Roughly 56 million years ago, a surge of volcanic activity released massive amounts of greenhouse gases into the atmosphere, causing Earth’s temperature to rapidly rise. A new investigation into this ancient global warming event offers insight into how forests respondedâand a stark warning for our future.
The study, published today in the journal Science, suggests that forests thinned dramatically during the Paleocene-Eocene Thermal Maximum (PETM), reducing their ability to store carbon. Scientists consider this warming event to be the best natural analog for human-driven climate change, but the rate of carbon release from burning fossil fuels is 10 times faster.
The researchers believe their findings could foreshadow major changes to modern forests that would exacerbate climate change, creating a dangerous feedback loop.
Reconstructing an ancient forest
Led by Regan Dunn, a paleobotanist at the Natural History Museum of Los Angeles County and an adjunct assistant professor at the University of Southern California, the researchers analyzed forest fossils from Wyoming’s Hanna Basin.
This exceptionally deep basin preserves many layers of coal, which means it contains rocks rich with organics. This allowed the team to examine the epidermal cells of fossilized leaves that date back to the PETM and figure out what the forest looked like during that time.
“These are cells in, basically, the skin of a leaf,” Dunn told Gizmodo. “They look like little puzzle pieces, and the shape of those cells is dependent on how much light that leaf received during development.”
She and her colleagues thought that the shape of these cells might be closely related to leaf area index, which measures how much foliage is in a forest canopy and therefore determines the amount of light that penetrates through.
They tested this idea using modern leaves from forests with known LAIs and found a strong correlation between cell shape and the amount of foliage. With this relationship established, they could use the shape of fossil leaf cells to estimate how much foliage was present in the Hanna Basin forest during the PETM.
“I wasn’t necessarily expecting to see what I found,” Dunn said. Plants consume carbon dioxide to make energy, so one could argue that a sharp increase in atmospheric CO2 would boost plant growth and create denser forests, she explained. But the findings showed that the consequences of rapid warming ultimately outweighed the benefits of increased CO2 availability, killing trees and thinning the canopy.
“The hotter things are, the more stressed trees are,” Dunn explained. Based on their analysis, the researchers found that the Hanna Basin forest canopy was, on average, 35% thinner during the PETM than it was before the warming event began. The sharpest decline occurred at the onset of the PETM, when LAI decreased by roughly 61%.
A grim window into Earth’s future
Dunn and her colleagues attributed the rapid decline in LAI at the start of the PETM to drought driven by rising temperatures and altered precipitation patterns. If that sounds familiar, it’s because that’s essentially what we’re seeing in many forests today.
Though carbon emissions have driven increasing global “greenness” since 1982, this pattern reversed around 2000 across roughly 90% of Earth’s vegetated surface, indicating that the productivity benefits of rising CO2 are being outpaced by heat and drought stress.
As a result, forest carbon storage is declining. If this trend continues, scientists estimate that the amount of carbon stored in their aboveground biomass could decline up to 20% by 2100.
As rising temperatures weaken one of Earthâs most important defenses against carbon emissions, even more heat could build up in the atmosphere, driving temperatures higher still. But there are some forests that offer hope.
“There’s a major reforestation going on in places like China, for instance,” Dunn said. “It’s not too late if we can recognize this now, if we realize that it’s going to take 100,000 years for recovery of this big carbon bubble that we’re putting into the air.”
Looking ahead, she hopes to investigate forest changes during the PETM at other locations around the world to get a better sense of the global response. “One of my primary interests is to understand what’s going on in our largest and most productive forests today,” Dunn said. “What happened during this interval in the tropics?”
Answering that question is especially important as the Amazon rainforest, often called the lungs of our planet, buckles under the strain of deforestation and global warming. As Earth’s climate enters uncharted territory, understanding the consequences of past warming events may be one of the best ways to understand our future.