As the world's average temperatures creep higher, marine animals are far more vulnerable to extinctions than their earthbound counterparts, according to a new analysis of more than 400 cold-blooded species.
With fewer ways to seek refuge from warming, ocean-dwelling species are disappearing from their habitats at twice the rate of those on land, notes the research published Wednesday in the journal Nature.
The study, led by researchers from New Jersey's Rutgers University, is the first to compare the impacts of higher temperatures in the ocean and on land for a range of cold-blooded wildlife, from fish and mollusks to lizards and dragonflies.
While previous research has suggested warm-blooded animals are better at adapting to climate change than cold-blooded ones, this study punctuates the special risk for sea creatures. As the oceans continue to absorb heat trapped in the atmosphere from carbon dioxide pollution, bringing waters to their warmest point in decades, undersea denizens don't have the luxury of ducking into a shady spot or a burrow.
"Marine animals live in an environment that, historically, hasn't changed temperature all that much," says Malin Pinsky, an ecologist and evolutionary biologist at Rutgers who led the research. "It's a bit like ocean animals are driving a narrow mountain road with temperature cliffs on either side."
The scientists calculated "thermal safety margins" for 88 marine and 318 terrestrial species, determining how much warming they can tolerate and how much exposure they have to those heat thresholds. The safety margins were slimmest near the equator for ocean dwellers and near the midlatitudes on land.
For many, the heat is already too much. At the warm edges of the marine species' ranges, the study found, more than half had disappeared from historical territory as a result of warming. The rate for these local extinctions is twice that seen on land.
"These impacts are already happening. It's not some abstract future problem," Pinsky says.
The narrow safety margins for tropical marine animals, such as colorful damselfish and cardinalfish, average about 10 degrees Celsius. "That sounds like a lot," Pinsky says, "but the key is that populations actually go extinct long before they experience 10 degrees of warming."
Even just a degree or half-degree boost, he adds, can lead to trouble finding food, reproducing, and other devastating effects. While some species will be able to migrate to new territory, others—coral and sea anemones, for example—can't move and will simply go extinct.
"This is a really heavy hitting paper because it contributes hard data to support the long-standing assumption that marine systems have some of the highest vulnerabilities to climatic warming," says Sarah Diamond, an ecologist and assistant professor at Case Western Reserve University in Cleveland, Ohio who did not work on the paper. "This is important because marine systems can get overlooked."
Most humans are landlubbers, after all—though many of our foods and jobs are tied to seaborne economies. Pinsky points to species such as Atlantic halibut, winter flounder, and ocean quahog that have disappeared from historical habitats and are important to fisheries.
In addition to cutting the greenhouse gas emissions that are causing climate change, he says that stopping overfishing, rebuilding overfished populations, and limiting ocean habitat destruction could help address species loss.
"Setting up networks of marine protected areas that act as stepping stones as species move to higher latitudes," he adds, "could help them cope with climate change going forward."
The Rutgers study reflects how important it is to measure not just temperature changes but how they affect animals, says Alex Gunderson, an assistant professor of ecology and evolutionary biology at Tulane University in New Orleans who did not work on the study.
And that includes those who live on the land.
"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," Gunderson points out.
"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."