Ocean creatures are twice as likely as land animals to lose their habitats due to climate change, according to a paper published Wednesday in Nature.

Marine species may be more vulnerable to extirpations—extinction from a local habitat—in part because they cannot seek refuge from extreme temperatures as easily as land animals, the study suggests.

Led by Malin Pinsky, an ecologist at Rutgers University, the authors calculated the heat tolerance of 318 terrestrial species, including butterflies, spiders, and lizards, and 88 marine animals, such as fish, molluscs, and crustaceans.

All 406 animals were included in the study because they are cold-blooded—they rely on external sources to regulate their body temperatures—and are particularly sensitive to climate shifts.

Pinsky and his colleagues calculated the “thermal safety margin” for each species, a metric that quantifies a range of survivable temperatures. The margin is the difference between the upper heat tolerance of a species and its body temperatures at both the hottest annual conditions it encounters and the coolest accessible “thermal refuge.”

A refuge could be shady forested areas in the case of land animals, or colder deeper waters in the oceans.

The team also projected different climate scenarios for the species’ habitats over the 21st century, to get a sense of the long-term vulnerabilities of each animal.

The results showed that marine species have much narrower thermal safety margins than land animals. Tropical ocean dwellers are particularly susceptible to climate-related extinction, with many already living in temperatures that infringe on their thermal maximums.

“The smaller [thermal safety margins] for marine species suggest that—all else being equal—ongoing warming may already have driven more frequent population extirpation in the ocean,” the authors said in the study.

“We tested for this effect [and] found that extirpations at the warm edges of species’ ranges were twice as common in the ocean (56 percent) as on land (27 percent),” they said.

Terrestrial creatures may fare slightly better, but they are not off the hook. Land animals will be especially at risk if human infrastructure continues to fragment their habitats, which would restrict access to the thermal refuge areas they need to chill out.

There are some limitations to the research, such as the lopsided terrestrial-to-marine ratio in the sample size. But the authors point out the fossil record validates the idea that climate change can cause ocean extinction events by pushing species beyond their thermal safety margins.

At the end of the Permian period, global warming led to a mass die-off of high-latitude marine animals, while the cooling era of the Late Ordovician wiped out scores of equatorial species.

“The palaeobiological record shows that climate change can cause serious local and global marine extinctions, but history need not repeat itself if humanity mitigates the effects of climate change,” the team said.