A marine heatwave off the west coast of North America in August 2019.

A marine heatwave off the west coast of North America in August 2019. NASA

As Waters Warm, Ocean Heatwaves Are Growing More Severe

With oceans absorbing more than 90 percent of global warming, marine heatwaves are becoming hotter and larger and are lasting longer. Scientists say the trend has major ecological consequences, from altering fish and plant populations to forcing whales into hazardous waters.

Off the coast of California this August a sea monster of record size was spotted: a patch of warm water that grew to the size of Canada, 9.8 million square kilometers simmering up to 4 degrees Celsius warmer than usual. “It’s off the chart,” says Andrew Leising, a fisheries oceanographer at the National Oceanic and Atmospheric Administration who is mapping the marine heatwave on his website, nicknamed the Blobtracker. By Leising’s reckoning, in September, the unglamorously-named “NEP20b” became the biggest-yet-spotted blob of warm water there since satellite records began in the early 1980s.

Researchers are now scrambling to chart or anticipate the impacts of the NEP20b blob on marine life, tracking how the step change in temperature throws ecosystems out of whack.

The phenomenon of a patch of abnormally warm water off the west coast of North America gained notoriety in 2014, when the first such “Blob” was spotted and given that name, after the horror movie creature that devoured everything in its path. That first Blob lasted years, from 2013 to 2016. It has been blamed for slicing some forage fish populations in half; starving seabirds; triggering a collapse in cod; shifting tuna as far north as Alaska; pushing whales into the path of crab fishing lines and ships; and allowing exotics, including glowing tropical sea pickles, to arrive in northern waters.

In 2019, a second blob emerged. With record-warm waters appearing again this year, some scientists believe the 2019 event, known as Blob 2.0, may have just never gone away. If NEP20b is now big enough and hot enough, Leising says, it may do what the 2013-2016 version did, creating its own microclimate that perpetuates the heat, forcing the warm water to stick around for years yet to come.

While NEP20b appears to be starting to shrink, as of early October it was still a massive 6 million square kilometers and more than 100 days old, says Leising.

Marine heatwaves have always been around. But they are becoming hotter, more noticeable, and more problematic.

“We might not see the impacts for years,” says Leising — especially with the paucity of data on fish stocks this year thanks to the ongoing pandemic, he notes. But these impacts could last decades.

On land, the effects of increasing heatwaves have been easy to see: a global record-breaking temperature of 129.9 degrees Fahrenheit (54.4 degrees C) hit Death Valley in California this August; hot summers have turned forests into tinderboxes that feed massive wildfires; crops have failed. In 2019, Europe had its hottest June on record. Those land-based heatwaves notoriously lead to food price spikes, heat exhaustion, and increased rates of death.

But while the land takes the bulk of the headlines, the ocean is taking the bulk of the heat. Water, covering two-thirds of the Earth’s surface, absorbs more than 90 percent of the energy from climate change, and has warmed more than 1 degree C on average over the past century. That warming is ramping up: 2019 saw warmer oceans than any year on record — at least in the top 2 kilometers.

Like land-based heatwaves, marine heatwaves have always been around. But they are becoming hotter, more noticeable, and more problematic. Infamous and destructive marine heatwaves hit the Mediterranean in 2003; western Australia in 2011, northern Australia in 2016… the list goes on. They can be caused or spun up by natural variability, like El Niño. But the warming planet is also contributing: One recent attribution study showed that anthropogenic climate change made seven recent high-impact marine heatwaves at least 20 times more likely.

Research on marine heatwaves is in its infancy. A precise definition for the phenomenon was only proposed in 2016: a discrete period of five days or more when the sea surface temperature is significantly warmer than the 30-year historical baseline for that place and time of year. Given that definition, which most now use, heatwaves are clearly becoming more frequent. One study showed that the count of annual marine heatwave days increased globally by over 50 percent from 1925 to 2016, with heatwaves becoming 34 percent more frequent and lasting 17 percent longer. That means that while the 1980s had about 25 heatwave days per year, that number jumped to 55 per year by the 2010s. Another study showed that by 2100, no matter whether humanity follows a high emissions path or a low one, the oceans will be in a “near-permanent heat-wave state.”

Sea surface temperature anomalies in the western Pacific Ocean in January 2009 vs. January 2019. Source: NIWA

But, some researchers are quick to point out, these are perhaps misleading statistics: They are true in large part simply because the average temperature of the oceans is going up (and going up fast; about 4.5 times faster now than 50 years ago). As any region warms from an historic baseline, the number of days spent over some benchmark temperature is also bound to increase.

A different and equally important question, says Michael Jacox, another NOAA scientist studying marine heatwaves, is whether the heatwaves in and of themselves are changing — whether each year, against the background of its new normal, is seeing larger, steeper, or more frequent waves above that new norm. It’s like rising sea levels, says Jacox: The slow upward creep of the ocean is important, as it will consume parts of the coast, but it’s also important to know if the ocean is getting feistier, with higher or more frequent storm surges. Both the long-term trend and the variability in extremes are important.

Untangling that variability is important for efforts to predict heatwaves. So far, says Jacox, the models that predict ocean temperature a year out aren’t good at spotting when heatwaves will start, but are better at predicting their evolution once formed. “We are making headway,” he says. “I don’t think we’re at the point where we can predict them. But once they show up the models predict if things will stay warm.”

For many of the creatures living in these waters, it often doesn’t matter why the temperature is rising, but simply whether it’s too hot. “Most animals do have absolute thermal limits, just like people,” says Leising, who was trained in biological oceanography. Many marine animals are cold-blooded, he notes, and initially they seem fine as the temperature goes up: they can swim faster, reproduce faster, using up more oxygen as they go. “Up until some breaking point at which they absolutely break down and die,” he says.

