ASU studies how marine heat waves disrupt global food supplies

In 2019, about 4,600 miles from Arizona, a marine heat wave cranked up the temperature in the waters around Hawaii. For several sweltering summer months, a low pressure system sat over the Pacific Ocean between Hawaii and California and led to decreased cooling winds and sea surface temperatures 4.5 degrees Fahrenheit higher than normal.

Scientists called it “The Blob 2.0.” The original “Blob” developed in 2013 as a strange pool of warm water off the coast of Alaska, then quickly expanded all the way to Mexico in a meteorological phenomenon that lingered until early 2016 and “was so persistent and unusual that it initially defied explanation,” according to NASA.

Marine life suffered in both “Blob” events. In the mid-2010s heat wave, higher ocean temperatures fueled the growth of less-nutritious types of algae. Populations of salmon and other important fish species plummeted, straining the Pacific fishing industry. Fin whales and sea otters started washing up dead while baby seals starved on shore for all to see. And nobody quite knew what was going on.

When another marine heat wave descended on the Pacific in 2019, though, researchers with the Arizona State University Global Futures Laboratory were ready. They recorded aerial imagery of coral reefs before and after the attack of Blob 2.0. Then they layered the results onto maps that enabled them to also consider the compounding impacts of stressors like pollution, sedimentation and climate change.

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Last week, the team published its findings in the scientific journal Proceedings of the National Academy of Sciences.

The results will help conservation efforts identify reefs with the best chance of survival and areas that would benefit the most from protection as marine biodiversity refugia, the researchers say.

They also say that protecting reefs will be incredibly important not only for the snorkeling enjoyment of colorful sea fan fans, but for the well-being of people living near and far from oceans all over the world. The health of coral reefs has been linked to the success of global fisheries, which employ and feed millions of people, the production of some medicines and the protection of coastlines from storm surges that drive patterns of human migration.

And some Arizona-based research is at the heart of it all.

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Researchers at ASU have developed a technique for mapping coral reefs that may help conserve them

Greg Asner, a principal investigator at Arizona State University who is based in Hawaii, takes spectral measurements of coral reefs after a heat wave.

Courtesy of Greg Asner and the ASU Global Futures Laboratory

The sway of the sea reaches far inland

Marine ecologist Greg Asner is employed by Arizona State University, as a professor and director of the Center for Global Discovery and Conservation Science. He loves his job and his boss and being a part of ASU’s Global Futures Laboratory. And he loves that he can do it while living full-time in the Hawaiian Islands. He says it’s part of the vision ASU president Michael Crow and Vice Provost Peter Schlosser have for leveraging Phoenix-based talent and resources to contribute to a more sustainable and habitable planet for all humankind.

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From Hawaii, Asner led the recently published effort to map, for the first time, coral reef mortality before and after a major marine heat wave.

“One of the reasons I like using Hawaii as a model for the world is that, if you look down at a map of Hawaii, there are areas of intense development and there are areas that are still completely wild, with no development. And so we can do a comparison,” Asner explained.

When they compared reef vulnerability to the 2019 heat wave, they found high levels of coral mortality in areas with more coastal development, which is associated with pollution and sediment entering and clouding up the water. While it was previously known that these factors stress reefs, the new research showed that a marine heat wave can be the final straw for coral already struggling to cope with human impacts.

“When you have kind of medium development, then a heat wave, it’s a one-two punch,” Asner said. “The system is starting to get stressed with development, but then you have a heat wave come through and the reefs just croak, they die.”

But as with most large-scale ecological systems, the outcomes depended on a range of factors. Blob 2.0 was highly spatially variable, meaning that the degree of warming over the duration of the 2019 heat wave was different in different parts of the Hawaiian Island waters, and so too were the coral losses.

Reefs found in deeper waters suffered less mortality from the heat wave’s increased temperatures and solar radiation, as did larger reefs that had higher total cover, suggesting there may be some safety in numbers and depth for coral species.

