A Marine Heat Wave Intensifies, with Risks for Wildlife, Hurricanes and California Wildfires

The last Pacific ‘warm blob’ killed seals and birds, shut down crab fisheries and also affected the land, worsening drought and wildfire risk in California.

A sickly harbor seal pup is examined at the Pacific Marine Mammal Center in March 2015 after it was rescued in Laguna Beach, California, amid a marine heat wave. Credit: Robyn Beck/AFP/Getty Images
A sickly seal pup is examined at California's Pacific Marine Mammal Center during the last big marine heat wave in the region, in 2015. During that ocean heat wave, a record numbers of starving baby sea lions also washed ashore. Now another is building off the coast. Credit: Robyn Beck/AFP/Getty Images

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An intensifying marine heat wave in the northeastern Pacific Ocean has triggered government warnings about harm to salmon and other fisheries along the U.S. West Coast, and it’s raising concerns about hurricane risks to the Hawaiian islands and wildfire risks in California.

The last time the region saw such a widespread and intense “warm blob,” in 2014-2015, the unusually warm ocean water boosted the growth of toxin-producing algae and suppressed the growth of small organisms at the base of the ocean food chain. The impacts rippled through ocean ecosystems, with mass die-offs of marine mammals and birds, the closure of crab and clam fisheries and warnings for sardine and anchovy fisheries because of poisoning concerns.

Young salmon had less to eat as they entered the ocean, and thousands of sea lions and seal pups ended up stranded on California beaches. The halt to crab fishing cost the industry an estimated $100 million.

“We’re seeing more intensity in the marine heat waves, higher high temperatures, and that would be more of a function of climate change,” said Andy Leising, a research scientist at NOAA Fisheries’ Southwest Fisheries Science Center in La Jolla, California. He said the baseline ocean temperatures are warming so fast that scientists are scrambling to keep up with measuring and classifying the events.


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The current marine heat wave covers a horseshoe-shaped area about the size of Alaska. It extends from the Gulf of Alaska down the coast of Western North America and westward to Hawaii. In the warmest areas, sea surface temperatures have reached about 5.4 degrees Fahrenheit above average.

Around Hawaii, the overheated ocean has also contributed to a string of high temperature records on land, including the warmest summer on record for Hawaii.

“Hawaii is literally sitting in the middle of the southern limb of this marine heat wave,” said University of Washington marine heat wave researcher Hillary Scannell. “If these ocean temperatures persist into the fall longer than the atmospheric forcing, I worry that these conditions could intensify any possible tropical storms that might develop in this region.”

If the marine heat wave continues to expand toward the U.S. West Coast, it could also raise the wildfire danger in California this autumn at the peak of the state’s wildfire season. The last northeastern Pacific warm blob contributed to both drought and wildfire conditions in California, said Daniel Swain, a climate scientist at the National Center for Atmospheric Research.

How Global Warming Plays a Role

Marine heat waves occur when sea surface temperatures in part of the ocean rise and stay above the expected seasonal temperatures for at least five days in a row. Scientists say these heat waves are forming more frequently, and they suspect that shifts in winds and ocean currents driven by global warming are a big part of the cause.

A 2018 study showed that, since 1925, marine heat waves have become 34 percent more frequent and they are lasting longer. The majority of marine heat waves, about 87 percent, can be attributed to human-caused global warming, the authors found.

Map: Recent Marine Heat Waves

The fact that about 93 percent of the heat trapped by greenhouse gas pollution ends up being stored in the oceans is a key contributor to marine heat waves, said Kevin Trenberth, a senior climate scientist with the National Center for Atmospheric Research.

Stagnant atmospheric patterns are also thought to drive marine heat waves. And overheated ocean areas can also have a feedback effect on the climate by affecting the atmosphere, said climate scientist Stefan Rahmstorf, with the Potsdam Institute for Climate Impact Research.

Large areas of above-average ocean temperatures, similar to below-normal temperatures, can shift the track of weather systems, he said. “Large-scale surface temperature patterns have an influence on the atmospheric circulation, for example on preferred places where high and low pressure areas form or where the meanders in the jet stream like to sit,” he said.

What Happens to the Coast?

The National Oceanic and Atmospheric Administration is tracking the current marine heat wave and has warned of disruptions to ocean ecosystems and fisheries along the West Coast.

Leising said this year’s warm blob could be as strong as the last and is already one of the most significant marine heat waves on record in the region because of its size and intense warmth.

So far, upwelling—vertical currents of cool water rising to the surface near the coast—has kept the heat from reaching the coast, but the winds that drive the upwelling there often die down in the fall. That has already started happening along the coast of Washington, where the warm water is starting to affect temperatures on land.

“It’s just starting to reach the California coast. From San Diego to Eureka, temperatures are warmer than average, and it’s not cooling down and staying muggy at night,” said Swain, who closely studied the 2014-2015 California heat waves and drought.

“The impacts really can be devastating, and we’re in uncharted territory because we haven’t seen back-to-back events like this in the Pacific,” he said. “Extreme and persistent marine heat waves have increased a lot, especially in the Northeast Pacific, and they do appear to be linked with persistent high pressure in the atmosphere.”

Comparison: A Marine Heat Wave Grows in the Pacific

Some of the effects of heat waves on the ocean food chain are subtle but significant.

“If you crank up the temperature on an invertebrate, it grows faster and develops faster but it ends up being smaller at adult age, sometimes only half as big. That’s really important because the fish that rely on them use a visual system to identify prey, and if they (the invertebrates) are too small, the fish won’t go after them,” Leising said.

During the last northeastern Pacific heat wave, the usual coastal phytoplankton and krill species were also replaced by species rarely seen near the coast, he said.

“If you just change what kinds of things are available, there is disruption to the ecosystem, and that kind of disruption is not so dependent on the intensity of the marine heat wave. The temperatures may not be lethal to fish directly, but the food base is gone. That’s the food for the sea lions, and they either died, or went north find better stuff,” Leising said.

Pushing Ecosystems Over the Edge

Along with immediate short-term impacts, the surge in marine heat waves will have significant long-term impacts on ocean ecosystems, said Dan Smale, with the Marine Biological Association of the UK, lead author of a recent study that documented how marine heat waves threaten global biodiversity.

Marine heat waves “are rapidly emerging as forceful agents of disturbance with the capacity to restructure entire ecosystems and disrupt the provision of ecological goods and services in coming decades,” he and other researchers concluded in the March 2019 paper.

Some of the oceans’ most fundamental ecosystems—coral reefs, beds of seagrass and kelp forests—are at a high risk from disturbance by marine heat waves. All those ecosystems are important nurseries for other species.

“Marine heat wave intensification is and will happen very quickly and it seems unlikely that most species can adapt quickly enough,” Smale said. “My feeling is that intensification will push many populations and ecosystems over the edge, leading to a loss of biodiversity and ecological functioning, at least in the short term.”

Published Sept. 17, 2019