SAN FRANCISCO – It could be the return of "The Blob," and scientists are worried.

A mass of warm water extending from Baja California in Mexico all the way to Alaska and the Bering Sea could result in death for many sea lions and salmon, as well as toxic algae blooms that can poison mussels, crabs and other sea life.

When it happened in 2014 it was dubbed “The Blob” and disrupted sea life between Southern California and Alaska. Now it's back.

The ocean heat wave began to form in June.

"Temperatures are about as warm as have ever been observed in any of these locations. It developed in mid-June and it's gotten really big really fast," said Nate Mantua, head of the Landscape Ecology Team at the National Oceanic and Atmospheric Administration's Southwest Fisheries Science Center in Santa Cruz, California.

The surface temperatures in the affected areas are 5 to 7 degrees above the long-term average. The so-called blob covers an area of 4 million square miles, or three times the size of Alaska. It's 165 to 325 feet deep, Mantua said.

The only other known major ocean heat wave in 2014 stuck around for two years. It covered a slightly bigger area of 4.5 million square miles of ocean.

The warming comes from a ridge of high pressure that keeps winds calm.

"The winds have been persistently weaker than they normally are this time of year," Mantua said. "If the winds stop, the surface warms. If the winds stop for a long time, that warm surface water just gets deeper and deeper because the sun’s warming it up."

Whether it will last as long as the 2014 heatwave is impossible to know, say scientists.

"If the weather patterns that led to its development change all of the sudden and we get some cold snaps and storms, then it could dissipate pretty quickly," said Stephanie Moore, a NOAA research oceanographer at the Northwest Fisheries Science Center in Seattle.

The heat waves are unprecedented. Satellite records go back about 40 years and ship observations 100 years, but nothing in the historical record has matched either of these two events, Mantua said.

Along the California coast, the blob-like warm areas are still about 1,000 miles away from shore, though the very fringes of the warm patch have been approaching some California coastal areas.

Northeast Pacific Marine Heatwave

In Washington state, the warm area has reached the shore, where it's causing a toxic algae bloom that has prompted health officials to caution against eating mussels from state beaches.

The bloom is exceptionally large, with unusual levels of an algae called alexandrium that produces saxitoxin, a deadly neurotoxin that can cause paralytic shellfish poisoning.

Mussels have been collected on Washington shores that contained more than 10 times the regulatory limit of the toxin for human consumption, Moore said.

“There’s no antidote. You’ve just got to get yourself to the hospital, get on a respirator and hope for the best,” she said.

The warm area that persisted off the West Coast in 2014 and 2015 wasn't something scientists had seen before.

With the arrival of this year's ocean heat wave, they've realized such events need a new name because they're probably likely to happen again. They've dubbed this one "The Northeast Pacific Marine Heatwave of 2019."

"It's not a very snazzy title," acknowledged Andrew Leising, a research oceanographer for NOAA in La Jolla, California, who has developed a system to track and measure the heat waves. "But it is one of the most significant events that we've seen."

Is climate change to blame?

Why any one climate event happens is always impossible to know. But there are global changes to the oceans that are likely affecting the marine heat waves.

One is that the world's oceans have warmed by about 1 degree because of human-caused global warming. "The oceans have absorbed about 90% of the excess heating that's being caused by greenhouse gases," Mantua said.

These two ocean heat waves, coming so close together, could portend a new abnormal-normal, where our old experience of what the oceans look like isn’t necessarily a good guide to the oceans of the future, he said.

“The natural phenomenon that have always caused one year to be different from the next may not be that stable," he said. "We’re fundamentally altering the heat balance across the whole planet.”

Why ocean heatwaves are bad news

That shifting heat balance won't bring good news for the ocean's inhabitants.

Warmer water isn't as nutrient-rich as the colder water that wells up from the bottom. So it doesn't support the same tiny plants and animals that sustain marine life.

In 2014, the marine heatwave significantly affected marine life up and down the West Coast. One particularly heart-wrenching example was what happened to thousands of sea lion pups on California's coast.

The oceans are filled with tiny crustaceans called copepods. When the Pacific Ocean warmed from 2014 through 2015, the "nice big fat copepods" that fish liked to eat got pushed out by skinny little ones that weren't as nutritionally satisfying, Leising said.

The fish moved north in search of better food. When sea lion mothers arrived with their pups, they found few fish to eat. The mothers typically leave the pups on shore while they eat their fill, then come back to nurse their babies on the beach.

"The females would go out looking for food but not find anything they liked to eat because the warm waters had changed things. So they just went north, leaving their babies behind on the shore," he said.

That year saw a record number of stranded sea lion pups. Only some of the pups could be rescued by animal rescue operations. The rest died.

One positive sign is that so far the heat wave is not as strong in Southern California as it was in 2014. That could mean sea lions and fur seals, all of which breed on islands off the southern coast, won't be as affected.

Other marine life that was harmed in 2014 included Guadalupe fur seals, whales, salmon, albacore, bluefin tuna and others.

There were also major blooms of toxic algae that poisoned crabs and other animals.

The marine heat wave also pushed warmer water toward the coast, resulting in a narrower "lane" of colder, food-rich waters that whales and sea turtles use to migrate north toward their feeding grounds in Alaska.

"They ended up going into the shipping lanes and we had more ship strikes and entanglement with fishing gear," Leising said.