This is the finding that gave researchers new hope. Although there is still much to be learned about the role factors like genetics, microcurrents, reef slope and wave frequency play in coral resilience to marine heat waves, the mapping effort allowed Asner and colleagues to identify regions where coral seem to have a good chance of surviving additional heat and light stress, if they’re protected. And protecting them, he said, is in everyone’s best interests.

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“It’s important to understand that coral reefs are critical to the economy and commerce,” Asner said. “The connections are pretty amazing. First of all, at least 25% of all species that live in the entire ocean spent part of their life in a coral reef. So even if you’re only connected to the ocean by eating fish, which is a large portion of humanity, you should care about what happens to coral reefs.”

Fish are friends, and food

Kirstin Holsman, a research fishery biologist with the National Oceanic and Atmospheric Administration and an author of one of the recent reports from the Intergovernmental Panel on Climate Change, agrees with Asner about the importance of coral reefs and research on how best to protect them.

“I think it’s really a big contribution,” Holsman said of Asner’s team’s recent work. “It has some innovation around machine learning, which is pretty exciting. And because these coral reef ecosystems are so delicate and sensitive to climate, that kind of work to tell how much of the change to coral reefs is due to climate impacts versus other factors is really critical for the future of these systems.”

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Oceans, though out-of-sight-out-of-mind for many land-locked residents, are central to our global climate, Holsman explained. Oceans have absorbed around 90% of the excess heat from human-caused climate change. This shifts the balance of ocean life and introduces uncertainty into an environment upon which a large proportion of humanity relies, whether for food, recreation, income or nourishment for the soul. What happens in the oceans also influences weather patterns everywhere.

“The oceans are a major component of the Earth’s system. You really have to have oceans in the equation when you’re trying to understand climate impacts,” Holsman explained.

Reefs of coral, which are animals themselves, are where much of the importance of the ocean to humans begins. They serve as nurseries for young fish, which become food and products and jobs for people. In 2019, the International Coral Reef Initiative estimated the economic value of the world’s coral reefs to be on the order of tens of billions of U.S. dollars each year.

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Coral reefs also buffer the impacts of tropical storms on land. When reefs die, storms systems cut further inland and erode away coastal dwellings.

“If an Arizonan doesn’t care about shorelines, they might care about the fact that we are seeing mass migration of people to Arizona from the coasts,” Asner said.

While marine heat waves are a natural part of the global climate system, scientists have determined that warming resulting from higher concentrations of greenhouse gases in the atmosphere, largely from burning fossil fuels, is related to more frequent and extreme “Blobs.” 

“Associated with those changes, there have been documented increases in disaster declarations by fisheries related to these climate impacts,” Holsman said. “Marine species, like people and other animals, have a hard limit of how much temperature they can tolerate.”

Both Holsman and Asner are hopeful in the face of massive marine system shifts, though. Holsman emphasized that the latest climate change report from the IPCC made large strides in outlining what to expect from future warming, which systems, species and peoples will be most vulnerable and what we can do to mitigate the impacts.

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“Humanity can do this,” she said. And research like Asner’s can help. Being able to predict which species in which locations will face the greatest consequences of marine heat waves will enable fisheries to anticipate challenges and shift their operations before disaster strikes. 

“There is a little bit of triage,” Holsman said. “But the more heads up we have, the better our planning can be.”

On-island, Asner’s optimism keeps him pulling on his wetsuit and slathering on his reef-safe sunscreen to help Arizona understand the oceans.

“Coral reefs and polar ice caps are the two frontline ecosystems that are being lost. I work on one of them. And it’s challenging because there isn’t a lot of time. But we have opportunities to focus our relatively limited conservation funds in areas that are persisting and that’s the way we’ll keep more species on the planet in the long run.”

Joan Meiners is the Climate News and Storytelling Reporter at The Arizona Republic and azcentral. Before becoming a journalist, she completed a Ph.D. in Ecology. Follow Joan on Twitter at @beecycles or email her at joan.meiners@arizonarepublic.com.