The amount of carbon dioxide in the atmosphere last year grew at the fastest rate in recorded history - a dramatic spike that scientists fear may indicate that Earth’s ecosystems are so stressed by warming they can no longer absorb much of the pollution humanity emits.

The National Oceanic and Atmospheric Administration’s Global Monitoring Laboratory on Monday released data showing the concentration of carbon dioxide in the atmosphere increased by 3.75 parts per million in 2024. That jump is 27 percent larger than the previous record increase, in 2015, and puts atmospheric carbon dioxide concentrations at a level not seen in at least 3 million years.

Though the vast majority of planet-warming gases come from people burning fossil fuels, a separate study released Tuesday suggests that last year’s sudden spike was likely driven by a different force: the deterioration of rainforests and other land ecosystems amid soaring global temperatures.

Coming during the hottest year scientists have ever seen, the surge in atmospheric CO2 indicates that humanity may be losing a crucial ally in the fight against planetary warming, experts say. The land and oceans have historically taken up about half of the greenhouse gases people emit. Without these places that absorb CO2 - known as carbon sinks - global temperature rise would be twice the roughly 1.3 degrees Celsius (2.3 degrees Fahrenheit) the world has already endured.

The preliminary analysis published Tuesday shows how extreme drought and raging wildfires unleashed huge amounts of carbon from forests last year, effectively canceling out any pollution they might have absorbed. While not final, the findings have sparked concern among scientists who have been tracking the buildup of greenhouse gases in the atmosphere for years.

“What this is showing is that climate change itself is having an impact on the terrestrial carbon sink,” said John Miller, a carbon cycle scientist at the Global Monitoring Laboratory.

The search for a source

Every week, NOAA monitoring stations from Hawaii to the South Pole sample the air for carbon dioxide and other gases. By combining those data from across the globe, researchers can determine what fraction of the atmosphere is filled with planet-warming pollution and how fast that proportion is increasing.

The agency’s estimate for 2024 shows that the CO2 growth rate sped up by more than 30 percent from the previous year.

“This is next-level high,” said Glen Peters, who studies the carbon cycle at the CICERO Center for International Climate Research in Norway. “So it raises that concern of ‘why is it much higher than expected?’”

The record growth can’t be blamed solely on burning fossil fuels, data suggest. Though emissions from coal, oil and gas hit an all-time high last year, according to the International Energy Agency, the record was a modest one. The growth of fossil fuel pollution has tapered off in recent years as renewable energy sources like solar and wind power have become cheaper and more widely available. Preliminary estimates from the Global Carbon Project suggest that fossil fuel emissions in 2024 increased about 0.8 percent over the previous year - not nearly enough to explain such a dramatic jump in greenhouse gas levels.

Nor do researchers think the rising carbon concentrations came from the ocean, which has historically absorbed about 25 percent of the pollution that people emit. Carbon dioxide usually moves from the atmosphere to the ocean when it is taken up by photosynthetic plankton or becomes dissolved in seawater; scientists have not yet observed significant changes in either process.

The only remaining explanation, Miller said, is a change in the amount of pollution being absorbed by land.

Carbon sinks on land are especially vulnerable, researchers say, because they rely on biological processes that can change a lot year to year. When growing conditions are good, plants will suck up billions of tons of carbon dioxide through photosynthesis. This carbon is then locked away in wood or buried in the soil, where it can stay sequestered for centuries.

On average, forests and other land-based ecosystems have taken up about 30 percent of the carbon emitted by humans in the last decade, according to the Intergovernmental Panel on Climate Change.

Yet these landscapes are increasingly disrupted by human and natural forces, said Nancy Harris, research manager for the Global Forest Watch program at World Resources Institute. Using satellite imagery and other monitoring tools, Harris and her colleagues have studied how a logging operation or wildfire can unleash hundreds of years of accumulated carbon in the space of a single day. Droughts may also prevent plants from performing photosynthesis, interfering with carbon uptake. Rising temperatures can accelerate decomposition in soils, releasing more carbon dioxide back into the air.

“From a selfish humanity perspective, we’ve continued to rely on these forests to buffer our emissions,” Harris said. “If we don’t have that sink to rely on, climate change is going to get even worse than it is now.”

Forest feedbacks

When 2024 began, the Earth was already in the midst of a natural climate pattern called El Niño, when hot conditions in the Pacific Ocean tend to send global average temperatures soaring.

El Niño events are known to be bad for terrestrial carbon storage, said Philippe Ciais, associate director of the Climate and Environmental Sciences Laboratory outside Paris. The warm Pacific waters tend to decrease rainfall in tropical forests, particularly the Amazon, leading to carbon-emitting wildfires and a decline in vegetation growth.

In a preliminary study published to the preprint server arXiv on Tuesday, Ciais and Peters worked with a team of international researchers to analyse sources and sinks for carbon over the course of the past El Niño event, which spanned roughly from July 2023 to June 2024. Though the analysis has not been published in an academic journal, it used well-established peer-reviewed methods to track how carbon moves through the atmosphere, oceans and land.

Using data from the European Center for Medium Range Weather Forecasts (ECMWF), they found that the dry conditions in tropical rainforests were much more extensive than during previous El Niños; at the height of the event, nearly a third of the Amazon was gripped by extreme drought.

Using multiple global vegetation models as well as data from NASA’s Orbiting Carbon Observatory, which measures carbon dioxide from space, the researchers estimated that the land took up about 2.24 fewer gigatons of carbon over the 12-month period - the rough equivalent of burning 9 billion pounds of coal.

Depending on how it’s calculated, the change either zeroed out the land sink or turned terrestrial ecosystems into a net source of carbon pollution.

“This tropical dryness is basically shutting down CO2 uptake,” Ciais said.

The analysis only extends through June, when the El Niño was declared over. But the scientist said he was even more concerned by what came next.

Though the end of the climate pattern typically signals the return of moisture, in the second half of last year the extreme drought in the Amazon surged to encompass nearly 40 percent of the rainforest. Across South America, many rivers fell to record-low levels. Wildfires ripped through the parched landscape, burning an area larger than California.

On the other side of the Atlantic, an equally severe drought had descended on the rainforests of central Africa. By midsummer, more than half of the region was experiencing “extreme” conditions, according to the ECMWF data. Satellite measurements showed that the forests were absorbing far less of the sun’s radiation than normal - an indication of trees dying or becoming too stressed to perform photosynthesis.

“It’s a bad cocktail of an El Niño followed by a very strong dry event, so the plants don’t get a break,” Ciais said. “This has no equivalent during previous Niño events.”

It is still unclear why the world’s forests suffered so intensely last year. It may be a simple case of bad luck - a severe El Niño event that happened to coincide with a randomly occurring drought. Or it could be a sign of an emerging climate feedback loop, in which rising temperatures trigger the release of more carbon, which then causes the planet to heat up further.

“The obvious question is, are we on the cusp of a tipping point in natural ecosystems?” said Rob Jackson, a climate scientist at Stanford University’s Woods Institute for the Environment. “I don’t think one year’s rise answers that question, so I’m not saying we are, but that’s the question on my mind.”

Scientists are still collecting data on last year’s emissions and anxiously waiting to see what happens to the atmosphere in 2025. Yet the past year should be a warning about the toll of continued planet-warming pollution, Miller said. As long as people continue to emit greenhouse gases, temperatures will continue to rise, and the risk of unleashing carbon stored in the Earth creeps ever closer.

“We’re very fortunate that we have these chemical and biological systems that are absorbing a huge amount of what we emit, but there’s no guarantee,” Miller said. “[2024] is an example of that.”