As well as challenges around prediction, it is also difficult to say what the impact is, so far, of the 2023 heatwaves. “It is difficult to project what the effects of these events are going to be because they are unprecedented,” says Dr Miller

“Our predictions are based on what’s happened in the past but we’ve never seen these conditions before. We’re in brand new territory, so we can only make our best guesses.”

Multiple stressors

Professor Tim Smyth—a colleague of Dr Miller—is head of science for marine biogeochemistry and observations at the Plymouth Marine Laboratory, with a brief that includes leading the Western Channel Observatory, which gathers data on “pretty much everything that there is for looking at ocean ecosystem health”.  The observatory boasts both breadth and longevity: it has been gathering ocean data since 1903, making it one of the longest time series available and offering a 120-year view on marine temperatures.

Looking at data from a station about 20 nautical miles offshore in the English Channel from 1903 through to the present day, Professor Smyth explains that when you look for temperature anomalies, “it's quite clear that over the last 20 years, we've not had very many cold anomalies [where temperatures drop below the 10th percentile for at least five consecutive days]. What we have had is a lot of these 90th percentile-plus anomalies.”  Since around the 1980s, there has been this “gradual warning” trend, he continues. “Looking at 2023 and that June-July heatwave, it was really quite a long way above the 90th percentile— actually at the maximum that we'd ever measured.”

It is not yet clear whether marine-heatwave trends will return to the more gradual increases seen previously or if the 2023 record-breakers are an acceleration—and an indication—of what the scientific community should come to expect.

Professor Smyth points out that you cannot separate marine heatwaves from other ocean stressors. “We're not just talking about temperature, we're talking about stressing the ocean in terms of nutrient loading, in terms of increasing nitrates and phosphates in the water, in terms of ocean chemistry. We’re stressing it in terms of oxygen levels,” he warns.

“If you continuously, and multilaterally, stress a system, when does that become an intolerable stress that it cannot recover from?”

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