The numbers of plankton and krill have crashed. The number of Pacific cod off Alaska decreased before the population finally collapsed. Hungry sea lions ran aground by the thousands and a huge number of seabirds died. With no krill to eat, the humpback whales turned into anchovies, looking for which they swam closer to shore and got entangled in fishing gear. Humpback whale births also dropped by 75% over the next six years. Toxic algal blooms have blocked crab fishing. The food web has shifted from one supported by tiny crustaceans to one dominated by nutritionally poor gelatinous organisms called pyrosomes that had never been recorded this far north.
Marine heatwaves are defined as sharp spikes in abnormally warm temperatures that last at least five days, although many persist for weeks or months. Fueled by the warming of the oceans caused by climate change, they can impact marine ecosystems for years after the water has cooled again. As the effects of climate change take root around the world, marine heatwaves are increasing in frequency and intensity, posing “a major concern” for ocean environments, says oceanographer Hillary Scannell.
Sea heat waves are on the rise
Just as scientists were coming to terms with ongoing events in the Northeast Pacific, a group of 15 ocean experts gathered in Perth, Western Australia to piece together emerging science on marine heatwaves. Their encounter was not provoked by the blob, whose full nature was just emerging, but by a warming event during the summer of 2010-11 in which the waters off Western Australia warmed to an astonishing temperature of 11 ° F, killing huge expanses of forest seaweed and large numbers of animals, from abalone and scallops to penguins.
At the time, says Eric Oliver, now of Dalhousie University in Nova Scotia, Canada, who was present at the meeting, science was in its relative infancy; since then, however, interest in the subject has exploded.
“I think the first use of the term marine heatwave was in 2010, 2011, something like that,” he says. “I knew all the newspapers that were going to come out. Now, I can’t keep track of it. ”
This increase in scientific studies reflects a growth in reports on marine heat wave events. In 2016, a wave of marine heat off the coast of Chile triggered algal blooms that devastated fish farms. From 2015 to 2019, a series of heat waves in the Mediterranean Sea led to multiple mass deaths of algae and coral, heralding what has been called a new normal in the region. In 2021 and 2022, New Zealand experienced the highest ocean temperatures ever recorded, resulting in bleaching of “millions” of sponges, according to reports.
The precise causes of these heat waves vary, although a warm climate makes them more frequent. The Western Australian event was triggered by a strengthening of the Leeuwen Current flowing south, which brought more warm water from the Indian Ocean. Similarly, a 2015-16 heatwave in the Tasman Sea between Australia and New Zealand was initiated by a strengthening of the East Australian current, which extends south of the Coral Sea.
In contrast, a 2019 study found that 60% of marine heat waves in the southwestern Atlantic, including one off Brazil in 2013-14, originated in high-pressure systems over the Indian Ocean. The blob was precipitated from what has been termed a “ridiculously persistent ridge” of high pressure parked over the North Pacific, preventing cooler air from generating storms that could shake the water. As a result, the water stratified and a hot anomalous layer was deposited on the surface.
Climate change is causing waves of marine heat
But there is an underlying element that makes such heat waves more frequent and more intense: climate change. The ocean absorbed 90 percent of the extra heat added to the atmosphere by burning fossil fuels, and as a result, the world’s highest 2,300 feet of oceans, where most of the absorbed heat is concentrated, warmed by about 1. 5 ° F on average since 1901. An ocean that is already warmer can reasonably be expected to be more susceptible to marine heatwaves, and indeed it appears to be.
A 2020 study in the journal Science concluded that marine heat waves have increased more than 20 times due to global warming. The authors found that in the first decade after satellites started recording ocean temperatures (i.e. after 1981), 27 major marine heat waves occurred, with an average duration of 32 days and an average temperature anomaly. peak of 8.5 ° F; in the 2010s there were 172, which lasted an average of 48 days with an average peak temperature of almost 10 ° F above the norm.
Much remains unclear about marine heatwaves. For example, explains Nicholas Bond, a researcher at the University of Washington and a Washington state climatologist, there is the question as to why so many persist for weeks or months. “There has to be something else going on that helps keep them,” he says. He notes that one explanation is that when the ocean’s surface warms, it radiates heat into the atmosphere which prevents cloud cover from forming, exposing seawater to more sunlight and further warming.
However, enough is known about marine heatwaves for scientists to be seriously concerned about their potential impacts. Of particular note is the fact that these impacts can last long after the heat waves have subsided. After three years of blobs, the waters of the Northeast Pacific began to cool in 2016; but years later, scientists are still determining the extent to which the region’s ecosystem is likely to fully revert to its pre-blob state. Similarly, notes Scannell, who is a data scientist with Jupiter Intelligence, Inc., following the 2010-11 Western Australia event, “many kelp forests have died and it literally takes decades before those ecosystems recover “.
Oliver is particularly concerned about the potential impact in tropical waters.
“I think that’s where it’s really troubling,” he says. Life in the tropics, she notes, is adapted to “a rather narrow temperature range. So this is where things can get really messy. We can have full shifts in tropical systems. That’s why people are so worried about coral reefs ”.
As destructive to marine ecosystems as marine heatwaves can be isolated, Bond notes that they are multiplying and intensifying at the same time that the ocean is facing a host of other pressures, making the potential impacts of heatwaves still more serious.
“In many parts of the global ocean, fishing is probably unsustainable,” he notes. “There is just incredible pressure on those ecosystems. And when you add to that the changes that occur due to events like marine heatwaves, changes that will reduce the productivity of these systems, it is very distressing. It’s something we’d better recognize before we see collapses that would have a tremendous impact. “