"Fire associated with human activity and drying is likely to be what eliminates the forest rather than the gradual stress of climate change," Mark Bush of the Florida Institute of Technology and US-based colleagues wrote in a study.

Examining the history of fire in Amazonia, they said people were the overwhelming cause of burning in the past 3,000 years with lightning strikes rarely igniting the wet forest. "The Amazon doesn't burn unless people burn it," Bush told Reuters.

A drier climate, more human settlements and burning to clear land for farming would bring risks of ever wider fire damage, they wrote in the British journal Philosophical Transactions of the Royal Society B devoted to the Amazon.

Indigenous people in the Amazon basin before Christopher Columbus sailed the Atlantic in 1492 "burned the forest to clear it for agriculture, and perhaps also to improve hunting," they wrote of charcoal records. "After the time of European contact, fires became much scarcer."

The UN Climate Panel predicted in a 2007 report that rising temperatures and drier soil would "lead to gradual replacement of tropical forest by savannah in eastern Amazonia" by 2050.

It also said there was a risk of a "significant" loss of the diversity of species of animals and plants because climate change could drive many to extinction. Its models did not assess fire risks.

"Fire is the greatest climate-linked threat to the Amazon forest," a team led by Jos Barlow of Lancaster University in England wrote in the same journal, adding that the ability of the forest to regrow after fires may have been repeated.

"Episodic fires can lead to drastic changes in forest structure and composition," the said.

But they said there was some hope because farming practices could be changed to avoid burning. Fire is "one of the few aspects of climate change mitigation over which we retain some direct control," they said.

Deforestation -- mainly from burning tropical forests -- is widely considered to contribute about 20 percent of world greenhouse gas emissions. Trees soak up heat-trapping carbon dioxide as they grow and release it when they burn or rot.

A study led by the University of Leeds said trees and creepers in intact parts of the Amazon forest grew faster in the 1980s and 1990s -- apparently spurred by climate change -- and so helped to brake the overall warming.