SAN DIEGO -- For plants, animals and marine life whose environment changes, their options are stark and simple: Move, adapt or die.
But when the marine life in question is a coral reef cemented to the ocean floor and the threat is climate change, the outlook appears grimmer, said scientists presenting new findings here at the annual meeting of the American Association for the Advancement of Science.
Even small temperature rises of 1 to 2 degrees Celsius in the ocean can make corals more prone to bleaching, a kind of sudden death that occurs when corals expel the algae that normally live inside them, providing them with food and their bright coloration.
Scientists are also worried that, as carbon dioxide emissions rise, the ocean will absorb greater amounts of the greenhouse gas, shifting the chemistry of seawater. As the ocean becomes more acidic, it will be harder for corals to grow. Eventually, ocean water could become corrosive, dissolving reefs faster than corals can grow.
The question now is whether reef-building corals have the capacity to adapt to those changes.
"Can reefs disappear? That's the question," said Joanie Kleypas of the National Center for Atmospheric Research. "We don't have a lot of information on this."
One example comes from the Galapagos Islands, where a strong El Niño event in the early 1980s bleached more than 95 percent of corals. Then a species of sea urchin moved in and started chewing the reef, she said.
"Reefs that were there for hundreds or thousands of years disappeared in a matter of about a decade," Kleypas said. "Some people think it's a glimpse into the future ... an example of what we might expect in a high-CO2 world."
But there is some evidence to suggest all coral reefs aren't created equal, said Stephen Palumbi, a professor of marine science at Stanford University. He's studying what appear to be unusually resilient reefs off the coast of Ofu Island in American Samoa.
Samoan reefs appear resilient
Summer low tides occur at the hottest point of the day, exposing reefs in Ofu Island's shallow lagoons to temperatures normally lethal to corals -- but these reefs survive.
The secret, Palumbi said, may be that the high temperatures last for a short period of time. Temperature recorders he has placed in the lagoons experience the high temperatures for up to four hours -- and then have a chance to recover.
He has performed experiments to see whether those regular, but brief, exposures to temperature stress have "cross-trained" the corals to withstand high temperatures for even longer periods.
"Our conclusions from these experiments, ongoing now, is that corals do show temperature acclimation -- that brief excursion to high temperature in these pools hardens them in some way," he said. "This transient exposure to heat may be an exercise or conditioning program, in a sense."
The question now is whether corals in other reefs share that ability or can acquire it as climate change warms the average ocean temperature.
"We have no idea whether this is a general ability that corals have," Palumbi said. "We don't know if it's genetically hard-wired [in the Ofu corals]. It gives us the idea there may be some ways to generate more resilient populations of corals."
But scientists understand even less about whether corals can adapt to more acidic waters, the scientist said, and that remains a looming question. Creating marine protected areas may help corals generally by removing stresses like pollution and overfishing, giving them a better chance in a changing climate.
But the trick will be identifying reefs that are more resilient, like those on Ofu Island.
"Finding and mapping highly resilient habitats is not something we do now, but we can learn to do it," Palumbi said. "It still doesn't solve the problem we have of being addicted to carbon dioxide -- it may slow down the degradation of marine ecosystems while we do solve the larger problem."
Limits to defenses against warmth and acidic waters
But there are limits to resilience, said Simon Donner of the University of British Columbia.
He has used a high-resolution climate model to project how corals will fare under a "business as usual" scenario in which greenhouse gas emissions keep rising at the current pace, and another in which the world is able to stabilize the level of CO2 in the atmosphere at 550 parts per million.
The news isn't good, according to Donner. The carbon dioxide that's already in the atmosphere is enough to cause ocean warming beyond the point at which coral bleaching becomes common, he said.
Greenhouse gases already in the atmosphere "will cause the majority of the world's coral reefs to experience harmfully frequent thermal anomalies," he said. "Adaptation could postpone the forecast by 50 to 70 years, but it's not a panacea."
Experiments in Bermuda also suggest rising temperatures won't be the only climate change corals will face in coming decades. As ocean waters absorb greater amounts of CO2 and become more acidic, it will be harder for reefs to keep growing, said Andreas Andersson of the Bermuda Institute of Ocean Sciences.
Andersson said his laboratory experiments show that Bermuda's corals, which are now in good shape, would begin to dissolve if the current level of carbon dioxide in the atmosphere doubled. The corrosive effects of water in those conditions would outpace Bermudan reefs' ability to grow.
The effect was most pronounced in winter, when there's less sunlight available to fuel reef growth.
"This is a very healthy reef, with very high coral cover in certain areas," he said. "We don't have major industries or pollution, so any changes we detect in Bermuda's coral reefs are probably due to global changes like warming and ocean acidification."
Andersson said it's hard to say when the balance between growth and dissolution might tip for Bermuda's reefs, but the results are disturbing. "This is a reversal in trend in a major biogeochemical process that's been in place for thousands of years, all as a result of human emissions of CO2," he said.
Aline Tribollet, scientist with the Institute of Research for Development based in the French colony of New Caledonia, said there's also a risk that rising carbon dioxide emissions could increase erosion caused by tiny organisms that bore into reefs.
Experiments she conducted in Hawaii show reefs will dissolve 48 percent faster when the level of CO2 in the atmosphere jumps from 400 parts per million to 750 ppm. To put that in perspective, the current level of CO2 in the atmosphere is 387 parts per million.
Corals may also have trouble coping as sea level rises and stirs up sediments, said Kimberley Yates of the U.S. Geological Survey. That can make water over reefs cloudier, reducing the sunlight that reef algae need to produce food.
"Most of the communities we've studied show significant amounts of sediment dissolution already," said Yates, who is based in St. Petersburg, Fla. "Many communities are currently not keeping up with sea level rise."