Coral reefs, whose biodiversity make them the oceans' "rainforests," this year showed the "most geographically widespread bleaching ever recorded," says the International Society for Reef Studies (ISRS). Bleaching is caused by the loss of algae living in the soft tissues of the coral animals. The coral may or may not recover.

Purple spots represent a 1ºC excess over seasonal averages; yellow and orange signal dangerous increases, up to 2 degrees. There's been more yellow for longer times this year than in the past 15 years in the Philippines.

NOAA

Alan Strong, a physical oceanographer with the National Oceanic and Atmospheric Administration in Camp Springs, Maryland, says that much of the damage results from higher-than-usual sea surface temperatures, especially in the Indian Ocean and Southeast Asia. These regions have experienced fewer storms and less wind this year, factors that provide corals relief from the hot sun and tepid waters.

Strong has constructed an online map of changes in sea surface temperatures to help researchers predict where bleaching might occur (psbsgi1.nesdis.noaa.gov:8080/PSB/EPS/SST/climohot.html). The more extreme warming causes "more and more types of coral to bleach," Strong says. Branching coral and corals close to the surface tend to be the most vulnerable, but deeper, more massive boulder corals also are affected when the warming becomes severe enough.

Rising temperatures aren't the only culprit. The connections between warm seas and bleaching "are not clear-cut in all cases," cautions outgoing ISRS President John Ogden of the Florida Institute of Oceanography in St. Petersburg. Pollution from river runoff and disease also take their toll, he says.