David Irons was driving past a beach in Whittier, Alaska, on New Year’s Day four years ago when something caught his eye. It was an endless line of white lumps near the water’s edge—piles of something that shouldn’t be there.

They were dead sea birds, and the bodies were everywhere. “I just couldn’t believe it,” said Irons, a recently retired biologist with the U.S. Fish and Wildlife Service. “We started counting them, and we just counted a section and we got to 1,500.”

In all, he and his wife, son and a friend found 8,000 dead birds on a beach about a mile long. A dead zone of common murres—a species known for its resilience.

For almost a year, people had been reporting finding dead common murres up and down the Pacific coastline, from California to Alaska. From the summer of 2015 through the spring of 2016, about 62,000 washed ashore, part of a mass species die-off that scientists are attributing to an extreme marine heat wave.

In a study published Wednesday in the journal PLOS ONE, a group of scientists from various state and federal agencies, universities and bird rescue organizations documented the die-off and concluded from the data that it was caused by a record-breaking ocean heat wave in 2014 through 2016 that triggered systemic changes throughout the ocean ecosystem.

The authors estimate that 1 million common murres died during the period, an event they called “unprecedented and astounding.”

The common murres weren’t the only species to experience mass die-offs during this time—tufted puffins, Cassin’s auklets, sea lions and baleen whales died, too. But what the scientists document is by far the largest die-off, one they say was caused by disturbances rippling across the food web, a result in part of ocean warming from climate change.

Oceans are warming at a rapidly increasing pace, a study published earlier this week showed, and last year registered the hottest ocean temperatures on record. As that heat builds up, it’s having devastating consequences.

“When I heard the numbers of birds being killed in California and Oregon and Washington and many areas of Alaska, as that unfolded, it was biblical to me,” said John Piatt, a biologist with the U.S. Geological Survey who was the lead author of the new paper on the bird deaths and has been studying common murres for 40 years.

“This bird doesn’t fail unless there aren’t enough high density patches of food to serve their high demand needs. And that’s rare,” Piatt said.

The Death Toll Grows

So what happened?

As reports came in from up and down the Pacific coast, Piatt was perplexed. Common murres are known for their ability to adapt. “Murres are the ultimate predator—they’re extremely well adapted, they can dive to 200 meters, and they live on the Continental Shelf,” he said. “Anywhere along there is their domain. And they’re the fastest flying sea bird.”

Yet murres were washing in with the tides—sometimes 10 birds at a time, sometimes 100.

After Irons’ discovery on New Year’s Day, everything changed, said Julia Parrish, a biologist at the University of Washington who leads the Coastal Observation and Seabird Survey Team and was a co-author of the study. Federal agencies started to get involved and were able to fly along the coastline and send more people to conduct surveys.

Surveys in the Gulf of Alaska conducted by the Interior Department turned up more than 20,000 dead murres, and the public reported 21,435 more to the department.

The scientists reached out to bird and rehabilitation centers from southern California to Alaska and found that, out of 66 that responded, 37 reported receiving injured or dead murres—a total of 3,365 birds. The body count ticked higher.

The Investigation

The first thing the scientists needed to know was whether these deaths indicated a danger for human health. Were the birds carrying a disease? A toxin?

Carcasses were shipped to the U.S. Geological Survey’s National Wildlife Health Center in Madison, Wisconsin. “They did all sorts of analyses for viral and bacterial diseases, toxins in the tissues,” Parrish said. “We’re trying to eliminate smoking guns. But all of those things—not found. No parasites, nothing we can hang our hat on. But there was lots of emaciation.”

Like many of his peers, Piatt was aware that these deaths were happening at the same time that the ocean was experiencing a record high heat wave, exacerbated by a ridge of high pressure on the West Coast that scientists were calling “the Blob.” But still, he wondered, “What could account for a decline in the food supply from California to the Bering Sea all at the same time?”

To get the answer, the scientists started ruling things out.

