Coral reefs are often described as the canary in a coal mine. They are sensitive to changes in temperature and pH, and climate change is raising ocean temperatures and making oceans more acidic.
Nearly one-fifth of the world’s reefs are already gone due to the combined effect of global warming, pollution and overfishing. A few years ago, an international study predicted widespread losses in the coming decades.
But a handful of new and ongoing studies are starting to suggest a less gloomy picture for the future of the world’s reefs. Many coral species are indeed sensitive to increases in temperature and acidity. But, there are other species which are more resilient to these changes.
One such study was published this past week by a team of Australian scientists. The scientist who led the study, ecologist Terry Hughes of James Cook University, explained that most studies on coral health involve measurements of overall coral cover. But Hughes and his colleagues took a closer, more detailed look by studying the abundance of individual coral species in the Great Barrier Reef, the world’s largest reef system.
The entire system consists of nearly 3,000 individual reefs and spans from 10°S to 23°S.
“We used the Great Barrier Reef as a natural laboratory because of its length,” Hughes said. “The Great Barrier Reef has a great temperature variance from the north to the south.”
The north of the reef in the summer is about 48 degrees warmer than the south in the winter.
After studying some 33 reefs in the entire system, they found that some coral species were more abundant in the north, where it is warmer. And some were more abundant in the south. Others were most abundant somewhere in the middle.
“The species mix is changing…very rapidly,” said Hughes.
Climate change won’t cause coral reefs to disappear. Instead, there will be “winners and losers,” explained Hughes.
“What we will end up with is a very different kind of reef with a very different mix of species," he said.
Stanford marine biologist Steven Palumbi is also looking into corals’ resilience to high temperatures. He recently returned from a trip to American Samoa and the Cook Islands, where he was conducting experiments on different coral species.
“As we look around the world, we realize that corals have been living in quite variable environments all this time,” Palumbi said.
For example, in the backwaters of American Samoa some corals thrive in temperatures between 92-93°F, which is generally considered too hot for corals.
“The question then is: how did those corals survive those temperatures?” explained Palumbi. “And what do we learn from that that could help coral reef management in the future.”
That’s what Palumbi was trying to learn on his recent trip, where he subjected different coral species to different temperatures. Watch a video (below) of his experimental set up.
Once he has identified the more resilient species, he wants to understand what contributes to their resilience.
But the resilience of some coral species to higher temperatures is no reason to rejoice, according to both Hughes and Palumbi.
It’s still unknown how the shift in the distribution of species with rising temperatures will affect entire reef ecosystems. For example, Hughes says some of the more vulnerable species are often also the ones that provide the best habitat for other marine species. So losing them could hurt populations of fish and other marine species.
But now that we know corals aren’t quite the canary in the coal mine, we should not give up on protecting our reefs, says Hughes.
“If it was really true that coral reefs were the canary in the coal mine and that they would all be dead in 20 years from climate change, then we would logically give up on protecting them,” Hughes said. “If reefs are to have any future, then we should simultaneously address the issues of pollution, overfishing and climate change.”
Read more of Palumbi's blogs and see some fun videos he made on his recent trip.