Study shows Pacific island reefs can match rising sea levels

Porites microatollsPorites microatollsCoral reefs are a lot more resilient than previously thought. At least according to a new study published yesterday that showed Pacific island coral reef can grow fast enough to match rising sea levels, even with increased ocean temperatures. Because they grow vertically on shallow reef flats, researchers observed that Porites microatolls coral is keeping pace with current sea level rise, but may have trouble under the worst-case IPCC scenarios. The Porites microatoll, whose growth is largely lateral and limited by exposure to air, is named for its resemblance to island atolls (see picture).

Researchers at the Florida Institute of Technology, who published their study in the Royal Society Open Science, say their findings provide the first evidence that “well-managed reefs will be able to keep up with sea-level rise through vertical growth.” However, if CO2 emissions rise past 670 parts per million (ppm), which may cause ocean temperatures to increase 2.2 degrees Celsius, reefs will have a hard time keeping up with the projected sea level rise.

Currently CO2 levels worldwide are 400 ppm (.o4 percent), but once they cross the 670 ppm threshold, the corresponding rise in ocean temperatures may hamper even a healthy reefs ability to survive. “Reefs will continue to keep up with sea-level rise if we reduce our emission of greenhouse gases,” said Florida Tech’s Rob van Woesik, a professor at FIT’s Department of Biological Sciences and the study’s lead author. “If reefs lose their capacity to keep up with sea-level rise they will drown.”

The study, which focused on Palau island in the western Pacific Ocean, was also co-authored by researchers from the University of Queensland and the Palau International Coral Reef Center. Palau is an island country that is part of the larger Pacific island group of Micronesia and relies on the reef system to break apart storm waves.

The researchers measured “570 reef-flat Porites microatolls (a type of coral) at 10 locations around Palau, which revealed recent vertical skeletal extension (78±13 mm) over the last 6–8 years,” consistent with the observed increase in sea level. The study’s authors then used the Intergovernmental Panel on Climate Change (IPCC) greenhouse-gas concentration trajectories from its 2014 fifth Assessment Report to model four different outcomes for the Porites microatolls. It showed that under the low- to mid-RCP scenarios, reef-coral growth will keep up with sea-level rise.”

These IPCC’s RCP pathways are primarily used for climate modeling and research and “describe four possible climate futures,” depending on how much greenhouse gases are emitted in the future.

However, under the IPCC’s worst-case RCP scenario, where “greenhouse gas concentrations exceed 670 ppm atmospheric CO2 levels with a concomitant increase of 2.2 degree Celsius in sea-surface temperatures by 2100,” their “predictions indicate that Porites microatolls will be unable to keep up with projected rates of sea-level rise.”

CO2 levels have increased at a rate of approximately 2 ppm/year since recordkeeping began. That means if current trends continue, CO2 levels will have increased only 170 ppm by the year 2100, much less than the 670 ppm or higher needed to affect the reef systems described in the study. There is also an annual fluctuation of about ~10 ppm that is negatively correlated with the Northern Hemisphere’s growing season (plants absorb CO2 during the spring/summer/early fall).

As reported here, other studies have also shown coral reefs are far more resilient then previously estimated. One 13-year study of coral reefs showed “them spontaneously recovering,” refuting the “often doomsday forecasts about the worldwide decline of the colorful marine habitat.” Tom Frazer, professor of aquatic ecology at the University of Florida and part of that study, told Reuters, “People have said these systems don’t have a chance. What we are saying is: ‘Hey, this is evidence they do have a chance.'”

Another study—funded by NOAA—showed that coral reefs can also adapt to warmer ocean temperatures through a variety of processes. Even after the great coral die-off in 1998 from a particular brutal El Niño, most of the coral reefs across the planet rebounded to their original numbers. Coral can also perish from a variety of issues: environmental stressors, such as elevated temperature; changes in salinity; high solar radiation; pollutants; and/or diseases.

This latest study from the Florida Institute of Technology will be good news to the thousands of people who populate these Pacific islands and rely on the intricate reef system for protection and tourists.


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