A period of intense, natural changes in climate caused coral reefs in the eastern Pacific to shut down thousands of years ago, and human-induced pollution could worsen the trend in the future, scientists said Thursday.

The study in the US journal Science points to sea temperature fluctuations -- brought on by the same phenomenon that causes El Nino and La Nina events every several years -- as the main cause for the coral die-off near the Panama coast.

The reef shutdown began 4,000 years ago and lasted about 2,500 years, said the research led by the Florida Institute of Technology and including experts from China and France.

"We were shocked to find that 2,500 years of reef growth were missing from the frameworks," said lead author Lauren Toth of FIT. "That gap represents the collapse of reef ecosystems for 40 percent of their total history."

Researchers did their analysis by driving 17-foot-long (five-meter-long) hollow pipes into dead frameworks of the coral reefs along Panama's Pacific coast.

They pulled out cross-sections of the reefs, and using radiocarbon dating and mass spectrometry techniques they were able to see what they described as a period of "hiatus" in their growth.

An examination of reef records in Australia and Japan showed similar gaps in growth, which Toth and co-authors believe is due primarily to historical changes in a phenomenon known as El Nino Southern Oscillation (ENSO).

"For Pacific reefs to have collapsed for such a long time and over such a large geographic scale, they must have experienced a major climatic disturbance. That disturbance was an intensified ENSO regime," Toth said.

Very strong El Nino events, bringing higher sea temperatures across the east-central equatorial Pacific, would likely have caused the initial collapse of the Panama corals due to bleaching-related die-off.

This ENSO activity began some 4,200 years ago and peaked about 3,000 years ago. Frequent abnormalities in water temperature would have made it impossible for the coral populations to recover, the study said.

The past may offer a hint of what is to come in the future, with eastern Pacific coral reefs again on the verge of collapse, but this time the situation is made worse by human-induced climate change.

"Climate change could again destroy coral-reef ecosystems, but this time the root cause would be the human assault on the environment and the collapse could be longer-lasting," said co-author Richard Aronson of FIT.

"Local issues like pollution and overfishing are major destructive forces and they need to be stopped, but they are trumped by climate change, which right now is the greatest threat to coral reefs."

However, researchers are hopeful that the world will take measures to curb pollution, allowing the reefs -- which have already proven their resilience -- to recover once more.