The world's endangered coral reefs, under stress because of global warming, are displaying resilience. U.S. scientists report that the reefs are adapting to higher temperatures by allying themselves with an organism that can tolerate warmer waters. But, scientists say coral adaptation may go only so far.

Scientific evidence indicates that coral reefs have deteriorated rapidly around the world, in the past few decades. According to a recent report by the Pew Center on Global Climate Change, climate warming and human activities such as pollution and overfishing are to blame. A co-author of the report, Joan Kleypas of the U.S. National Center for Atmospheric Research, says there has been significant coral degradation from disease and bleaching, a condition where higher water temperatures kill the protective algae that cover the reefs.

"Currently, I think the estimate is that about a quarter of the reefs are severely damaged on the planet," she said. "Most people agree that we're probably going to see extinctions of some species and that the composition of coral communities is going to change. That in the end is going to change the way coral reefs function and provide things that are valuable to humans."

Against this gloomy backdrop stand two separate reports in the journal Nature revealing signs that the corals are beginning to evolve their way out of trouble. They show that corals in the Pacific and Indian Oceans have adapted to higher ocean temperatures by playing host to a different kind of protective algae than they are normally associated with, one that tolerates warmer water.

The leader of one of the studies is Andrew Baker of Columbia University in New York City.

"We concluded that the ability of corals to flexibly associate with different types of algae actually gives them a rather cunning mechanism by which they can respond to climate change by adjusting their physiologies rather quickly, much more quickly than if they were forced to do this by conventional evolutionary means," he said.

Mr. Baker began surveying coral reefs in 1995. He says corals in Panama, the Persian Gulf, and Kenya that had been bleached during the ocean warming event called El Nino in the late 1990s were covered by more of the heat tolerant algae in 2001 than they were six years earlier. In contrast, corals in the Red Sea and Mauritius that did not suffer bleaching had only a tiny percentage of this same algae.

The second study by University of Guam scientist Rob Rowan analyzed the chemistry of this algae and concludes that it is a high-temperature specialist. He writes that if other coral species can host it or similar types of algae, they also might adapt to warmer habitats relatively easily.

But Andrew Baker at Columbia University says there is an environmental drawback for the coral that host them.

"The corals that contain these heat tolerant algae actually end up growing more slowly than corals that don't," he said. "So what this means is that while these corals are recovering from the affects of global climate change, they may not be resistant to a whole bunch of other stresses, things like overfishing and habitat destruction and nutrient pollution."

And just how resistant to climate change can the coral become? Although the studies show they can survive increases of two to three degrees Celsius with the heat tolerant algae, Joan Kleypas says there might be a limit on how much more adjustment they can make, but no one knows for sure.

"The reason we can't make predictions this is going to save the corals is because the temperature changes that we are seeing are likely to be much greater than what than corals have experienced over the last few thousand years at least," she said. "So their capacity to adapt probably has its own limits. Just because they are showing this adaptive response, we don't know how much it's going to help them adapt."

Nevertheless, says Ms. Kleypas, every bit of adaptation helps the endangered coral reefs in their time of stress.