As Mark Kaufman reports for Mashable, the analysis—centered on around 400 cold-blooded species, including fish, mollusks, crustaceans and lizards—suggests marine creatures are ill-equipped to adapt to rising temperatures and, unlike land animals that can seek shelter in the shade or a burrow, largely unable to escape the heat.
“You don't have anywhere to go,” Natalya Gallo, a marine ecologist at the Scripps Institution of Oceanography who was not involved in the study, tells Kaufman. “Maybe you can hide under a kelp leaf, but the entire water around you has warmed.”
Speaking with National Geographic’s Christina Nunez, lead author Malin Pinsky, an ecologist and evolutionary biologist at Rutgers University in New Jersey, further explains that ocean dwellers “live in an environment that, historically, hasn’t changed temperature all that much.”
Given that cold-blooded creatures rely on their surroundings to regulate body temperature, relatively stable marine ecosystems have actually made their inhabitants more susceptible to significant temperature changes. And while ocean temperatures are still much lower than those on land, as Anthony J. Richardson and David S. Schoeman point out in an accompanying Nature News and Views piece, marine heat waves, increased carbon dioxide pollution and other products of global warming are driving Earth’s oceans to higher temperatures than ever before.
To assess the threat posed by warming waters, Pinsky and her colleagues calculated “thermal safety margins” for 318 terrestrial and 88 marine animals. According to Motherboard’s Becky Ferreira, this measure represents the difference between a species’ upper heat tolerance and its body temperature at both full heat exposure and in “thermal refuge,” or cooled down sanctuaries ranging from shady forests to the depths of the ocean.
The team found that safety margins were slimmest for ocean dwellers living near the equator and land dwellers living near the midlatitudes. Crucially, Nunez writes, the data revealed that more than half of marine species at the higher end of their safety margins had disappeared from their historical habitats—a phenomenon known as local extinction—due to warming. Comparatively, around a quarter of land animals had abandoned their homes in favor of cooler environments.
On average, tropical marine creatures have a safety margin of 10 degrees Celsius. “That sounds like a lot,” Pinsky tells Nunez, “but the key is that populations actually go extinct long before they experience 10 degrees of warming.” In fact, Pinsky notes, just a degree or half-degree shift can dramatically impact such animals’ food-finding skills and reproduction abilities.
While some marine creatures can escape the heat by migrating to colder waters, others have fewer options: As Mashable’s Kaufman observes, surface-dwelling fish can’t simply move to the deep ocean and expect to thrive or even survive. The same is true of marine animals living in the shallow waters off of continental shelves, Bob Berwyn adds for InsideClimate News. And these species, as well as ones forced to flee their long-time habitats, are far from obscure ones likely to have no impact on humans’ livelihood; many, including halibut and winter flounder, serve as key food sources for coastal communities.
“This affects our dinner plates in many cases,” Pinsky says to Kaufman.
Berwyn highlights several examples of animals reaching or surpassing their heat threshold. Coral reef-dwelling damselfish and cardinalfish, for example, have started to disappear from some areas, hampering the health of these already threatened ecosystems. Summer flounder, once native to the North Carolina coast, have moved to cooler waters, forcing fishermen to travel some 600 miles further north than before in order to catch them.
Although the new study emphasizes marine dwellers’ plight to an extent little-seen in academia, Alex Gunderson, an ecologist and evolutionary biologist at New Orleans’ Tulane University, is quick to point out that terrestrial creatures remain at risk, too: As he tells National Geographic’s Nunez, “Land animals are at lower risk than marine animals only if they can find cool shaded spots to avoid direct sunlight and wait out extreme heat.”
Building on the researchers’ call to lower greenhouse gas emissions, stop overfishing and limit ocean habitat destruction, Gunderson concludes, “The results of this study are a further wake-up call that we need to protect forests and other natural environments because of the temperature buffer that they provide wildlife in a warming world.”