Climate change's "evil twin" -- ocean acidification -- has been increasing at a rate unexpected by scientists, says Dr. Jane Lubchenco, head of the National Oceanic and Atmospheric Administration (NOAA).

Lubchenco told he Associated Press that surface waters, where excess carbon dioxide from the atmosphere has been concentrating, "are changing much more rapidly than initial calculations have suggested." She warns, "It's yet another reason to be very seriously concerned about the amount of carbon dioxide that is in the atmosphere now and the additional amount we continue to put out."

Lubchenco made the comments while at the 12th International Coral Reef Symposium in the Australian city of Cairns, where thousands of scientists are meeting and calling for action to save the world's coral reefs.

"The carbon dioxide that we have put in the atmosphere will continue to be absorbed by oceans for decades," Lubchenco added. "It is going to be a long time before we can stabilize and turn around the direction of change simply because it's a big atmosphere and it's a big ocean."

A study published in March in the journal Science found that the Earth's oceans are becoming more acidic at a faster rate than at any time in the past 300 million years due to increased levels of CO2 in the atmosphere.

At the conference, scientists warned of the urgency to act on climate change.

"There is a window of opportunity for the world to act on climate change, but it is closing rapidly," said Terry Hughes, convener of the symposium.

Jeremy Jackson, senior scientist at the Smithsonian Institution in the United States, says that while climate change is bringing ocean acidification, it is also causing droughts and sea rise, so "what's good for reefs is also critically important for people and we should wake up to that fact," he said. "The future of coral reefs isn't a marine version of tree-hugging but a central problem for humanity."

International Society for Reef Studies president Robert Richmond urges immediate action to save the earth.

"The scientific community has an enormous amount of research showing we have a problem. But right now, we are like doctors diagnosing a patient's disease, but not prescribing any effective cures," he said.

"We have to start more actively engaging the process and supporting public officials with real-world prescriptions for success."