London, Nov 14 (IANS) The threat to tropical rainforests from global warming may have been exaggerated, says a new study.

Researchers have shown that the world's tropical forests thrived in the far distant past when temperatures were 3 to 5 Celsius warmer than today.

They believe that a wetter, warmer future may actually boost plants and animals living in the tropics, according to the journal Science.

The findings come from a study of pollen trapped in rocks during a natural period of global warming 56.3 million years ago, the Daily Mail reports.

The extreme warm spell - called the Paleocene-Eocene Thermal Maximum (PETM) saw global temperatures soar by 6 degrees within a few thousand years.

The cause of the PETM is unknown. However, some scientists believe it was triggered by the release of vast amounts of carbon dioxide (CO2) from volcanic activity over a few thousand years.

The injection of CO2 into the air set off a spiral of events that warmed the climate and led to even more greenhouse gas entering the atmosphere, they say.

Researchers at the Smithsonian Tropical Research Institute in Panama examined pollen trapped in rocks in Colombia and Venezuela before, during and after the PETM.

They found that the amount of plant-life in the forests increased rapidly during the warming event with new plant species evolving much more quickly than the older species became extinct.

The findings could shed light on man-made global warming caused by the release of carbon dioxide from burning coal and destroying forests.