Ocean thermostat protects corals?

Written byEdyta Zielinska

| 2 min read

In a time when all coral news is bad news, a new study that will be published online Saturday in __Geophysical Research Letters__ (read the press release linkurl:here);http://www.ucar.edu/news/releases/2008/coral.jsp suggests that areas of open ocean can act as a natural thermostat, protecting corals from bleaching by preventing surface water temperature from going up. (You can read more about coral bleaching and about the other effects of global warming on the biome in our linkurl:January issue.);http://www.the-scientist.com/2008/01/1/36/100/ The theory of ocean thermostats is a controversial one, says lead author Joan Kleypas from the National Center for Atmospheric Research in Boulder, CO. The theory goes that in the open ocean, increases in surface water temperature lead to more evaporation. The process of evaporation itself cools the water, but the cloud cover that develops also acts as shade, blocking further heating. But this feedback doesn't function everywhere. "You'll have evaporation over a swimming pool," explained Kleypas, "But you're not going to get cloud formation above it." The authors examined satellite data of surface temperatures from the years 1950-2006 and found a region of particularly warm waters in the western Pacific with temperature increases at half the rate of other areas. They compared the satellite data to a model predicting how surface temperatures, carbon dioxide levels, solar variability and other factors interact. Even as other factors changed, the model predicted the same stable temperatures they'd observed in the data, suggesting a mechanism such as the ocean thermostat was at play, Kleypas explained. Comparison with a global dataset of coral bleaching showed that fewer bleaching events had occurred in that ocean region, suggesting that waters that were warmer but more stable might protect against coral bleaching. It's not perfect protection, says Kleypas. Big shifts in temperature, caused by monsoons, for example, could override the thermostat regulation, and corals in this region are actually more vulnerable because they can tolerate a smaller range of temperature shifts. Most corals commonly bleach after water temperature change by 1-2 degrees Celsius. In these corals, bleaching events were observed after a change of 0.2-0.3 degrees. The researchers then projected whether the temperatures predicted by the thermostat hypothesis would remain stable as carbon dioxide levels increase. Now for the bad news: They won't, said Kleypas. Kleypas is the first to admit that her data correlating warm temperatures to healthier corals don't prove the thermostat theory, but the pattern is suggestive, she said. And understanding such a protective mechanism could help conservationists determine "where reefs are most likely to succeed," she says.

Ocean thermostat protects corals?

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