Climate change is wreaking havoc on coral reefs, heating and acidifying the waters in which they live. But some corals are actually benefiting from a warming world, according to a new study. In cooler regions, climate change has led to faster growth of a coral that's key to many reefs in the Indian Ocean. That growth has more than compensated for projected negative effects of acidification, leading to a net gain for the reefs. It is not clear, however, whether these findings apply to other types of coral, and too much warming could eventually kill them.
Excess carbon dioxide in the air is a double whammy for corals. The increase of this greenhouse gas translates into hotter seas, leading to more bleaching events  in which corals lose the algal partners that sustain them. This and acidification can make it harder for corals to grow or even maintain their current size; sometimes they die. The future of coral reefs, centers of marine biodiversity, is a big worry.
Still, it has been difficult for scientists to get a handle on how acidification and rising sea-surface temperatures affect reefs because growth can be so variable and hard to measure long-term. So Timothy Cooper, a marine biologist at the Australian Institute of Marine Science in Crawley, and his colleagues turned to coral growth bands, which, like tree rings, can be drilled out to reveal the history of the organism. The researchers collected 27 drill cores from a long-lived coral called Porites, which grows up to 5 meters high and is a common reef-building coral in the Indo-Pacific. They sampled six locations spanning 1000 kilometers along the Western Australia coast. By counting backward on the bands, they were able match up the amount of annual growth from 1900 to 2010 with prerecorded sea-surface temperatures for each year.
If acidification were a problem, as some lab studies have indicated it should be, there should have been a decline in coral growth in recent times for all the sites studied. But only one site showed that trend. Instead, at the southernmost—coolest—reef, there was a 23% increase in growth rate since 1900 , Cooper and his colleagues report in the 3 February issue of Science. Two other southern sites had increases of about 9% and 5% as well. The temperature is cool enough in those regions that it typically limits coral growth rates, so warming has been helpful, Cooper says. The more northern locations didn't have much change in growth rate.
For now, "the effects of temperature are outweighing the effects of acidification," says Nancy Knowlton, a coral biologist at the Smithsonian National Museum of Natural History in Washington, D.C., who was not involved with the study. These findings contrast with results from the Great Barrier Reef , where growth seems to have been slowing since the 1990s.
Roberto Iglesias Prieto, a coral ecophysiologist at the National Autonomous University of México in Cancún, praises the study for including coral samples from high latitudes, where the effects of acidification are expected to be stronger. But marine ecologist Joanie Kleypas of the National Center for Atmospheric Research in Boulder, Colorado, cautions that this work assumes that the waters at these locations have experienced ocean acidification at the same rate as the open ocean. Other work, she points out, indicates that acidification can be quite variable. The study did not have any direct measurements of ocean acidity.
In addition, other research has shown that the coral studied by Cooper is relatively insensitive to acidification. "We still know very little about the response of hundreds of other species to climate change," notes Peter Mumby, a marine ecologist at the University of Queensland in Australia, who was not connected to the study. "It is difficult to generalize the specific response of corals to overall climate change."