A series of fossilised forests the size of small cities have provided prehistoric evidence of how tropical rainforests are destroyed by global warming.

The fossil remains represent the first rainforests grown on the planet and their demise more than 300million years ago “points to the future” of the modern-day Amazon.

Six petrified forests, dating from 303.9 million to 309 million years ago, have been discovered in coalmines in the United States. Because they straddle a period of intense global warming researchers have been able to see the effects of climate change on an ancient landscape.

One forest that stretched 10,000 hectares (100 sq km) is the largest fossil forest yet found, dwarfing a 1,000ha forest that was announced last year as the biggest.

Howard Falcon-Lang, of the University of Bristol, said that the forests were frozen in time and show changes in the tree cover before and after the global warming began.

Fossils reveal that the landscape now deep beneath Illinois and Kentucky was covered in huge club moss trees, horsetails and ferns 309 million years ago. Once global warming had taken place 306.5 million years ago, the landscape altered enormously and the trees were replaced with “weedy ferns”.

“These are the remains of the first rainforests to evolve on our planet,” Dr Falcon-Lang said at the British Association yesterday. “They had lush rainforest vegetation, not dissimilar to the Amazonian rainforest. These are the largest fossil forests in the world. It's quite extraordinary to find a forest landscape preserved for miles.”

The forests were buried during earthquakes and the vegetation was swiftly preserved as the sea rushed in and buried it under sediment. Proof of their existence can now be seen in more than 50 mines where the coal seams have been dug out.

Walking along the mine tunnels was an extraordinary experience, Dr Falcon-Lang said: “The coal represents the soil on which this rainforest was growing. The trees are on the roof. You can see roots hanging down.”

He said it appeared that the huge trees suffered enormous stress and died out when faced by global warming. “We are beginning to show there appears to be a threshold in ancient rainforest systems beyond which the whole system begins to unravel quite quickly,” he said.

“The rainforest dramatically collapses during this period of warming. This was very, very extreme global warming. Giant club moss trees vanished overnight to be replaced by rather weedy fern vegetation. All this points to the fate of the Amazon.”

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