56-Million-Year-Old Fossil Forests Reveal How Trees Respond to Rapid Warming

Fifty-six million years ago, a massive pulse of carbon entered Earth's atmosphere during the Paleocene-Eocene Thermal Maximum (PETM), a period of rapid global warming that offers one of the best ancient comparisons for understanding how forests respond to fast climate change. In research published in Science, scientists led by paleobotanist Dr. Regan Dunn of the Natural History Museum of Los Angeles County reconstructed forest canopies from that period using fossilized leaf cells for the first time.

"We are putting CO2 into the atmosphere faster than any known natural process," Dunn said. "The Earth has never experienced a carbon release at the pace we're creating today. The PETM gives us our best window into how Earth's climate and ecosystems respond to a massive carbon injection before humans began reshaping the planet."

The reconstruction showed that large areas of Earth grew browner during the PETM, as volcanically driven carbon emissions raised atmospheric CO2 and global temperatures while reducing rainfall. Plant species shifted northward, forest canopy cover decreased, erosion intensified and the terrestrial water cycle was disrupted. To make the reconstruction, Dunn's team examined microscopic leaf cuticle fragments preserved in sedimentary rocks from the Hanna Basin in Wyoming, measuring cell shapes that reveal how much sunlight each leaf received and therefore how dense the forest canopy above it was.

Dr. Dunn said tree-mortality events are occurring across Earth today due to warming temperatures, drought stress, pathogens, insects and wildfires, a pattern she said has occurred in nearly all of Earth's five major extinction events.