Fossil study: prehistoric CO2 surge withered forests, not greened them

Trump officials have argued burning fossil fuels benefits forests

A study published this week in the journal Science analyzed fossilized leaf cells from a period roughly 56 million years ago and found that forests initially thickened as carbon dioxide surged, then collapsed, shedding about 60% of their standing biomass. The researchers said the finding, which they describe as having “chilling implications” for the present, offers a sobering precedent for what may lie ahead as the planet’s carbon emissions accelerate.

The PETM — when vast amounts of carbon were released over a 130,000-year span and the world heated by as much as 8°C (14°F) — is the closest analog to today’s fossil-fuel-driven warming available in the geologic record, the researchers wrote. Atmospheric concentrations of CO2 have since risen by more than 50% in little over a century, the paper noted, a pace unmatched in either historic or prehistoric records. Although the ancient carbon release came from volcanic activity and unfolded over many millennia, while current emissions are produced by the deliberate combustion of coal, oil and gas on a timescale of decades, the authors said the response of the world’s forests offers a sobering preview.

Lead author Regan Dunn, a paleontologist at the La Brea Tar Pits and the Natural History Museum of Los Angeles County, and her colleagues reconstructed ancient forest canopy density by analyzing fossilized leaf cuticle fragments — the outermost layer of cells on a leaf — recovered from PETM-era geological deposits. The cuticles preserve how much sunlight reached the forest floor, letting the team chart how canopies advanced and retreated over time.

“What we see is that as the carbon dioxide ramped up, the forests became very dense but then subsequently collapsed, opening up dramatically,” Dunn said. “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 paper directly addresses claims by members of the Trump administration who have argued that rising CO2 benefits the environment. Interior Secretary Doug Burgum told Fox Business earlier this year that “CO2 was never a pollutant” and that plants “thrive with more CO2.” Energy Secretary Chris Wright told the same network last year that a warmer planet with more CO2 is “better for growing plants.” The new research, Dunn said, shows that trees require much more than CO2 to thrive: extra CO2 traps heat, stresses trees through extreme temperatures and nutrient depletion, fuels pest and disease outbreaks, and dries out vegetation that then ignites into the kind of wildfires that have engulfed parts of Canada and western Europe this summer.

“Forests are some of our best friends and allies but we are already seeing them die back, be cut down and consumed by wildfires,” Dunn said. “It’s happening quickly, every summer the change seems faster and faster.”

“Once the climate becomes too warm plants aren’t able to handle it, it affects their physiology and their relationship with water,” Dunn added. “Once they can’t survive because the CO2 and temperatures are too high, that’s when you see forests die back.” Dunn said there was “quite a lot of tree mortality” now occurring in Europe, North America and the Amazon, and called for swift cuts to carbon emissions to keep the world from following the PETM path.

Trevor Keenan, a scientist at the University of California, Berkeley, who was not involved in the research, called the paper a “genuine advance” that provides a record over tens of thousands of years showing the knock-on impacts of thinning forests. “The headline is that warming won,” Keenan said. “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 comparison to the present is sobering on two fronts. “The ancient carbon release happened over thousands of years, ours is happening over decades,” he said. “Faster forcing gives ecosystems less room to adjust. Second, those forests faced climate change alone. Today’s forests are dealing with 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.”

The switch from greening to browning forests occurred around 2000, the paper states, when the impacts of drought and heat stress overtook the benefits of extra CO2. Since then, evidence has accumulated of escalating global tree mortality, including trees in the tropics dying of dehydration.

“There is still time to prevent the worst outcomes and best thing we can do is keep fossil fuels in the ground,” Dunn said. “It’s important for people to understand that once we pass thresholds it takes a long time to recover from them. The 130,000 years of the PETM change on human timescales is basically forever.”