As global temperatures rise and rainfall patterns change, the dark stone temple, which dates from the 9th century, could deteriorate faster than normal, Marsis Sutopo, head of the Borobudur Heritage Conservation Institute, told Reuters.

"We are racing against the weather," Sutopo said.

"Changing climate will have an impact on temple conservation efforts. Warmer temperature could theoretically cause more fissures and cracks in the stones," he said, adding that acid rain has already eroded many of the reliefs.

"Humidity allows moss and algae to grow on the stones already more than 1,000 years old. The stones have been exposed to the heat and humidity for so long, they have reached a critical point where deterioration is going to happen faster," he said.

"We suspect changing climate will make it happen faster."

NIRVANA

Borobudur, near Java's ancient royal capital Yogyakarta, dates back to around 800 AD, long before Islam became the dominant religion in the world's most populous Muslim nation.

It represents a Buddhist view of the universe, comprising a series of square and circular terraces that allow visitors to move upward from the everyday world to a large bell-shaped stupa representing nirvana.

Steep stairways lead to the wide-open terraces, where stone-lattice stupas contain statues of Buddha overlooking the tropical green plain and its distant volcanoes.

The monument was neglected and abandoned for almost a thousand years before it was rediscovered beneath volcanic ash and jungle in the 1800s when a survey team investigated talk of a great ruin in central Java.

Borobudur's conservation began during Dutch colonial times thanks to the efforts of a Dutch scientist, Van Erp, between 1907 and 1911.

But the most extensive and complex restoration work took place between the mid-1970s and early 1980s, and involved taking out each of the stones for cleaning and then reassembling them in the original layout. Waterproof layers and channels were also installed inside to protect the temple's reliefs from rainwater.

Conservationists say Borobudur is just one of many world heritage sites, including the Tibetan monasteries in the Himalayas and the cultural monuments of Greece, that are threatened by global warming, although it isn't necessarily endangered by the effects of climate change.

"One of the big problems is the deterioration of the stones, much exacerbated by early conservation efforts. Warming and humidity changes have added to the fungus," said Richard Engelhardt, a Bangkok-based regional adviser at UNESCO for culture in Asia and the Pacific.

Although Borobudur was not affected by the 2006 earthquake in Yogyakarta which killed over 5,000 people, conservationists say the increasing frequency of earthquakes is also a challenge.

"The stones on the reliefs have not been affixed to the basic structure, so in case of a quake they could fall apart," Sutopo said. "Indonesia is part of the Pacific Ring of Fire. In the long run, quakes could destabilise the temple structure."