A single heatwave “can have ecosystem impacts that resonate for decades,” says one scientist.

Even sedentary organisms like kelps or corals might be able to move or genetically adapt to new conditions, generation-to-generation, if (and that’s an important “if”) the temperature changes slowly enough. But heatwaves are the most rapid spurts of temporary temperature change, and bring the highest of high temperatures. “The tops go into uncharted territory,” says Thomas Wernberg, a marine ecologist at the University of Western Australia who has studied the impacts of recent heatwaves there.

Those uncharted highs can hit the base of the food chain. Heatwaves are known to shift the kinds of plankton that thrive in a given patch of water, for example, boosting harmful or toxic algae blooms like “red tide.” The zooplankton that thrive in cooler waters also tend to be bigger, richer in fats, and more packed with calories than the ones that thrive in warmer waters; hot water effectively breeds junk food for species like salmon.

Temperature spikes can have a dramatic impact even if they are relatively small. Corals, for example, typically bleach when temperatures hike up just a degree above their normal maximum. A 2016 heatwave across northern Australia dramatically affected more than 90 percent of the Great Barrier Reef, leaving swaths of dead coral skeletons.

When a hot spell hit the water off western Australia in 2010-2011, Wernberg remembers, things changed fast. “For months we had temperatures several degrees higher than anything seen in over 100 years at least. Probably 250 years,” says Wernberg, who had been studying kelp forests off that coast for a decade. In early 2011, his dives north of Perth brought a shock: “We dropped in and said, ‘Ho, what has happened here? This is massive!’ It was just the same everywhere. Not a single kelp in sight.” Low-lying turf seaweeds had replaced the kelp, and the fish population was different too. Along Australia’s west coast, 43 percent of the kelps were gone over a vast area of 2,300 square kilometers. Even in 2020, Wernberg says, after many intervening non-heatwave-years, there has been basically no recovery north of Perth. The whole ecosystem has shifted.

A 2016 heatwave off the coast of Australia caused mass coral bleaching that affected 90 percent the Great Barrier Reef.

A 2016 heatwave off the coast of Australia caused mass coral bleaching that affected 90 percent the Great Barrier Reef. Oregon State University

The hints that heatwaves can trigger tipping points are disturbing. This means a single heatwave “can have ecosystem impacts that resonate for decades,” says NOAA’s Elliott Hazen, a fisheries ecologist. There have been shifts off the coast of California from a warm spell in the 1990s, Hazen notes, that have persisted: voracious Humboldt squid, for example, still ply waters as far north as Vancouver Island although they used to be a far more southern species. The 2013-2016 Blob also seems to have caused lasting ecological shifts, he adds. “More and more it looks like we don’t see signs of going back to pre-2014.”

No single definition of a heatwave works to capture the ecosystem impacts for all species, notes Jacox. For corals and kelps, it makes sense to talk about whether it’s hotter than the historic norm. But for highly mobile species like tuna, he notes, there’s no point in calculating the heat stress of where a fish originated. Instead, he argues, what’s important is how far those species will have to go to find more amenable conditions. And this map too will change in a warmer world, according to Jacox’s recent study.

In the North Pacific, for example, the high north is warming faster than the global average. That means fish in that region will have to swim further north out of a heatwave in search of cooler waters, food or comfort in the future. Around the equator, by contrast, average warming is predicted to be quite intense in a narrow band, so fish won’t have to go far to find cooler conditions during a heatwave.

There’s a lot to figure out about any given heatwave — why it started, how high the temperature will rise, how long it will persist, how close to shore and how far the heat extends, and how deep it goes, which affects how nutrients mix from top to bottom. Add all that to the variability in species mobility and the location of things like shipping lanes, and you get a complex picture where ecosystems and economies collide in sometimes unexpected ways.

What’s needed is dynamic ocean management: tools and policies that can shift on-the-fly as conditions change.

Earlier this year, researchers charted how the original 2013-2016 Blob triggered a chain of events with disastrous consequences for whales: The warmth spurred harmful algae blooms that made shellfish toxic, delaying 2015’s crab fishery. Meanwhile, the warmth also made for low numbers of krill, forcing whales to move closer to shore in search of anchovies. By the time crabbing opened in spring 2016, the fishers hit the water right on top of the whales, leading to a record number of entanglements (more than 70 in 2016; prior to 2013 the annual average was about 10). “The boat strikes were bad too,” Leising notes, as the whales also ended up funneled into the shipping lane. “Whales got basically impaled on ships and were getting dragged through the ocean.”

This is the sort of thing researchers have to untangle in order to help fisheries managers deal with future conditions.

What’s needed, says Hazen, is dynamic ocean management: tools and policies that can shift on-the-fly as conditions change. Some of this is already being done. In the U.S., EcoCast produces something like a weather forecast map showing where swordfish and bycatch species are predicted to be, helping fishers to pick suitable hunting grounds that will avoid accidentally devastating sensitive species. WhaleWatch similarly maps out where there is a high risk of ship strikes of whales so captains may know to slow down. Both of these tools, Hazen says, were invented precisely because of marine heatwaves and the need to accommodate them in decision-making.

Scientists have worked to communicate with fisheries managers about the perils of heatwaves, says Leising. But as for the details of telling them exactly how policies should change for specific stocks, “we’re not there yet. We don’t have enough data.” Leising points out that for the west coast of North America, the two anomalously big blobs of 2013-2016 and 2019-now are just two events, one of which is still unfolding — not nearly enough to make predictions about specific species.

What they do know, Leising says, is that these two events are extreme outliers from what came before — and a sign of things to come.