Woods Hole Researchers: Global Climate Change Cause of Coral Stress

In a pioneering use of computed tomography (CT) scans, scientists at Woods Hole Oceanographic Institution (WHOI) have discovered that carbon dioxide (CO2)-induced global warming is in the process of killing off a major coral species in the Red Sea.

As summer sea surface temperatures have remained about 1.5 degrees Celsius above ambient over the last 10 years, growth of the coral, Diploastrea heliopora, has declined by 30 percent and “could cease growing altogether by 2070” or sooner, they reported in the July 16 issue of the journal Science.

“The warming in the Red Sea and the resultant decline in the health of this coral is a clear regional impact of global warming,” said Neal E. Cantin, a WHOI postdoctoral investigator and co-lead researcher on the project. In the 1980s, he said, “the average summer [water] temperatures were below 30 degrees Celsius. In 2008 they were approaching 31 degrees.”

Cantin and WHOI Research Specialist Anne L. Cohen, the other lead investigator, said the findings were unexpected because D. heliopora did not exhibit one of the typical signs of thermal stress: bleaching. “These corals looked healthy,” said Cohen.

But CT scanning of the coral's skeletal structure in the laboratory revealed “the secrets that the skeletons are hiding,” she said. “The CT scans reveal that these corals have actually been under chronic stress for the last 10 years, and that the rates of growth were the lowest in 2008,” the final year of the study.

The other WHOI researchers who participated in the study are climate dynamicist Kristopher B. Karnauskas, coral biologist Ann M. Tarrant and chemical oceanographer Daniel C. McCorkle.

Cantin explained: “With CT scanning we are able to work with a complete 3-D reconstruction of the entire core. We can then make digital slices from the core, as many times as we need to in order to continually visualize the annual density bands. CT scanning is the evolution of X-ray.”

The researchers scanned six skeletal cores of D. heliopora and were able to pinpoint two high-density growth bands, indicating high thermal stress in 1998 and 2001. This correlates with an abrupt drop in skeletal growth after 1998, which has continued steadily since then.

The corals are building skeleton, or calcifying, at a progressively slower rate because they are losing symbiotic algae that live in the coral tissue. By performing photosynthesis, the algae provide the fuel for the corals to make new skeleton.

But, says Cohen, “when the corals are thermally stressed, they lose algae and many will eventually starve and die. When corals lose enough algae, they actually turn white, and that’s what bleaching is. We think these corals are on their way to bleaching.”

The same corals had a similar reaction to a “warm event” in 1941-42 but recovered within three years as the ocean cooled. The recovery was possible because that warming episode was probably triggered by El Nino, a natural, short-term climate anomaly.

In contrast, the current warming trend—which Cantin says has been going on since 1980--“is due to human-induced climate change,” he says, and appears unlikely to be slowed or reversed before coral health deteriorates further.

The scientists point out that the results show that, at least in this case, the culprit is sea surface temperatures and not ocean acidification, another effect of carbon dioxide emissions that has become an increasing concern for scientists.

“We were able to pinpoint temperature as the driver of the declining growth rates because we have long records of skeletal growth going back to around 1930,” Cohen said, “and we were able to correlate skeletal growth with temperature records that span the same time period. We were also able to rule out ocean acidification because we have actual measurements of the aragonite saturation state of seawater--a measure of acidity--at our study sites.

She cautions against drawing conclusions about other coral species based on these results. “This study reports the impact of rising temperature on one coral species,” she says. “It’s an important reef-building coral in the Red Sea, but there are about 250 species of stony corals in this region and we have no idea what the other species are doing. Some might be doing much worse; some might be doing a little better in terms of thermal tolerances. We need much more of this type of work to be able to predict what the coral reefs will look like over the next few decades.”

The work was funded by King Abdullah University of Science and Technology.

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