Forests Lost 60% of Their Canopy When CO2 Spiked 56 Million Years Ago

Forests Lost 60% of Their Canopy When CO2 Spiked 56 Million Years Ago — Recovery Took 100,000 Years

A study published in Science on August 13 reconstructs forest canopies through the Paleocene-Eocene Thermal Maximum using fossilized leaf cells from Wyoming's Hanna Basin. Canopy cover collapsed by roughly 61 percent within a few thousand years and stayed far below baseline for tens of thousands of years — a stark parallel as CO2 rises today.

Key Takeaways

A study published in Science on August 13 reconstructs forest canopies through the Paleocene-Eocene Thermal Maximum using fossilized leaf cells from Wyoming's Hanna Basin.
Canopy cover collapsed by roughly 61 percent within a few thousand years and stayed far below baseline for tens of thousands of years — a stark parallel as CO2 rises today.

AI synthesis4 sources7

Fifty-six million years ago, a sharp pulse of carbon dioxide heated the planet by 5–9 °C in an event known as the Paleocene-Eocene Thermal Maximum (PETM). A new reconstruction, published in Science on August 13, shows what that ancient warming did to forests — and the picture is sobering for a world adding CO2 at an even faster rate today.

The team, led by researchers including paleoclimatologist Regan Dunn of the Natural History Museum of Los Angeles County, analyzed fossilized leaf cells from sediment cores in Wyoming's Hanna Basin. Because plant cells grow differently in sun-exposed versus shaded leaves, the cell shapes encode information about how open the forest canopy was. From those fossils, the researchers estimated a 'leaf area index' — a measure of canopy greenness — across the PETM.

The result: after an initial boost, in which the fertilizing effect of extra CO2 briefly thickened the canopy, the forest collapsed. Within a few thousand years, leaf area plummeted by about 61 percent, and the canopy stayed roughly 35 percent below its pre-event baseline for tens of thousands of years. Recovery took more than 100,000 years. The likely sequence: a volcanic pulse of CO2 first fertilized plant growth, then the accumulating heat overwhelmed it. 'Too much carbon dioxide raises temperatures beyond the limits that plants can function,' Dunn says.

The PETM's CO2 release is thought to be roughly equivalent to total human emissions projected by the end of this century under a pessimistic scenario — which is why the authors say the ancient collapse has direct parallels with today's satellite-observed browning of forests worldwide. 'We're seeing that now all over the world. Many forests are in states of decline,' Dunn said. The study adds hard data to the open question of whether CO2 fertilization will save forests — or be swamped by the heat.