An Alarming Rise in Ocean Heatwaves


Heatwaves in the ocean? We interchange “weather” and “climate” in the context of our atmosphere, yet it’s the oceans that are the engines of Earth’s climate. There are not one but two dynamic fluid systems circulating above its solid surface, yet we know relatively little about the one that’s arguably more important in regulating the world’s heat distribution. For example, the average air temperature difference between the tropics and the poles is 86° F. Take the ocean’s moderating effects out of the equation, and the difference would be more than twice as much.

Hotter now than they’ve ever been, the oceans have absorbed around 90 percent of the excess heat that humans have caused, but they can also give it back. One effect of a heated ocean surface is that it will stratify and not mix as much with colder layers below. Less mixing means the surface layer absorbs less carbon dioxide from the atmosphere because it’s already saturated. This in turn amplifies global warming.

Only recently have we begun to talk about marine heatwaves. Most people don’t fully understand their implications. Remember the blob? It was an extreme marine heatwave that lasted over 200 days, covered 10 percent of the North Pacific, and was unprecedented in 150 years of record-keeping. It existed periodically from May 2014 to January 2017, corresponding with an intense El Niño, and was notable in its geographical extent, depth, and persistence. It covered the ocean from the Gulf of Alaska all the way to Southern California. And it was catastrophic.


Let’s start with what it did to oceanic marine life. Scientists found that the first thing to change was metabolism. This led to a cascading series of ecosystem failures. To understand it, let’s compare the murre, a warm-blooded seabird with a high metabolism, to the Pacific cod, a cold-blooded groundfish. A murre needs to eat 56 percent of its body weight every day, which translates to catching 60 to 120 small, high-fat forage fish per day.

Murre with forage fish (courtesy USFS)

If a murre can’t find any food for 3 to 5 days, it will die of starvation. But they’re great hunters, so successful that they are one of the more common seabirds. Under normal North Pacific conditions, a cod the same size as a murre needs to eat only 0.4 to 1.5 percent of its body mass daily, or about 1 to 3 high-quality forage fish. That’s a big difference.

What happens when the ocean warms up? It’s complicated, but basically, the cold-blooded fish get hungrier. For every couple of degrees warmer, their metabolisms double, meaning they have to double their intake. So, forage fish gobble up plankton, and large fish gobble up forage fish, leaving high-metabolism birds, whales, sea lions, and other warm-blooded wildlife with far less food. When that happens, die-offs occur.

Photo: USFWS-Common-murre-group-RLowe

Tipping point? 

There’s a 62 percent chance of the fading La Niña shifting to an El Niño in the Pacific by June or July, and a 40 percent chance that it will be a strong one. Recent research shows that vertical ocean currents, known as upwelling and downwelling, are affected by differences in temperature. This can slow the horizontal ocean currents that we are more familiar with, such as the Atlantic’s Gulf Stream. In fact, the Gulf Stream may be approaching a tipping point. When El Niño’s warming effects slow or completely stop the upwelling of colder, nutrient-rich waters near coasts, it also dampens the growth of the spring and fall blooms of plant and animal plankton, by reducing their numbers, size, and caloric value. This in turn forces fish, seabirds, and marine mammals to move offshore to find food or die. Many don’t survive. In addition to marine heatwaves, El Niño is also connected with coral bleaching, floods, and droughts worldwide.

Dramatic effects

In 2017, scientists found that Pacific cod populations had declined 71 percent in less than two years. That’s a $103 million dollar fishery, which is 29 percent of the groundfish harvest value in the Gulf of Alaska. Also that year, the physical size and condition of those fish were at record lows. Die-offs of tufted puffins, Cassin’s auklets, and murres coincided not only with the lack of food but also with a harmful, heatwave-induced algal bloom, the largest and most widespread in recorded history. Like a double whammy it shut down the West Coast Dungeness crab fishery, and extended from California to the Aleutian Islands, producing the highest concentration ever recorded of domoic acid, a potent neurotoxin. Paralytic shellfish poisoning was not uncommon, including in some endangered murrelet nestlings whose parents unwittingly fed them forage fish infected with the harmful algae. Common murres experienced the most dramatic die-offs; on a single mile-long beach near Whittier, Alaska, a team found 8,000 dead birds, mostly murres.

At least 62,000 dead murres were found washed ashore in the Gulf of Alaska and Bering Sea. Because most dead birds never reach the beaches, scientists estimated a total die-off of one million murres. It was, in the scientists’ words, “unprecedented and astounding.” They estimated that as much as one quarter of all Alaskan breeding-age murres died of starvation. There are no such estimates for Washington, Oregon, or California populations. It remains to be seen whether murres will recover to their former abundance, given predictions for more marine heatwaves. Even under favorable circumstances, recovery can take decades.

Heatwaves coming

The record-breaking atmospheric heatwave from June 23-28, 2021 was one of the most extreme ever recorded globally. It coincided with exceptionally low tides and caused an unprecedented shellfish die-off that was compared to the effects of a forest fire.

Last year, 2022, had already marked the hottest temperatures ever recorded in our already profoundly altered ocean ecosystems, but since March 2023, average sea surface temperatures have been climbing to record highs, as shown in the dark line in the graph below.

Globally, the annual number of marine heatwave days doubled between 1986 and 2016. Future projections indicate an accelerating number of marine heatwave days by the end of the century. Their size is predicted to be be 21 times larger than in preindustrial times and forecast to last an average of 112 days. Maximum sea surface anomalies are predicted to reach 4.5° F, which is enormous.

Magnifying effects

Nearer to land, impacts from forest cutting can warm adjacent waters by several degrees, adversely altering salmon habitat. A February 2023 study found higher stream temperatures in watersheds—by more than 7° F—where there had been more logging. As trees are cut down along waterways, small streams are exposed to more direct sunlight. Logging across watersheds can change the way water flows throughout the whole system. This reduces the ability of ecosystems to survive and recover from adverse weather cycles.

There’s still hope

No one knows how intense the coming El Niño might be, because the word “normal” now feels like an anachronism, and the effects of marine heatwaves aren’t limited to oceans. Still, the window has not closed on hope. While it is indeed a race against time, if enough people disavow the sense of fatalism that grips too many of us, and if we can subscribe to the idea that what we all do as individuals matters in terms of at least reducing the worst impacts (and this includes holding elected officials accountable), then a livable albeit less comfortable future can still be possible. The emergency we’re facing can no longer be denied.

You can see her other work at This article first appeared in Rainshadow Journal.

Karen Sullivan
Karen Sullivan
Karen Sullivan is a Port Townsend writer and poet, former ship captain, marine biologist, and spokesperson for a federal agency. She is a member of the Rainshadow Journal collective and is at work on a book. You can see her other work at


Please enter your comment!
Please enter your name here

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Comments Policy

Please be respectful. No personal attacks. Your comment should add something to the topic discussion or it will not be published. All comments are reviewed before being published. Comments are the opinions of their contributors and not those of Post alley or its editors.