About 4,100 years ago, coral reefs in Panama violently collapsed and ceased growing for the next 2,500 years. Intrigued, a Florida graduate student, her adviser and a team of researchers set out to discover why.

By analyzing the chemical signatures of six coral reef cores taken from multiple sites in the Pacific Ocean around Panama, the scientists found an extreme weather event associated with what we would call La Niña today triggered the reef collapse. A series of events similar to El Niño continued to suppress the reef for the next two millenniums.

"What ultimately caused these reefs to collapse was the extreme frequency of these extreme events," said Lauren Toth, who conducted the research as a graduate student at the Florida Institute of Technology.

The study, published this week in the journal Nature Climate Change, shows at the time of the collapse the water was very cool, rainfall had increased and upwelling was stronger in the region.

Using historical records to determine a reef's environmental thresholds—the range of conditions it has survived through and what conditions push it past its tipping point—can help scientists predict how current conditions altered by climate change could spell peril for reefs, Toth said.

"This is the best way we can approach the long-term response to reefs and climate change, by looking at this historical record," she said.

Toth added that coral researchers are often missing the link between climate variations and the long-term health of a reef.

"We can measure these instantaneous impacts of climate change," she said. "We can see coral bleaching, some effects of changing pH, the increases in coral diseases, but we can't yet predict how this will translate over time."

Reaching a temperature limit?
The chemical analysis of the coral fossils showed temperature was a key cause of reef collapse. As climate change warms oceans, Toth said it's possible we could be approaching an upper temperature threshold.

Using historical records to try to better understand what we're up against in the future can be effective with coral reefs because they are particularly vulnerable to climate and oceanographic changes relative to other ocean ecosystems, said Jeremy Jackson, a professor of oceanography at the Scripps Institution of Oceanography in La Jolla, Calif., and a senior scientist at the Smithsonian Tropical Research Institute in Panama.

But he cautioned that eastern Pacific coral reefs, like those in Panama, are vastly different from the Great Barrier Reef or Caribbean reefs, so conclusions from the Panama reef data cannot necessarily be extrapolated to coral reefs worldwide.

Panama reefs have always been "marginal reefs," meaning they have low diversity and are isolated. Reefs in the area experience a huge tidal range—20 feet on average throughout the day. That suspends sediment in the water, leaving it murky and not conducive to reef growth.

Their geographical location, however, does mean they have one strength.

"These coral reefs are like canaries in a coal mine for ENSO variation," Jackson said.

The El Niño-Southern Oscillation, or ENSO, is a cycle of temperature variations between warm ocean water that forms in the east-central Equatorial Pacific and the atmosphere. La Niña is often called the cold phase of ENSO and El Niño the warm phase. These deviations from normal surface temperatures have large impacts not only on ocean processes but also on global weather and climate, according to the website of the International Research Institute for Climate and Society at Columbia University.

Big knowledge gaps remain
How climate change will impact the ENSO cycle is complicated and a contentious subject among scientists, but a study published in January 2014 found that although the overall total number of El Niños is projected to stay constant, the number of super-El Niños is likely to double. These cycles could be disastrous for reef ecosystems. Sixteen percent of the world's coral was lost during the last big El Niño in 1997-98, with some countries like the Maldives losing up to 90 percent of their reefs.

Kim Cobb, an associate professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology and one of the co-authors of the new paper, said researchers now understand what was going on during that time period when the reef collapsed, but they still don't know exactly why the reef reacted in such a persistent, dramatic way.

"We are looking at a book 200 pages long, but we only have 10 to 20 pages," she said. "The inference to what we know from modern-day reef ecology is that changes, whether hot or cold, can cause bleaching events and stress."

Both Toth and Cobb said it would be premature to use this research to forecast future collapse of reefs around the world, but it's a "gentle reminder that whatever happens in the past can happen in the future."

"This did occur, and happened because of changes that were associated with cool, persistent altered conditions, and coral reefs of tomorrow will be subject to dramatic changes," Cobb said. "This was an inferred cooling; going forward, we're looking at a warming."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500