It used to be rare to see blue whales, like this one spotted near Long Beach, along the California coast. (Photo: David McNew/Getty Images)

In 2011, Dan Smale was a post-doc in western Australia working on a kelp forest ecology project when water temperatures in the region suddenly spiked. Kelp forests, sea grass meadows, coral reefs, and many of the animals that call these ecosystems home began to die off. For several months, Smale and his colleagues tracked the impacts of the record-breaking water temperatures in western Australia, and began to wonder just how widespread marine heat waves are, and what effects they might be having on marine ecosystems elsewhere in the world. Eventually Smale teamed up with an international team of researchers that included oceanographers, climate modelers, fisheries scientists, and ecologists to find out.

In a study published today in Nature Climate Change, Smale, now a research fellow at the Marine Biological Association of the United Kingdom, and his colleagues show that marine heat waves are becoming more frequent and intense, putting extra pressure on ocean ecosystems that are already stressed by climate change.

The study pulled together data from all over the world and across multiple levels of marine ecosystems, allowing the authors to draw broad conclusions about ocean heat waves while highlighting the varied ways the events can affect different organisms. Nick Bond, an associate professor at the University of Washington and the state's climatologist, who was not involved in the paper, called the work an "important milestone" in the study of marine heat waves, notable for its scale and broad conclusions.

Marine heat waves, like their atmospheric counterparts, are a natural phenomenon, defined as a spike in ocean temperatures above the average for a particular region for at least five consecutive days. Because marine heat waves are relative to average ocean temperatures, they can occur anywhere and during any season.

Sea surface temperatures normally fluctuate, rising or falling with the winds, currents, or phase of El Niño. But since 1925, when the instrumental record begins, the number of days that break that heat wave threshold has increased dramatically: Analyzing temperature data from ships and satellites, Smale and his co-authors found that there were more than 50 percent more heat wave days between 1987 and 2016 than there were between 1925 and 1954. The data also showed that, not only are marine heat waves occurring more frequently today, they're also lasting longer.

That's because, much like the atmosphere, the oceans have been warming, which makes natural heat waves both more likely to occur and more destructive when they do. "There's a really strong link with anthropogenic climate change," Smale says, "because as the oceans absorb more excess heat, they become warmer on average, and then when these marine heat waves happen they're happening from a higher starting temperature, so the absolute temperatures experienced during these events are some of the highest on record."

"I think it's a really important idea to think about things from an event scale rather than just mean changes," Bold says. "That kind of thinking has been prevalent in the meteorological and atmospheric science community, where with things like heat waves, it's not the mean temperatures going up that is important to human health, it is that when the heat waves come they're that much more intense. That's when the problems are. It's not in the slow changes but what that means for the severity and the frequency of the more extreme events."

To that end, Smale's team looked more closely at eight specific marine heat waves to evaluate the ecological impacts of the temperature spikes, and found, perhaps unsurprisingly, that stationary plants and animals, such as kelp forests and corals, tended to be hit harder than species that could easily migrate or those living in colder regions. The 2011 heat wave off of western Australia killed off an entire kelp forest, for example. But the meta-analysis of all eight heat wave events revealed a net positive response for fish species, which the authors attributed to invasions of tropical species into normally temperate zone.

Shortly after a mass of warm water, dubbed "the Blob," blanketed the coastal region of the Pacific Northwest around 2015, tropical species of fish began showing up in fishermen's nets. Ocean sunfish, the massive, disc-shaped fish that tend to stick to warmer, subtropical waters were spotted as far north as the Gulf of Alaska, Bold says. (Just last week a southern species of sunfish known as a hoodwinker, which has only been spotted in the Northern Hemisphere on one other occasion, washed up on Santa Barbara, California's shores, but scientists have yet to speculate on why the animal was swimming so far north.)

Those "charismatic megafauna" get all the attention, but what's even more significant is the changes that happen at the base of the food web, according to Bold. Many animals feed on zooplankton, the tiny animals and larvae that drift throughout the water column. Cold water plankton tend to be bigger, more calorie dense, and richer in fats, Bold says, but when the water warms, that can lead to an overabundance of subtropical zooplankton. "It's not that it's too warm for [the fish], it's more that they're not finding the food that they prefer, the prey that they've adapted to," Bold says, likening a marine heat wave to a land-based one that harms more humans not through heatstroke, but rather by killing off our crops.

It's not yet clear how long-lasting the effects of marine heat waves can be. Even after the Blob subsided, sightings of tropical species in the Pacific Northwest persisted. Similarly, a turf seaweed field that took hold after the kelp forest withered off of Australia remained even after temperatures returned to normal.

"It's like any abrupt ecological disturbance," Smale says. "You're going to have winners and losers." Smale and his colleagues found that species living toward the colder reaches of their habitat ranges tended to fare better than those at the warmer ends.

Indeed, according to Bold, Sockeye Salmon runs in Bristol Bay, Alaska, at the northern end of their range, have thrived as the water of the Bering Sea has warmed, while Sockeye in the Gulf of Alaska, toward the middle of the fish's range, haven't done nearly as well.

Going forward, Smale and his colleagues are looking for ways to forecast the location, severity, and duration of marine heat waves to predict where their impacts will be greatest—and maybe even do something about it. "If there's a regional fishery that targets a particular species that is quite vulnerable to these events, then perhaps there can be some adaptive management where you reduce fishing pressure," Smale says.

In the age of climate change, it's not just marine species that need to adapt. "Certainly theres going to be changes with climate change to marine communities, but still the sun is going to shine, and plankton is going to grow, and things are going to eat that plankton, so it's not like the oceans are going to become the dead sea," Bold says. "It's just that, as a consequence of what we're doing to the oceans, there's going to be different marine communities in different places than what we're used to. Obviously that is a problem because we're sort of set up for what the climate is now rather than what it is going to be in the future."