(AP) -- Corals at remote atolls northwest of the main Hawaiian islands suffered some bleaching this summer as ocean temperatures rose to higher-than-normal levels for a couple of weeks, but they were spared the large-scale mass bleaching observed this year in Indonesia and other parts of Southeast Asia, scientists said Wednesday.
Corals appear white or "bleached" when the ocean becomes too hot and they expel the algae they rely on to survive. Corals may recover if the algae returns, but they're still significantly weaker and more vulnerable to disease.
Thirty percent of the Kure atoll reef and one-fifth of the Pearl and Hermes atoll reef bleached, according to scientists who spent the past month on a research cruise in the Papahanaumokuakea Marine National Monument. Corals at other atolls inside the monument were unaffected.
"There were certain areas where the bleaching was kind of severe, where it was just a white carpet essentially," said Peter Vroom, chief scientist of the cruise, which returned to Oahu on Wednesday.
"Other areas you would see a coral head that's had a lot of color, but maybe a quarter of it would be white," Vroom said.
Scientists say it's unclear what the long-term effect of this summer's bleaching will be. Corals in the monument - which account for 69 percent of all coral under U.S. jurisdiction - were exposed to only two weeks of slightly above normal temperatures.
Rusty Brainard, chief of the NOAA Pacific Islands Fisheries Science Center's coral reef ecosystem division, said coral starts dying after about two months of being exposed to higher-than-normal temperatures.
In May, marine biologists working for the Wildlife Conservation Society observed coral bleaching off Indonesia's Aceh province when surface waters there peaked at 93 degrees - 7 degrees higher than long-term averages. Subsequent surveys found 80 percent of the bleached corals had died. Warmer temperatures this year also affected reefs in Malaysia, Thailand, Vietnam and Sri Lanka.
In the Caribbean, scientists said this week that corals are being exposed to water temperatures higher than those reported during a record bleaching period five years ago. They warned corals there could start dying in coming weeks.
Brainard said the Hawaii and Southeast Asia bleaching events are related in that they both occurred as global temperatures hit records from January through September.
"The common denominator is more on a planetary scale," Brainard said. "There's more heat on the planet this year in the first nine months than any other year."
He said bleaching hadn't even been observed in corals until 20 years ago.
"Then with these El Nino events we started seeing more bleaching. And now these are occurring - this is one of the symptoms of global warming," he said. "Even these small events, I think what they indicate is the whole scale is shifting."
What the Hawaii corals experienced was significantly more mild than Southeast Asia. But being bleached could hurt the corals over time.
Heidi Schuttenberg, the National Oceanic and Atmospheric Administration's research coordinator for the monument, predicted the bleached corals would be sick by the time scientists return for another research trip next year.
"When corals are bleaching, they're essentially starving," Schuttenberg said. "They're very weak so even if they survive their event, they're much more vulnerable to disease, and they have much lower reproductive capacity."
Explore further: Tourists evacuated amid Iceland volcano concerns