New research into fossilised leaves dating back around 56 million years suggests that extremely high carbon dioxide levels can ultimately cause forests to decline rather than flourish. The findings challenge claims that rising CO2 concentrations from fossil fuel use will simply make the Earth greener by stimulating plant growth.
Ancient Climate Event Offers Warning for Modern Forests
Scientists examined fossilised leaf cells from the Paleocene-Eocene Thermal Maximum (PETM), an episode of rapid global warming approximately 56 million years ago.
During the PETM, enormous quantities of carbon entered the atmosphere over roughly 130,000 years, contributing to global temperature increases of as much as 8C.
Although the pace of modern carbon emissions from burning coal, oil and gas is significantly faster, researchers regard the PETM as one of the closest geological comparisons available for understanding how ecosystems may respond to major increases in atmospheric carbon.
The new study found that forests initially became extremely dense as carbon dioxide concentrations increased. However, that expansion was followed by a dramatic collapse in vegetation.
Researchers Find Major Loss of Forest Biomass
“What we see is that as the carbon dioxide ramped up, the forests became very dense but then subsequently collapsed, opening up dramatically,” said Regan Dunn, a researcher at La Brea Tar Pits and the Natural History Museum of Los Angeles County, who led the study.
“Around 60% of the biomass of the standing vegetation was gone; it was a huge loss. This should be a concern because the rate of carbon release has never been higher than now.”
The researchers said the prehistoric evidence indicates that prolonged warming and elevated atmospheric carbon can eventually overwhelm any initial benefit plants receive from additional CO2.
Dunn added: “Forests are some of our best friends and allies but we are already seeing them die back, be cut down and consumed by wildfires. It’s happening quickly; every summer the change seems faster and faster.”
Why More Carbon Dioxide Does Not Necessarily Mean More Trees
Carbon dioxide is essential for photosynthesis. Trees and other plants use carbon alongside sunlight and water to produce energy and build their structures.
Since the Industrial Revolution, however, widespread burning of fossil fuels has dramatically increased atmospheric CO2 concentrations. Levels have risen by more than 50% in little over a century.
Additional carbon dioxide can initially stimulate plant growth under some conditions. This phenomenon, sometimes described as the CO2 fertilisation effect, has contributed to increased vegetation in certain parts of the world.
However, the fossil record suggests that this effect has important limits.
Heat and Environmental Stress Can Override CO2 Benefits
Higher carbon dioxide concentrations also contribute to global warming, bringing environmental pressures that can undermine plant growth.
Extreme heat, drought, changing rainfall patterns and wildfires can all place forests under significant stress. When those pressures become severe enough, researchers suggest they can outweigh the advantages plants gain from having more carbon dioxide available.
The PETM evidence provides a long-term example of how this process may unfold. Forests initially responded to increased carbon levels with denser growth before experiencing widespread decline as climatic conditions changed.
For the UK, the findings are relevant to wider debates over woodland restoration and nature-based climate measures. British forests and newly planted woodland are expected to play a role in absorbing carbon, but their effectiveness depends on trees remaining healthy as temperatures and weather patterns change.
Fossil Leaves Provide Insight Into Future Climate Risks
Fragments of ancient leaves allowed researchers to reconstruct the structure of forest canopies during the PETM. By examining microscopic characteristics preserved in the fossils, scientists were able to estimate how vegetation responded as atmospheric carbon and temperatures increased.
The resulting picture does not support the idea that continually increasing carbon dioxide will automatically produce a greener planet.
Instead, the research indicates that forests may reach a point where warming and associated environmental disruption lead to substantial losses in vegetation and stored carbon.
That could have broader consequences because forests are among the world’s most important natural carbon stores. Their decline can reduce the amount of CO2 removed from the atmosphere while potentially releasing carbon already held in trees and soils.
Ancient Forest Collapse Carries Modern Climate Warning
The PETM unfolded over thousands of years, while today’s fossil fuel emissions are accumulating at a far faster pace. Scientists therefore caution that modern forests could face environmental changes for which there is little precedent in the geological record.
The fossil evidence suggests that additional carbon dioxide may temporarily encourage plant growth, but it cannot indefinitely protect forests from rising temperatures and other climate stresses.
Rather than guaranteeing a greener world, sustained high carbon emissions could weaken one of the planet’s most important natural defences against climate change, leaving forests increasingly vulnerable as global temperatures rise.
Thomas Hardy is a contributor to OE Mag, covering news, politics, business, technology, sport, entertainment, and lifestyle. He focuses on clear, accurate reporting and useful information that helps readers stay informed about current affairs and developments that matter to them. His work highlights relevant stories, emerging trends, and key issues, presenting them in a balanced, accessible, and reader-friendly way.