Heather McCarty, of the Central Bering Sea Fishermen’s Association, which manages community fisheries allocations for St. Paul, said in February that the city’s tax revenues went from about $2.5 million two years ago to approximately $200,000 this year. “It was all snow crab all the time,” she said at the time. “[Now] they have about a year’s worth of reserves that will allow them to survive with the municipal services relatively intact, but, after that, it’s anybody’s guess how they’ll actually pay for really basic things.”

Not long after the snow crab season was canceled, Bob Foy, science and research director of the National Oceanic and Atmospheric Administration’s Alaska Fisheries Science Center, estimated that billions of crabs had been lost in just a few months’ time. “We don’t have a smoking gun, if you will,” Foy said of the collapse. “Except the heat wave.”

The St. Paul community, commercial fishers, and scientists like Foy had been growing increasingly worried about the Bering Sea’s marine ecosystem since 2013, when a sustained period of light winds led to the creation of a massive hot spot in the eastern Pacific Ocean. “The Blob,” as the swath of warm surface water was dubbed, turned out to not be a fleeting anomaly but a ballooning crisis. Over the next three years, it encompassed much of the North American West Coast, an area of about 3 million square miles.

The world’s oceans have absorbed about 90 percent of the excess atmospheric heat generated by carbon dioxide emissions, which has manifested as an average sea surface temperature increase of 0.14 degrees per decade. When wind patterns weaken or shift, so too do ocean currents, gyres, and eddies — processes that essentially serve as the oceans’ circulation system. Dennis McGillicuddy Jr., deputy director of the Woods Hole Center for Oceans and Human Health, describes the warming of both water and air as a kind of ever intensifying feedback loop. “The wind patterns are heavily impacted by the distribution of heat over the Earth, and since most of the heat is in the ocean, changes in the currents are going to change the heat distribution, which then feeds back on the winds,” he says. “So, it really is a very tightly coupled system.”

As waters warm and currents shift, prey species like krill decline in abundance or move to cooler water. The whales and salmon that feed on them must follow or face starvation. What may appear to be a single issue — a warming atmosphere — becomes a complex tangle stretching across ecosystems.

Unusually high spikes in ocean surface temperatures like The Blob are becoming all too common — according to NOAA, since 2012, strong or severe marine heat waves have become 50 percent more frequent. El Niño, a warming phenomenon driven by a sustained period of shifting winds along the equatorial Pacific, is probably the best-known producer of marine heat waves. During the 2016 El Niño event, the South Pacific islands and Australia’s Great Barrier Reef experienced catastrophic coral bleaching. Last summer’s extreme heat across Europe triggered a marine heat wave in the Mediterranean that caused mass die-offs of sponges, sea stars, and mollusks. In the North Atlantic waters off New England and eastern Canada, rising water temperatures have been dramatic and long-lasting, with cod, haddock, and lobster departing for colder waters to the northeast. Facing drops in traditional prey, the North Atlantic’s large whales are increasingly chasing less nutritious food sources closer to shore, where they are at more risk of injury from fishing gear entanglement, vessel strikes, and other human interactions.

In some parts of the Gulf of Alaska, surface temperatures one year after the emergence of The Blob had risen by as much as 7 degrees Fahrenheit. Trouble quickly cascaded across the gulf’s ecosystem. Algal blooms poisoned shellfish. Krill and forage fish numbers declined, causing whales, cod, and other predator species to shift their migratory patterns in a desperate search for food. Between 2018 and 2019, NOAA recorded sustained periods of surface water temperatures of over 38 degrees F in the Gulf of Alaska, roughly 2 degrees higher than the average over the past two decades. By then, the dangerously warm water had crept through the wide passes of the eastern Aleutian Islands and began mixing with the cold waters of the Bering Sea.

Until recently, research on marine heat waves has centered primarily on ocean surface temperatures. Observational data from satellites, buoys, and research ships have historically been focused on this area of the water column because it is an important predictor for storms and weather patterns like El Niño and La Niña. Heating or cooling in the upper layer of oceans is also a key driver of distribution shifts in species important to fin-fisheries, like tuna, salmon, and menhaden. But there is mounting evidence that heat waves can occur throughout the water column, including at the seafloor, where myriad fish and crustaceans, such as the snow crab, live.

In March, a team of scientists from NOAA, University of Colorado, and the National Center for Atmospheric Research published a study that focused on “bottom marine heat waves” along North America’s continental shelves. The researchers found these events can occur simultaneously with surface heat waves and sometimes persist even longer. The team also learned that, when a bottom marine heat wave is underway, there might be little evidence of it at the top of the water column. “That means it can be happening without managers realizing it until the impacts start to show,” the study’s lead author, Dillon Amaya, said.

