Fossils show carbon emissions caused 60% forest loss 56m years ago

A new study of 56-million-year-old fossilized leaves shows that massive carbon emissions caused ancient forests to collapse, not flourish, challenging claims by Trump administration officials that fossil fuel pollution benefits plant life. The research, published in Science on August 13, 2026, reconstructed forest changes during the Paleocene-Eocene Thermal Maximum (PETM), when huge amounts of carbon were released over 130,000 years and global temperatures rose by up to 8°C (14°F).

The findings indicate that as carbon dioxide levels rose, forests initially became denser but then suffered a dramatic die-off, with about 60% of standing vegetation biomass lost. Regan Dunn, a researcher at La Brea Tar Pits and the Natural History Museum of Los Angeles County who led the study, said the forests "collapsed, opening up dramatically" and that the loss "should be a concern because the rate of carbon release has never been higher than now."

The study directly counters statements by Interior Secretary Doug Burgum and Energy Secretary Chris Wright, who have argued that more CO2 helps plants. Burgum told Fox Business earlier this year that "CO2 was never a pollutant" and that plants "thrive with more CO2," while Wright claimed on the same network last year that "a warmer planet with more CO2 is better for growing plants." Experts have rejected such claims as false, and the new paper shows that trees require much more than CO2 to thrive.

The switch from greening to browning forests occurred around 2000, according to the paper, when drought and heat stress overtook the benefits of extra CO2. Since then, evidence has mounted of escalating global tree mortality, including dehydration deaths in tropical forests. Dunn said there is "quite a lot of tree mortality" now occurring in Europe, North America and the Amazon, and that swift cuts to carbon emissions are needed to avoid repeating the PETM path.

Dunn and her colleagues analyzed fossilized leaf cuticle fragments to determine forest canopy density during the PETM, allowing them to track forest advance and retreat over time. In that era, forests covered a much larger extent of the world than today, with no polar ice and higher seas. However, the PETM changes occurred over many thousands of years via natural volcanic activity, whereas current climate change is much faster and caused by human combustion of fossil fuels.

Trevor Keenan, a scientist at the University of California, Berkeley, who was not involved in the research, called the paper a "genuine advance" and said its headline is that "warming won." He noted that today's forests face the same warming while also being fragmented, logged, converted to agriculture and exposed to altered fire regimes, making the ancient precedent deeply worrying. Dunn said there is still time to prevent the worst outcomes, but "once we pass thresholds it takes a long time to recover from them," with the PETM's 130,000 years of change being "basically forever" on human timescales.