"We're also interested to see whether rising levels of carbon dioxide—which can be detected in the fossils—were driving intense global warming in much the same way they are at the moment," Falcon-Lang said.
The prehistoric rain forests provide an important record that can help scientists forecast how today's topical forest regions, such as the Amazon, could respond to future global warming, he added.
The fossil trees indicate there is a certain, sometimes abrupt, climate threshold, after which rain forests reorganize to form a new, stable system, Falcon-Lang said.
Major earthquakes likely flooded Illinois's underground forests with prehistoric seawater, preserving them.
So far few traces of animal fossils have been found among the petrified jungle, as animal and plant remains tend to fossilize under different conditions, the study team notes.
But insect fossils found elsewhere reveal the Carboniferous forests were ruled over by giant bugs such as centipedes, some six feet (two meters) in length, and mammoth cockroaches and scorpions as much as three feet (one meter) long.
The ancient forests were found on top of existing coal seams. The coal represents the peaty soil in which they were growing, Falcon-Lang said.
Jane Francis is a professor of paleoclimatology at Leeds University, in the U.K.
Francis, who wasn't involved in the study, said the latest discoveries fit with other fossil evidence indicating that vegetation responded globally to climate warming some 300 million years ago.
Forest-fossil clues about climate change are most marked from the Southern Hemisphere, where the polar ice existed, she said.
North America at the time, however, was located in tropical latitudes.
The ice melting and subsequent drying of the climate is well known but it has never really been found in Northern Hemisphere forests before, Francis said. "So what you're seeing [in North America] is probably a more distant signal of the same climate change."
She would expect to see similar changes in rain forest vegetation affected by future climate change, she added.
Francis, however, notes that the geography of Earth's landmasses and oceans were very different in the Carboniferous period, making it difficult to draw modern-day climate parallels.
"Climate change then was also much greater than we're going through today, but who knows where it might go in a hundred years," she said.
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