Sea creatures, especially those that live in shallower water near the coasts, are much more vulnerable to global warming than land animals, new research shows. The scientists found that local populations of marine animals are disappearing at double the rate of land-based species.

That's because marine animals like fish, crabs and lobster are already more likely to be living near the threshold of life-threatening temperatures, and because in the ocean, there are fewer places to hide from extreme heat, said Malin Pinsky, lead author of a new study published Wednesday in the journal Nature.

"These results are stunning, in part because the impacts of climate change on ocean life were virtually ignored just a decade ago," said Pinsky, an ocean researcher at Rutgers University. The study took a close look at cold-blooded marine species whose body temperatures are dependent on their surroundings.

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Some fish can move poleward to cooler waters, but for others, those thermal refuges will be inaccessible because the cooler areas are too far away or because shallow water habitat along continental shelves is not continuous. That can affect people in developing countries that depend heavily on fish as a daily source of food.

Understanding which creatures are most at risk allows scientists and fisheries managers to better allocate resources for conservation, Pinsky said.

"We already know terrestrial species are highly vulnerable to climate change," he said, "and now we see that marine species are even more vulnerable."

Some Fish Already Reaching Thermal Limits

Locally caught fish are an important source of protein for about half the world's population, and the new study shows that some of those species near the equator are among the most vulnerable to global warming because they already live near the edge of their heat tolerance.

"We're heading into uncharted territories. We're already seeing species disappear from places they've been for generations and longer," Pinsky said.

For example, damselfish and cardinalfish, two small species that live on coral reefs, already live near their thermal limits and have started to disappear from some areas, which contributes to the overall decline of coral reef health.

Off the coast of North Carolina, summer flounder are another example, Pinsky said. They have moved so far to find cooler waters that it's had a big effect on fisheries, with boats having to travel more than 600 miles farther north to catch the species.

Cardinalfish in some areas are living near their thermal limits. Credit: Jens Petersen/CC-BY-3.0

"Our conclusions are based on global research across more than 500 species, from lizards and fish to spiders and crabs," he said. "We calculated safe temperatures for 88 marine and 294 land species, found the coolest temperatures available to each species during the hottest parts of the year, and identified whether warming had driven population loss for 159 species."

Of the marine species they studied, 56 percent experienced a range contraction due to global warming, compared to 27 percent of the land species.

Fish species won't be able to evolve fast enough to keep up, so the likely impacts include significant local extinctions that would leave some coastal communities in developing countries scrambling to feed themselves, he added.

Stuck in Warming Water with No Refuge

"The interesting thing with this research is the comparison between land and ocean animals. It's never been done this way," said Denmark-based ocean researcher Mark Payne, who was not involved in the study.

"Fish don't have refuges. On land, a lizard can crawl under a rock and get shade, but there's nothing like that in the ocean. Basically, you're sitting there floating around in this soup of warm water with nowhere to go," he said.

Payne said that particularly applies to the fish living along continental shelves, which are also the species most accessible for coastal communities. While some ocean-going species can dive down into deeper and cooler water, coastal fish that live in shallow water don't have that option. As a result, some coastal areas in the tropics will turn into ocean deserts, nearly devoid of fish.

The Risk of Extreme Ocean Heat Waves

The new paper also reflects how scientists are thinking about climate change in new ways.

"What's going to do the damage to fish in the ocean are extreme events, when temperatures spike for a month or two. Even if the temperatures return to normal, the damage is done for the next 10 years," Payne said. "Many of the changes will happen quickly and suddenly in response to marine heat waves, and you just don't come back from these things quickly, especially long-lived species.

"In the tropics, there are no species from even hotter areas to come in. Some parts of the ocean will become uninhabitable, an ocean desert."

Several recent intense ocean heat waves around the world have already had serious consequences for ocean ecosystems, killing coral reefs, seabirds and seagrass and leading to harmful invasions by non-native species. That resulted in significant financial loss for fisheries and aquaculture last summer after a marine heat wave warmed the oceans around Denmark up to 8 degrees Celsius above average, Payne said.

Pinsky said the findings can help fisheries managers plan conservation measures by helping identify areas where important food fish may be able to live as the oceans continue to warm. The information can show where to establish fishing restrictions or marine protected areas to bolster populations.