Adding huge quantities of carbon dioxide into the atmosphere is set to cause the Earth’s forests to wither and die rather than flourish, according to a new study that counters erroneous claims that fossil fuel pollution will green the planet.
An analysis of fossilized leaf cells from a period known as the Paleocene-Eocene Thermal Maximum (or PETM), which occurred about ۵۶ million years ago, has helped reconstruct how the world’s forests responded when planet-heating gases were previously being released at a rapid rate.
The current galloping pace of carbon emissions from the burning of fossil fuels is unmatched in historic and prehistoric records but the PETM, when huge amounts of carbon were released over a ۱۳۰,۰۰۰-year period and the world heated up by as much as ۸C (۱۴F), is the closest comparison.
The authors of the new paper found this prehistoric change caused a retreat in the forests that blanketed the Paleocene-Eocene world, causing them to die off as the Earth browned rather than greened, a finding they state has “chilling implications” for today’s world.
“What we see is that as the carbon dioxide ramped up, the forests became very dense but then subsequently collapsed, opening up dramatically,” said Regan Dunn, a researcher at La Brea Tar Pits and the Natural History Museum of Los Angeles County, who led the study.
“Around ۶۰% 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.”
Dun added: “Forests are some of our best friends and allies but we are already seeing them die back, be cut down and consumed by wildfires. It’s happening quickly, every summer the change seems faster and faster.”
Plants like trees require carbon to build their structures and, along with water and sunlight, provide food. Once the world started to burn coal, oil and gas in huge volumes from the industrial revolution onwards, the amount of emitted carbon dioxide surged at an unprecedented rate, with atmospheric concentrations of CO۲ rising by more than ۵۰% in little over a century.
This increase in carbon did initially help boost some plant life, a change that has been seized upon by those who question the facts of climate science. Prominent figures in Donald Trump’s administration have argued that burning fossil fuels is good for the environment.
“CO۲ was never a pollutant,” Doug Burgum, the interior secretary, told Fox Business earlier this year as the federal government moved to erase its own power to address the climate crisis. “When we breathe, we emit CO۲. Plants need CO۲ to survive and grow. They thrive with more CO۲.”
On the same TV network last year, Chris Wright, Trump’s energy secretary, said that there were “pluses” to global heating as well as negatives. “A warmer planet with more CO۲ is better for growing plants,” Wright claimed.
Experts have previously rejected such claims as false, with the new research paper, published in Science, showing that the Earth’s past experience demonstrates that trees require much more than CO۲ to thrive.
Extra carbon dioxide traps more heat, causing trees to struggle amid extreme temperatures and a lack of key nutrients. Hotter conditions can also trigger outbreaks of pests and disease, weakening trees further, and dry out vegetation that then explodes into huge wildfires, such as those that have engulfed much of Canada and western Europe this summer.
The switch from greening forests to browning forests occurred around ۲۰۰۰, the new paper states, when the impacts of drought and heat stress overtook the benefits of extra CO۲. Since then, evidence has piled up of escalating global tree mortality, including trees in the tropics dying of dehydration.
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“Once the climate becomes too warm plants aren’t able to handle it, it affects their physiology and their relationship with water,” said Dunn. “Once they can’t survive because the CO۲ and temperatures are too high, that’s when you see forests die back.”
There was “quite a lot of tree mortality” now occurring in Europe, North America and the Amazon, Dunn said, requiring swift cuts to carbon emissions to ensure that the world doesn’t barrel down the same path as it did ۵۶m years ago.
“There is still time to prevent the worst outcomes and best thing we can do is keep fossil fuels in the ground,” she said. “It’s important for people to understand that once we pass thresholds it takes a long time to recover from them. The ۱۳۰,۰۰۰ years of the PETM change on human timescales is basically forever.”
Dunn and her colleagues ascertained the extent of ancient forests by analyzing fossilized cells of leaves called leaf cuticle fragments. This enabled them to ascertain the density of the forest canopy during the PETM, allowing them to calculate the advance and retreat of forests over time.
In that past era, forests covered a much larger extent of the world than now, with no ice at the poles and much higher seas than today. But while the PETM changes occurred over many thousands of years via carbon released by the natural forces of volcanic activity, current climate change is much faster and caused by our deliberate combustion of fossil fuels.
The paper is a “genuine advance” and provides a record over tens of thousands of years that shows the knock-on impacts of thinning forests, according to Trevor Keenan, a scientist at the University of California, Berkeley, who was not involved in the new research.
“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.”
This is worrying because the pace of change today, he added. “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.”
