One of Earth’s most vital carbon sinks is faltering. Can we save it?

Some climate change skeptics often argue that as carbon dioxide levels increase, plant life thrives. Their belief is that the use of fossil fuels will lead to a more verdant planet, better able to absorb carbon, making concerns over climate change exaggerated.

This perspective does hold a grain of truth, as land ecosystems like forests and grasslands have been flourishing for decades, effectively absorbing significant amounts of CO₂ from the atmosphere.

The revelation in the 1960s that land acts as a net carbon sink was unexpected for many ecologists, who presumed that CO₂ removed from the atmosphere would simply be matched by decomposition or combustion. “There shouldn’t be sinks. Everything that grows, dies,” comments Scott Denning, an atmospheric scientist from Colorado State University.

This surprising development has historically mitigated between 25% and 33% of human-generated CO₂ emissions each year, temporarily slowing the effects of climate change until more concrete actions can be taken. However, the fundamental flaw in the reasoning of climate deniers lies in the reality that this carbon sink is not sustainable indefinitely. With ecological disturbances fueled by climate change and the inherent limits of Earth, we can expect this sink to reach saturation within this century.

Alarmingly, evidence suggests that we may already be nearing this critical threshold. In both 2023 and 2024, analyses indicate that the land carbon sink has seemingly vanished. Researchers are now investigating how varied ecosystems contribute to this precarious balance, from Arctic tundras to tropical rainforests, in an effort to determine whether this is indeed the conclusion of Earth’s land carbon sink—and what strategies might help to sustain it.

The land carbon sink

Imagine this sink as a vast swimming pool. The water within it represents the approximately 4 trillion tons of carbon held in plants, animals, microbes, and decaying organic matter in the soil. As plants grow and absorb CO₂ through photosynthesis, carbon trickles into the pool; concurrently, carbon escapes as organisms die or decompose. As long as the inflow exceeds the outflow, this cycle functions as a carbon sink, reducing atmospheric CO₂ levels.

This concept is now widely accepted in climate discussions, although it sparked controversy at its inception. Much of this understanding stemmed from climate scientist Charles David Keeling’s meticulous measurements of atmospheric CO₂ at the Mauna Loa Observatory in Hawaii. The resultant Keeling Curve showed that while CO₂ levels in the atmosphere continually rose, they weren’t increasing as rapidly as they would if all emissions were trapped in the atmosphere. Researchers were left wondering: where was this carbon going?

Initially, it was presumed that the ocean’s surface waters, where CO₂ naturally dissolves, served as the primary sink for the “missing” CO₂. “The first carbon scientists were oceanographers,” explains David Schimel from NASA’s Jet Propulsion Laboratory. However, models quickly indicated that even the vast oceans alone couldn’t account for all that CO₂, suggesting land ecosystems were also contributing significantly.

Land ecologists, however, found this conclusion difficult to reconcile, as they believed that the high levels of deforestation, urban development, and increased farming would limit carbon absorption. In the 1970s, during a time when Joni Mitchell was lamenting over development, they firmly believed that the land was a significant source of CO₂.

The existence of a land carbon sink also contradicted the idea that ecosystems naturally balance, implying that growth and death reach equilibrium. They thought, “green stuff grows, green stuff dies, green stuff rots, the CO₂ doesn’t change,” according to Denning. “It was hard to visualize sustaining more growth than death and decay over extended periods.”

<pYet the data unequivocally demonstrated that this was, in fact, occurring. Researchers deployed ships and planes to enhance their CO₂ measurements alongside isotopic analyses to track carbon between sinks and sources. This data, congruent with the composition of ancient atmosphere trapped in Antarctic ice cores, led to improved Earth system models. By the 1980s, all evidence pointed towards a persistent carbon sink on land, absorbing about 25% of annual human CO₂ emissions—comparable to the ocean’s carbon sink. One study posited that without this terrestrial sink, global temperatures would be 0.3°C higher.

The factors responsible for the sink’s existence are still under debate, but four key contributors are widely recognized. The foremost is the higher CO₂ levels that enhance photosynthesis in plants. This effect is compounded by nutrient runoff, such as fertilizers entering ecosystems. Denning notes, “Humans are inadvertently fertilizing the biosphere extensively.” Another factor is the recovery of forests previously cut down or burned for agricultural use. For example, reforesting areas of Appalachia has sustained a robust carbon sink. Furthermore, the increasing temperatures in the Arctic have extended the growing season and led to unexpected greening of various regions.

Agricultural pollution

Nonetheless, the unyielding rules of ecology impose limits on how much these factors can amplify the sink. CO₂ can only stimulate growth if plants also have sufficient access to other necessary resources. In controlled greenhouse conditions, plants can flourish with increased CO₂, but field studies show that trees exposed to elevated CO₂ in natural ecosystems exhibit much more moderate growth due to stresses like limited water and shortages of nitrogen and phosphorus in the soil. Nutrient pollution might compensate for some of these deficiencies, but its effects are localized to areas of intensive agriculture where too much accumulates. Forests undergoing recovery act as a strong sink initially, but their capacity diminishes as they mature, especially if they are disturbed through logging or fire.

Indeed, the performance of the land carbon sink fluctuates for reasons that frequently remain unclear. Between 2007 and 2016, the sink strengthened to the point of sequestering roughly one-third of our CO₂ emissions annually. This growth was not completely understood, states Peter Reich, an ecologist from the University of Michigan, leading to differing opinions on the future viability of the land carbon sink.

Incorporating climate change into these discussions clarifies that the sink’s longevity is not assured. Most Earth system models suggest that while CO₂ increases photosynthesis, climate impacts worsen over time, undermining the sink. However, predicting when the sink will falter remains a challenge. “I hesitate to assign a specific date,” states Ana Bastos, a climatologist from the University of Leipzig.

