Ancient Carbon Surge Shows Forests Can Wither, Not Green, Under Rapid CO2 Rise

A new study in *Science* reconstructs forest dynamics during the Paleocene-Eocene Thermal Maximum (PETM) — a 130,000-year interval about 56 million years ago when massive carbon release raised global temperatures by up to 8 °C (14 °F). Fossil leaf-cuticle cells from Wyoming show canopy density peaked before warming, then collapsed, losing roughly 60 % of standing vegetation. The authors argue the PETM is a natural analogue for today’s emissions, which are occurring at a rate roughly ten times faster than the ancient event.

The Guardian

During the PETM, volcanic activity likely injected large quantities of CO2, driving prolonged greenhouse warming. While elevated CO2 initially promoted dense canopies, subsequent heat, drought and nutrient stress triggered widespread tree mortality, canopy opening, and altered river sedimentation. Modern forests face a similar mix of rapid warming, moisture stress, pest outbreaks and fire, compounded by logging and agriculture.

The Guardian

• PETM carbon release spanned ~130,000 years, raising temperatures by as much as 8 °C.

The Guardian

• Fossil leaf analysis indicates a 60 % reduction in canopy biomass early in the PETM.

The Conversation

• Recovery of forest structure required over 100,000 years, far exceeding a human lifespan.

The Conversation

• Current atmospheric CO2 has risen > 50 % in just over a century, a rate unmatched in the geological record.

The Guardian

She warns that the “thresholds” crossed during the PETM suggest modern emissions could push today’s forests into a long-term, low-canopy state unless fossil-fuel use is sharply reduced. Trevor Keenan, a scientist at UC Berkeley not involved in the research, calls the findings a “genuine advance,” noting that “the headline is that warming won.” He stresses the ancient carbon release unfolded over millennia, whereas today’s forcing occurs over decades.

The Guardian

• “The headline is that warming won,” — Trevor Keenan, UC Berkeley

The Guardian

The PETM record shows that even when CO2 fertilization initially boosts growth, excessive warming and associated stressors can reverse those gains, leading to extensive forest die-back. Modern forests already exhibit rising mortality in Europe, North America and the Amazon, linked to heat, drought and wildfire. Continued emissions could replicate the PETM trajectory, reducing carbon sequestration, altering water cycles and diminishing biodiversity.

The Guardian

Scientists call for immediate, large-scale reductions in fossil-fuel combustion to keep atmospheric CO2 below the thresholds that triggered ancient forest collapse. The study underscores the urgency of aligning climate policy with the long-term resilience limits of forest ecosystems.

The Conversation