Carbon dioxide-induced global warming is slowly killing off a major coral species in the Red Sea, scientists at Woods Hole Oceanographic Institution (WHOI) have discovered.
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 per cent and “could cease growing altogether by 2070” or sooner, the researchers report in 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 computed tomography (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.
Cohen credits Cantin with “pioneering” the technique for this type of oceanographic research.
She said: “He really took it to another level. What Neal really did was to adapt the imaging software, previously developed for bodies, specifically for our coral needs. This was an excruciatingly difficult task but it certainly paid off. We could not have used conventional techniques on this coral. The skeletal architecture is too complicated.”
Co-author Karnauskas concurs that there is little doubt that the Red Sea phenomenon is attributable to long-term climate change.
He said: “El Nino events typically last about one year, and in a few rare cases last for two years. El Nino–and its ”cold” counterpart, La Nina–are quite well known with a very distinct signature in the Pacific Ocean, where they originate. El Nino and La Nina events have been occurring for millions of years, and the past few decades have been no exception.
“Therefore, there is no way El Nino could account for a ”trend” that persists for decades. These are simply superimposed upon the human/CO2-induced warming trend. There is probably nobody in the scientific community who would argue the rising temperatures in the Red Sea are related to El Nino. So, in the past few decades, the Red Sea temperature has been going up just like the global mean temperature, and the corals are suffering accordingly.”
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 CO2 emissions that has become an increasing concern for scientists.
Cohen said: “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 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.”
Cohen also cautions against drawing conclusions about other coral species based on these results.
She said: “This study reports the impact of rising temperature on one coral species. 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.”
But now, for D. heliopora, the outlook appears bleak.
“The data in hand suggest that without immediate, aggressive global intervention to reduce carbon emissions,” the researchers conclude in their report, “the pressures of predicted annual heat stress will most certainly result in further deterioration of coral health in the central Red Sea over the next century.”