The first question: Could the fish that the murres eat have moved elsewhere in response to the warmer water? It’s well understood that fish respond in specific ways when the ocean temperature changes, sometimes moving north, south or deeper down. “But the thing is, murres can go anywhere in a matter of hours,” Piatt said.

The researchers also looked into whether overfishing could be the answer, but that didn’t hold water, either.

Next, they investigated whether the fish were surviving from egg to larvae. Some juvenile stock were failing, sure, but not enough to explain the large number of starving birds.

As Piatt kept looking into it, he said he got pushback from some in the field who wanted to know how, in the absence of a clear explanation, he could still believe it was a single event that caused the birds to starve to death. But Piatt said, “There has never been such a thing. You really think that it’s a coincidence that they’re dying down there and dying up here? It’s connected.”

Finally, Some Answers

Piatt began researching how water temperature can change the food supply. He started looking from the bottom up: What were the food sources that the fish were eating? He found that as the water had warmed, phytoplankton and zooplankton, the smallest ocean organisms that provide the base of the food web, had changed.

“The older, fatter, nutritionally richer zooplankton were replaced by southerly or offshore species that weren’t as big and nutritionally rich,” Piatt said. “This was observed in the Gulf of Alaska and off California and in various studies.”

At the same time, Piatt dove into studies that found that when water gets hotter, fish like cod, flounder, pollock and hake respond by increasing their metabolism. “If you turn up the temperature by a couple degrees, they have to double their food intake,” Piatt said. “It’s a huge deal.”

It turns out that those fish feed on the same prey as the murres.

And the murres have an Achilles heel: They have to eat more than half their body mass every day. Based on their normal diet in Alaska, that’s typically 60-120 fatty forage fish every day—double that, if only leaner prey is available. By comparison, cod of similar size to a murre would only need to eat about 0.4-1.5 percent of their body mass per day—just 1 to 3 fatty fish.

The double-whammy caused by warming waters—less nutritionally rich food sources and more competition for the food available—is what Piatt and his co-authors hypothesize led to the common murres deaths.

If murres can’t fully meet their food demand every day, their body condition begins to decline quickly. “If they can’t find any food for 3-5 days, they will die of starvation,” the authors write in the new study.

What Caused the Marine Heat Wave?

The extreme heat in the ocean from 2014-2016 was a result of several factors. Piatt describes it like a step ladder:

• At the base is the ocean getting warmer due to global warming. Global warming contributed about 25 percent of the warming in the heat wave.
   
• Next, the Pacific Decadal Oscillation, a recurring pattern of ocean-atmosphere climate variability that leads to periods of warming in the mid-latitude Pacific—that contributed about 35 percent of the heat.
   
• As part of that, there was a strong El Niño from 2015-2016, which led to warming from California’s coast up to Alaska’s.

“If you remove all those signals you’ll see about a quarter of the heat is still unaccounted for, said Piatt. “That’s the Blob.” The Blob developed when a ridge of high pressure formed over the land on the northwestern coast of North America and blocked airflow from the Pacific to the interior, trapping heat over the ocean.

“It was the biggest marine heat wave so far on record,” said Thomas Frölicher, a climate scientist at the University of Bern in Switzerland who was not involved in the new study. “Usually, we are used to heat waves over land. They are much smaller in size, and they do not last as long. In the ocean, this heat wave lasted two or three years.”

In the past 35 years, marine heat waves have doubled in frequency, Frölicher said. And as global temperatures continue to rise, they will become even more commonplace.

“If we follow a high-greenhouse-gas-emissions scenario, these heat waves will become 50 times more frequent than preindustrial times” by 2100, Frölicher said. A low-emissions scenario, consistent with the Paris climate agreement, would still see 20 times more heat waves, he said.

“What that means is that in some regions, they will become permanent heat waves,” he said. The mass deaths of common murres suggests what that may look like. “This gives us some insight into the future.”

Correction: This story has been edited to correct the breakdown of factors contributing to the heat wave.