This difficulty arises from the intricate dynamics of how climate change diminishes the planet’s capacity to absorb CO₂. For instance, excessive heat may trigger droughts and wildfires, while extreme rainfall may increase decomposition rates, with microbes thriving as a result. Consequently, many feedback loops can exacerbate this decline. Wildfires not only emit considerable carbon but also produce smoke that can hinder plant growth by blocking sunlight. In temperate forests, diminished snowfall can reduce growth rates by exposing roots to environmental extremes. Furthermore, in the Arctic, increased vegetation growth can be offset by melting permafrost, which releases additional CO₂ and methane as dormant microbes become active. Such thawing may even destabilize trees, causing them to tilt in phenomenon known as “drunken forests.”

Numerous factors can alter the balance from carbon sink to carbon source. For example, coastal flooding aggravated by rising sea levels can render trees toxic due to salt. Additionally, the decline of seed-dispersing animals like primates can hinder ecosystem regrowth.

Despite this, carbon sinks have surprisingly remained resilient against climate change—until recently. A study by Pierre Friedlingstein of the University of Exeter estimated that climate-related impacts have diminished the overall strength of ocean and land carbon sinks by about 15% since 1960 compared to a scenario without climate change. In a different study published in August, Schimel and his colleagues found that sink-facilitating factors contributed an additional 38 billion tons of carbon storage on land from 2001 to 2021, although climate stressors counterbalanced this by over 8 billion tons. “However, this analysis doesn’t account for the complexity that’s emerging now,” Schimel adds.

Extreme weather

In both 2023 and 2024—record-setting years for heat—preliminary findings suggest that climate-induced extremes nearly eradicated the land carbon sink. The sink was at least 50% below average during 2023, primarily due to widespread wildfires and sluggish growth in northern ecosystems, combined with extreme heat, droughts, and fires in the Amazon later that year.

Evidence indicates the sink may have been weaker in 2024, potentially hitting its lowest point in over ten years—although these figures are still sought. An analysis suggests that this reduction was more related to heightened temperatures and excessive moisture hastening decomposition than to aridity. In any case, this led to the most significant single-year increase in atmospheric CO₂ on record, even while fossil fuel emissions remained stable. “Very short-term extreme events can have a tremendous impact on the land sink,” asserts Bastos.

However, researchers advise caution against interpreting the land sink’s sharp downturn over just two volatile years as a lasting trend, especially considering that 2023 and 2024 experienced drastic temperatures due to a strong El Niño weather pattern. Richard Birdsey from the Woodwell Climate Research Center in Massachusetts highlights that a similar significant decline in the sink occurred during a prior El Niño event a decade ago, only for it to rebound. “These numbers come with considerable uncertainty. I would wait to see a few more years of data,” remarks Reich. Nevertheless, he acknowledges the concerning trends that could signal the land sink’s demise. “It’s certainly possible,” he cautions. “I worry that it is.”

A closer inspection of the land carbon sink’s elements reveals early signs of a prolonged decline. For example, Melissa Rose from the World Resources Institute and her team noted a steady drop in the global forest carbon sink since 2001, largely attributed to deforestation. During 2023 and 2024, wildfires exacerbated this decline, reducing the forest sink to its lowest level in two decades and propelling feedback loops that drive climate change. “We are witnessing these changes unfold before our eyes, and it’s happening more swiftly than anticipated,” states Rose.

In December 2024, researchers reported that the Arctic tundra, after millennia of acting as a carbon sink, had shifted to a carbon source due to fire activity and melting permafrost. Meanwhile, the Amazon rainforest, which has been precariously close to becoming a continuous carbon source for over ten years, continues to face alarming threats. The ocean carbon sink has also seen a noticeable decrease since 2021, triggered by unprecedented marine heatwaves, albeit the shifts on land are more impactful.

The implications of the land carbon sink’s potential disappearance for climate initiatives would be enormous. Many nations rely on the ongoing efficiency of their carbon sinks to fulfill emissions commitments under the Paris Agreement, which aims to cap long-term global warming at 1.5°C. If these sinks vanish sooner than anticipated, nations will need to implement more drastic emission reductions elsewhere. For example, Europe is currently grappling with a sharp decline in its forest carbon sink—a situation exacerbated by overharvesting following the Russian invasion of Ukraine, along with drought, heat, and pestilence—leading the bloc significantly off course from meeting its 2030 emission goals.

Fortunately, multiple effective strategies exist to either preserve the carbon sink or decelerate its decline, despite the accelerating pace of climate change. Constantin Zohner, an ecologist specializing in climate change at ETH Zurich, emphasizes that prioritizing the protection, restoration, and management of ecosystems is essential. If current forests are allowed to grow without disturbance, models indicate a potential sequestration of 228 billion tons of carbon as they reach maturity over decades—an equivalent of roughly one-third of all carbon emissions accumulated so far. An additional 87 billion tons could be captured by restoring forests in areas where they once thrived, excluding urban regions and land currently dedicated to agriculture.

Moreover, improved ecosystem management could potentially enhance the land carbon sink by several billion tons annually, according to estimates from Yichun Xie and colleagues at Eastern Michigan University. This could be achieved through strategies such as prescribed burns to minimize major wildfires, implementing sustainable farming practices like cover cropping and rotational grazing, and adopting more prudent timber harvesting methods. “There are substantial opportunities, but these measures need to be embedded in our economic and policy frameworks,” asserts Reich.

The forthcoming years may prove critical in determining the fate of these complex carbon exchanges on our planet. Should we strive to retain the unexpected benefit of a land carbon sink, we must stop “removing the allies that we have,” says Zohner—and endeavor to extend the cycle of growth over death for as long as possible.