If there are large reductions in carbon dioxide emissions, coral reefs may be able to adapt better to moderate climate changes and survive till the end of this century.

In a new study funded by NOAA and conducted by the agency's scientists and its academic partners, researchers found that coral reefs have already adapted to climatic changes that have taken place so far to some extent. However, if there are large reductions in carbon dioxide emissions, they may be able to adapt better to other climatic changes, thus, increasing their chances of survival till the end of this century.

"Earlier modeling work suggested that coral reefs would be gone by the middle of this century. Our study shows that if corals can adapt to warming that has occurred over the past 40 to 60 years, some coral reefs may persist through the end of this century," said study lead author Cheryl Logan, Ph.D., an assistant professor in California State University Monterey Bay's Division of Science and Environmental Policy.

Warming of ocean waters results in a potentially fatal process known as coral "bleaching" where reef-building corals begin ejecting algae present inside their tissues. Since the algae provide the coral with most of its food supply, prolonged bleaching can lead to the death of corals. This bleaching takes place when the water temperatures rise by 1 to 2 degree Celsius above normal summertime temperatures. For the study, researchers looked into a range of possible coral adaptive responses to rising temperatures. They found that coral reefs are more resilient to climate changes than previously stated because previous studies ignored the possibility of adaption.

Researchers also found that if there are large reductions in carbon dioxide emissions, coral reefs will be able to reduce coral bleaching induced by rise in temperatures by 20 to 80 percent of levels expected by the year 2100.

"The hope this work brings is only achieved if there is significant reduction of human-related emissions of heat-trapping gases," said Mark Eakin, Ph.D., who serves as director of the NOAA Coral Reef Watch monitoring program, which tracks bleaching events worldwide. "Adaptation provides no significant slowing in the loss of coral reefs if we continue to increase our rate of fossil fuel use. Not all species will be able to adapt fast enough or to the same extent, so coral communities will look and function differently than they do today."

This study only focuses on ocean warming being a threat to coral reefs. However, various other factors that threaten the long-term existence of coral reefs and their species have been documented by other studies. These include coral disease, acidification, sedimentation, sea-level rise, pollution, storm damage, destructive fishing practices, and direct harvest for ornamental trade.

A 2000 report suggested that more than 20 percent of the ocean's reef systems were lost to high water temperatures during the El Niño and La Niña events in 1998 and 1999. Another study revealed an 80 percent loss of coral cover in the Caribbean, where reefs are among the most biodiverse ecosystems in the world.