Extreme heat is wilting and burning forests, making it harder to curb climate change

Earth’s lands lost much of their ability to absorb the carbon dioxide humans pumped into the air last year, according to a new study that is causing concern among climate scientists that a crucial damper on climate change underwent an unprecedented deterioration.

Temperatures in 2023 were so high - and the droughts and wildfires that came with them were so severe - that forests in various parts of the world wilted and burned enough to have degraded the ability of the land to lock away carbon dioxide and act as a check on global warming, the study said.

The scientists behind the research, which focuses on 2023, caution that their findings are preliminary. But the work represents a disturbing data point - one that, if it turns into a trend, spells trouble for the planet and the people on it.

“We have to be, of course, careful because it’s just one year,” said Philippe Ciais, a scientist at France’s Laboratory of Climate and Environmental Sciences who co-authored the new research.

But the results, he added, are still “worrying.” If extreme warming continues, society risks losing “the best friend of humanity” in Earth’s land.

Earth’s continents act as what is known as a carbon sink. The carbon dioxide that humans emit through activities such as burning fossil fuels and making cement encourages the growth of plants, which in turn absorb a portion of those greenhouse gases and lock them in wood and soil. Without this help from forests, climate change would be worse than what is already occurring.

“This is a significant issue, because we are benefiting from the uptake of carbon,” said Robert Rohde, chief scientist for Berkeley Earth, who was not involved in the research. “Otherwise, carbon dioxide levels in the atmosphere would rise even faster and drive up temperatures even faster.”

Ciais and his colleagues saw that the concentration of CO2 measured at an observatory on Mauna Loa in Hawaii and elsewhere spiked in 2023, even though global fossil fuel emissions increased only modestly last year in comparison. That mismatch suggests that there was an “unprecedented weakening” in the Earth’s ability to absorb carbon, the researchers wrote.

The scientists then used satellite data and models for vegetative growth to try to pinpoint where the carbon sink was weakening. The team spotted abnormal losses of carbon in the drought-stricken Amazon and Southeast Asia as well as in the boreal forests of Canada, where record-breaking wildfires burned through tens of millions of acres.

The paper, which has not yet been peer-reviewed, was posted last week on the science preprint site arXiv. The researchers plan to present their findings next week at a scientific conference in Brazil.

One big question overhanging this research is whether the results represent a one-year blip - or the start of troubling long-term trend.

“If it’s the new normal, then climate mitigation will be even harder than it is now,” said Rob Jackson, a climate scientist at Stanford and author of the book “Into the Clear Blue Sky,” who also was not involved in the research. “We expect the land sink to slow eventually, but I hoped it wouldn’t happen so soon. If it slows this early, we’re in trouble.”

Another key question, Jackson said, is whether the drop is due to the start last year of El Niño, a naturally occurring climate pattern that has been associated with carbon losses on land.

Ciais, the researcher behind the study, said the tropical rainforests of South America and Asia have a better chance to bounce back than the snowy woodlands in North America, given how slowly those northern forests grow.

“We have a strong buildup of evidence to predict that this northern sink is not going to become very strong again anytime in the future as this extreme warming continues,” Ciais said.

The lost ability of some northern land to lock away carbon may also be a sign that a significant amount of organic material frozen in permafrost is thawing, according to Woodwell Climate Research Center ecologist Richard Houghton. Scientists have long worried about potentially catastrophic releases of greenhouse gases from thawing in and around the Arctic Circle.

“It has always amazed me that the fraction of global carbon emissions taken up by land systems has remained so stable,” he said.

Computer models disagree on when land will cease acting as a carbon sink, Rohde added - whether it will be soon, or well past the end of this century.

“The worry is that we are approaching a level at which the ecosystem is getting harmed by our temperature changes to such a degree that it’s no longer helping us out by absorbing carbon,” Rohde said. “Because we don’t know when that is, something like this where we see it declining in real time is worrisome.”