In the Bering Sea, the first sign of likely trouble on the seafloor was in the winter of 2018-19, when the Gulf of Alaska’s surface temperatures reached record highs. “In 2018, 2019, we saw far and away the lowest sea ice extent on record, and far and away the highest temperatures, in the Bering Sea,” says Mike Litzow, who heads NOAA’s Shellfish Assessment Program in Kodiak, Alaska and monitors snow crab populations. Though Bering Sea crab fishers have known it intuitively for decades, in 2008, Litzow and his colleague, Franz Mueter, compiled the first empirical evidence connecting sea ice with snow crab abundance. The species prefers temperatures of about 35 degrees F and below — when sea ice begins to melt, cold, dense water falls to the bottom and remains there through the summer, creating ideal living conditions for snow crab. “Snow crab are an Arctic animal, and in Alaska they only exist in waters that are seasonally ice covered,” Litzow says. “And areas with ice on the surface in the winter are much colder on the bottom in the summer.”

When I had spoken to Zavadil, the St. Paul city manager, in February, he was still holding out hope that the Bering Sea ice would show up. But when I talked with him again in May, he recalled months of dramatic swings between snow and rain, which is not characteristic of winter at such a high latitude. “We never did see the ice this year,” he said.

Making the impacts of warmer water along the seafloor even more acute is the fact that snow crabs are a “pulse fishery,” meaning they seem to experience natural boom-and-bust cycles. No one, including scientists like Litzow, is quite sure why this happens. (Blue crabs in the Eastern U.S. have similar fluctuations in abundance.) To add to the mystery, in 2018, when so many species in the Gulf of Alaska were undergoing massive die-offs, the Bering Sea’s snow crab population had one of its biggest recruitments — or baby booms — ever recorded. Unlike past booms, however, this time, none of the juveniles survived to adulthood. “What we had this year is all these animals that were still immature, still small, just disappear,” Litzow says. “This was totally unprecedented.” He noted that loss estimates range from 10 to 40 billion animals, and no age group was spared.

The fear is that, as water temperatures continue to climb, the snow crab’s boom-and-bust cycles might become too intense to sustain a viable fishery. Says McCarty, of the Central Bering Sea Fishermen’s Association: “When is it not a disaster anymore? When is it just status quo?”

Another vexing, unanswered riddle is what, exactly, is killing the crabs. While overheated water is the obvious proxy, Litzow says, the actual cause of death remains an open question. There are clues, though, beginning with metabolism. In his lab in Kodiak, Litzow and his team have observed that a snow crab’s metabolic rate increases dramatically with just few degrees of temperature increase. As with humans, a sustained period of high metabolism leads to energy exhaustion; one early study found that snow crabs stop feeding altogether in temperatures above 53.6 degrees F). It is also likely that, when the Gulf of Alaska heated to unsustainable levels, groundfish like Pacific cod fled north to the Bering Sea, thus increasing predation pressure. Perhaps, Litzow says, warmer water intensifies the crabs’ vulnerability to diseases. Maybe it’s a mix of all these factors. “We know it’s really not the snow crab itself,” he says, “but the web of connections that make up the ecosystem it lives in.”

Since the Bering Sea’s snow crab fishery became St. Paul’s primary source of revenue in the 1980s, the city has learned to prepare for this creature’s boom-and-bust cycles by establishing an emergency fund. But in the past, says Cory Lescher, science advisor for Alaska Bering Sea Crabbers, a trade group, the community could rely on other fisheries, like king and bairdi crab and halibut, “to weather the storm and get them through the next couple of years.” These days, though, the kings are virtually gone; the bairdi are diminished to the point that quotas aren’t high enough to pay the bills; and the halibut have been in decline for the past decade. “The scale of this,” Lescher continues, “is something we’ve never seen.”

In February, Zavadil had said that, in order not to completely exhaust its emergency fund. St. Paul was going to scale back on basic community services. It would need a volunteer ambulance driver and could no longer pay for a medical transport plane to fly in regularly from the mainland. But those cuts and others would hardly be enough. “We can only continue to dip into that for so long before it’s all gone,” Zavadil said. (St. Paul was approved for funding as part of an Alaska-wide federal disaster declaration in the wake of the snow crab collapse, but the community has not yet received its share of the money.)

While he said he was hopeful that the snow crab would return — an encouraging number of juveniles have been observed in recent survey trawls — Zavadil pointed out that “we’re working to plan for economic diversification.” The island’s small tourism industry is one hopeful alternative. St. Paul is a key stopover for rare migratory birds, and when we spoke in May, the first planeload of birders had landed a few days earlier. Some small cruise ships would arrive at the height of summer. The city had imposed a modest $12 wharf fee and was thinking about adding a tourism tax to rental cars. “That by far does not make up for any of the tax dollars we get from the crab fishery,” he said. “But it’s helpful.”

He described a recent “community open house,” at which members of the tribal government put big Post-it notes on the wall for residents to write down the things they liked about living in St. Paul and the things they felt were challenging about living there. Some of the biggest concerns were about the school. Would they be able to get and keep good teachers? Would the kids stick around after graduation or move away in search of work? Zavadil described his neighbors as hopeful yet worried.

“We’re doing our best just to try to make it through this,” he said, “and make sure that St. Paul’s still a place that people want to call home, can make a living, and have a sustainable economic future.”