In a pioneering use of computed1 tomography (CT) scans, scientists at Woods Hole Oceanographic Institution (WHOI) have discovered that carbon dioxide (CO2)-induced global warming is in the process of killing2 off a major coral species in the Red Sea. As summer sea surface temperatures have remained about 1.5 degrees Celsius3 above ambient（周围的，外界的） over the last 10 years, growth of the coral, Diploastrea heliopora, has declined by 30% and "could cease growing altogether by 2070" or sooner, they report 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 investigator4 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 thermal5 stress: bleaching6. "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 chronic7 stress（慢性压力） 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 dynamicist8 Kristopher B. Karnauskas, coral biologist Ann M. Tarrant and chemical oceanographer Daniel C. McCorkle.
Cohen and WHOI graduate student Casey Saenger had previously9 used CT scanning to quantify skeletal growth in Atlantic corals, but she credits Cantin with "pioneering" the technique for this type of oceanographic（海洋学的） research. "He really took it to another level," she said. "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."
Historically, scientists have used x-rays to examine coral skeletons, which display annual growth bands much like tree rings, Cantin explained. But that method usually entails10 cutting into the skeleton, he said. CT allows non-invasive 3-D observation of the skeletons and bands.
"The biggest advantage we have over x ray is that we can scan intact（完整的） cores without cutting the core into thin slices," said Cantin. "Since corals do not grow in a straight line, when the core is cut, inevitably11 the growth axis12 will be lost from a thin cut. Maintaining the vertical13 growth axis is crucial for us to visualize14 the annual density15 banding patterns.
"With CT scanning we are able to work with a complete 3-D reconstruction16 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."
With CT, adds Cohen, "We have a 3-D visualization17 of the skeleton from which we can make 'virtual' cuts on the computer that have the exact thickness, orientation18 and location that we need for a particular coral to get the most precise measurements. X-ray requires that we cut the core 'blind' beforehand, before we know what the orientation of growth is. Whole cores can be sacrificed this way. With CAT scanning, our cores are imaged intact, nothing else is required. This is a huge leap forward over x-ray."
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