Corals Are Adapting to Higher Temperatures, But Likely Not Fast Enough

Oct 24, 2019

Evidence from the Hawaii Institute of Marine Biology shows that corals are already naturally adapting to higher temperatures. But the rate of increase may be too rapid for them to keep up.

Walk through the grounds of the Hawaii Institute of Marine Biology on Coconut Island and you’ll see dozens of tanks. They’re holding samples of marine life being studied by scientists. Right now, many contain transplanted coral fragments being used as part of an exploration of coral bleaching.

Marine scientists had issued dire predictions for Hawaii’s coral reefs after the state began breaking heat records in May. But as summer highs begin to subside, it appears the corals have may avoided the most severe forecasts issued earlier in the year.  

Cooler water temperatures have returned to the islands just in time to prevent extreme bleaching, a process when stressed corals turn almost pure white. Bleaching is not necessarily fatal, but the condition does often precede death.

Ku’ulei Rodgers leads the Coral Reef Ecology Lab on Coconut Island. Hawaii has seen two major bleaching events in last two decades, one in 1996 and another in 2014, and Rodgers says no two bleaching events are the same.

Critically, prior bleaching events were spaced 20 to 30 years apart, giving coral colonies time to recover. But as global temperatures rise, bleaching events appear to be happening more frequently, meaning that this year’s moderate bleaching may continue to worsen.  

“The oceanographers are predicting that this temperature is going to continue throughout the year and into next year. So possibly next summer might be a worse year,” Rodgers said outside her ocean-side lab.

Rodgers and her team are using tanks as controlled spaces to experiment with different water conditions. Altering variables like temperature and nutrient levels, they can test how different coral species respond to environmental stress.

Bleached corals in Kaneohe Bay.
Credit Shreya Yadav / Madin Lab/Hawaii Institute of Marine Biology

Rows of corals are visible in each tank. Some have been exposed to the same conditions currently being experienced by wild coral colonies in Kaneohe Bay. Others are in water heated 2 degrees Celsius above the normal ambient level. Yet more are sitting in water with higher than normal nutrient levels.

It’s apparent that corals in the heated tanks are not faring well. But some species are showing more resilience than others.

The team has also collected evidence that wild corals have already been adapting to warmer conditions. Using historical records, they have recreated conditions from previous decades in the tanks. The results appear to indicate that corals growing in Kaneohe Bay today are more resilient to high water temperatures than colonies from the 1970’s, all through natural adaptation. But Rodgers says, that adaptation may still not be enough.

“It's happening too fast. The temperatures are increasing faster than these corals can adapt.”

The goal of this research is to better identify what environmental factors stress corals, and then drive public policy accordingly. But excess heat remain one of the greatest threats and Rodgers cautions that no other changes can substitute for reducing the atmospheric carbon emissions driving temperatures higher each year.  

To read the first in this two-part look at coral bleaching research on Coconut Island, go to hawaiipublicradio.org.