AI-written summary of reporting by The Guardian US. No human editor reviewed this. AI can misread or omit facts — read the original, linked below.
ABSTRACT

A study published in the journal Science, analyzing fossilized leaf cells from the Paleocene-Eocene Thermal Maximum (PETM) roughly 56 million years ago, found that a rapid rise in carbon dioxide caused forests to initially densify and then collapse, with about 60% of standing vegetation biomass lost, according to the study's lead author.

The Guardian US

Study finds ancient warming event caused forest collapse, not growth

Study finds ancient warming event caused forest collapse, not growth
Aug 13, 2026, 2:00 PM

A study published in the journal Science, analyzing fossilized leaf cells from the Paleocene-Eocene Thermal Maximum (PETM) roughly 56 million years ago, found that a rapid rise in carbon dioxide caused forests to initially densify and then collapse, with about 60% of standing vegetation biomass lost, according to the study's lead author.

Context

The PETM, during which large amounts of carbon were released over a 130,000-year period and global temperatures rose by as much as 8C (14F), is the closest known prehistoric analogue to current carbon emissions, according to the source. The study's authors state that the current rate of carbon emissions from fossil fuel combustion is unmatched in historic and prehistoric records.

Regan Dunn, a researcher at La Brea Tar Pits and the Natural History Museum of Los Angeles County who led the study, said the team reconstructed ancient forest canopy density by analyzing fossilized leaf cuticle fragments — cells from the outer layer of leaves — which allowed them to calculate how forest cover advanced and retreated across the PETM period.

The paper states that the shift from forests expanding to forests declining occurred around 2000, when the effects of drought and heat stress overtook any benefit from elevated CO2 levels. The source notes that extra carbon dioxide traps additional heat, which can cause trees to struggle under extreme temperatures, nutrient deficits, pest outbreaks, and wildfire conditions.

The source notes that atmospheric CO2 concentrations have risen by more than 50% in little over a century since industrialization, and that this increase initially helped boost some plant life — a pattern cited by some officials in the Trump administration as evidence that fossil fuel emissions benefit the environment. Interior Secretary Doug Burgum told Fox Business that "CO2 was never a pollutant" and that "plants need CO2 to survive and grow. They thrive with more CO2." Energy Secretary Chris Wright said on the same network that "a warmer planet with more CO2 is better for growing plants."

Trevor Keenan, a scientist at the University of California, Berkeley, who was not involved in the study, described it as a "genuine advance" providing a record spanning tens of thousands of years. Keenan said the ancient carbon release occurred over thousands of years, while the current release is happening over decades, giving ecosystems less time to adjust. He also noted that today's forests face warming while simultaneously being fragmented, logged, converted to agriculture, and exposed to altered fire regimes.

All Perspectives
Regan Dunn, study lead author: "What we see is that as the carbon dioxide ramped up, the forests became very dense but then subsequently collapsed, opening up dramatically. 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." Dunn added: "Once the climate becomes too warm plants aren't able to handle it, it affects their physiology and their relationship with water. Once they can't survive because the CO2 and temperatures are too high, that's when you see forests die back." She said there is "still time to prevent the worst outcomes" and that "the best thing we can do is keep fossil fuels in the ground."
Trevor Keenan, University of California, Berkeley (not involved in study): "The headline is that warming won. Carbon dioxide rose sharply, and any benefit that gave the forests was not enough to stop canopies from opening up and staying open for tens of thousands of years." Keenan said the faster pace of current emissions gives ecosystems less room to adjust, and that today's forests face the same warming "while also being fragmented, logged, converted to agriculture and exposed to altered fire regimes. If forests thinned under the slower and cleaner version of this experiment, that is not a reassuring precedent."
Doug Burgum, US Interior Secretary: "CO2 was never a pollutant. When we breathe, we emit CO2. Plants need CO2 to survive and grow. They thrive with more CO2."
Chris Wright, US Energy Secretary: Wright said there were "pluses" to global heating as well as negatives, and claimed: "A warmer planet with more CO2 is better for growing plants."
Gaps & Unknowns
  • The source does not state the full title or publication date of the paper within the journal Science.
  • The source does not identify the other co-authors of the study beyond Dunn.
  • The source does not state the specific geographic location or locations from which the fossilized leaf cuticle samples were drawn, beyond a reference to the Hanna Basin, Wyoming, in a photo caption.
  • The source does not specify the CO2 concentration levels recorded during the PETM compared with present-day levels.
  • The source does not state whether the paper was peer-reviewed or describe its methodology beyond the analysis of fossilized leaf cuticle fragments.
  • The source does not include any response from Burgum's or Wright's offices to the study's findings.
Sources & Further Reading
  1. The Guardian US — original

Read the original at The Guardian US

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