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A blob of hot water in the Pacific Ocean killed a million seabirds, scientists say

(CNN) As many as one million seabirds died at sea in less than 12 months in one of the largest mass die-offs in recorded history -- and researchers say warm ocean waters are to blame.

The birds, a fish-eating species called the common murre, were severely emaciated and appeared to have died of starvation between the summer of 2015 and the spring of 2016, washing up along North America's west coast, from California to Alaska.

Now, scientists say they know what caused it: a huge section of warm ocean water in the northeast Pacific Ocean dubbed "the Blob."

A years-long severe marine heat wave first began in 2013, and intensified during the summer of 2015 due to a powerful weather phenomenon called El Nino, which lasted through 2016.

The heat wave created the Blob -- a 1,000-mile (1,600 km) stretch of ocean that was warmed by 3 to 6 degrees Celsius (5.4 to 10.8 Fahrenheit). A high-pressure ridge calmed the ocean waters -- meaning heat stays in the water, without storms to help cool it down.

Dead common murres washed up in Whittier, Alaska, on January 7, 2016.

Those few degrees of warming wreaked havoc on the region's marine ecosystems. There was a huge drop in the production of microscopic algae that feed a range of animals, from shrimp to whales. The warmth caused a massive bloom of harmful algae along the west coast, that killed many animals and cost fisheries millions of dollars in lost income.

Other animals that experienced mass die-offs include sea lions, tufted puffins, and baleen whales. But none of them compared to the murres in scale.

A common murre -- a fish-eating seabird of the North Pacific.

About 62,000 dead or dying murres washed up on shore -- but the total number of deaths is likely to be closer to one million since only a small fraction of birds that die at sea wash up, said researchers from the University of Washington, who published the study in the journal Plos One on Wednesday.

Alaska saw the most birds washed up -- in Prince William Sound, to the south of the state, more than 4,600 bird carcasses were found every kilometer (0.62 miles), the study said.

The murres likely starved to death because the Blob caused more competition for fewer small prey. The warming increased the metabolism of predatory fish like salmon, cod, and halibut -- meaning they were eating more than usual. These fish eat the same small fish as the murres, and there simply wasn't enough to go around.

A colony of common murres.

The Blob devastated the murres' population. With insufficient food, breeding colonies across the entire region had reproductive difficulties for years afterward, the study said. Not only did the population decline dramatically, but the murres couldn't replenish those numbers.

During the 2015 breeding season, three colonies didn't produce a single chick. That number went up to 12 colonies in the 2016 season -- and in reality it could be even higher, since researchers only monitor a quarter of all colonies.

"The magnitude and scale of this failure has no precedent," said lead researcher John Piatt in a University of Washington press release. "It was astonishing and alarming, and a red-flag warning about the tremendous impact sustained ocean warming can have on the marine ecosystem."

The study warned that it remains unknown how long it would take for the population to recover -- or if it would recover at all, "in light of predicted global warming trends and the associated likelihood of more frequent heatwaves."

There have been several other marine heat waves emerging in recent months. In September 2019, the University of Washington researchers discovered one almost as big as the Blob, forming off the coast of Washington state -- and they're bracing for its potential effects.

Another blob has also formed off the eastern coast of New Zealand. This blob is so big it's detectable from space -- it's about a million square kilometers (400,000 sq miles), an area larger than the size of Texas.

It's especially rare to see a patch of warm ocean water over such a large area, but scientists say global climate change is making these phenomena more common.

From 1982 to 2016, there was an 82% rise in the number of heat wave days on the global ocean surface, according to a 2018 study. That's because heat waves are increasing in both frequency and duration, with the highest level of maritime heat wave activity occurring in the North Atlantic.