Just 1°C of ocean warming can upend marine ecosystems

How will climate change affect the world’s oceans? Scientists have barely begun to chart how warming waters will affect marine ecosystems. Now, marine ecologists working off the coast of the Antarctic Peninsula have shown that a sudden 1°C rise in seawater temperature—a change expected to arrive within half a century—drastically alters ocean communities. Species diversity plummeted, and certain creatures, such as tiny, colonial “moss animals,” came to dominate the sea floor.

“I think that we have proof that we've entered a new age, a heat age,” says Mark Urban, an ecologist at the University of Connecticut in Storrs, who was not involved in the study. For lead author Gail Ashton, a marine ecologist at the Smithsonian Environmental Research Center in Tiburon, California, the study “adds to evidence that we're going to see very significant changes fairly soon.”

To predict how biological communities respond to climate change, scientists typically test their theories organism by organism, removing plants and animals from their ocean homes and examining how they react in heated tanks. But such an approach can only take researchers so far, Urban says.

So Ashton and her team decided to take the heat into the ocean itself. In the first-ever study of its kind, they encased electric heating elements in hand-sized plastic boxes. Then, they nestled the boxes into parts of the sea floor near the Rothera Research Station in Antarctica, where they warmed a thin layer of water to 1°C or 2°C above the ambient temperature. Those increases correspond to the expected global temperature rise in shallow seas within the next 50 and 100 years, respectively, by the Intergovernmental Panel on Climate Change, assuming no additional efforts to reduce greenhouse gas emissions are put in place.

Current Biology . What’s more, overall species diversity plummeted by 50%. For 9 months, the researchers monitored the distribution and growth of marine species that settled on the heated boxes and compared them with those that lived above unheated boxes. All it took was one look to notice the difference, Ashton says. In the 1°C increase experiment, a single species of moss animal, Fenestrulina rugula, doubled its growth rate. Within 2 months, the calciferous shelves it created dominated the community , the team reports today in. What’s more, overall species diversity plummeted by 50%.