Climate change and human-induced impacts are damaging the world's coral reefs, say two NOAA scientists who attended the International Coral Reef Symposium in Bali last week.

"Sea surface temperatures throughout the tropics have shown dramatic increases over the past two decades, as much as half a degree per decade," said NOAA Oceanographer Alan E. Strong. "This is ten times what we are observing globally. As a result, the concern for coral reefs is how much of this increase will continue over the ensuing decades."

Coral bleaching occurs when the symbiotic algae that live in corals become stressed and expelled. This turns coral white, leaving them in an unhealthy state. Research presented at the conference revealed that corals died in large numbers or were severely compromised after the extensive 1998 bleaching event and that rising sea temperatures have been responsible for large-scale bleaching and mortality events. Results also showed that this effect was exacerbated by other factors such as high light levels and human-related stress.

Discussions at the conference also highlighted the fact that climate interactions with coral reefs are highly complex, and that we need to understand much more than the El Niño-Southern Oscillation, such as decadal scale climate variability. While dramatic climate changes have occurred during past geological periods, we are able to explain these by natural phenomena. However, the changes we are currently witnessing can be explained only on the basis of human-induced impacts, said Mark Eakin, who heads NOAA's paleoclimatology program.

"Coral records from the last 400 years show consistent patterns of climate change in the last century. Recent paleoclimatic research and climate models indicate that this warming is due to human causes" said Eakin. Work presented at the conference by other researchers further demonstrated that bleaching events like those seen in the 1990s do not appear to have happened in the last few thousands of years.