Earths natural carbon sink nearly collapsed in 2024

For decades, the world’s forests, grasslands, and wetlands have quietly pulled roughly one-third of the carbon dioxide that people pour into the sky. That safety net is fraying, and the latest numbers suggest it could be tearing faster than most climate models imagined.

Preliminary satellite and atmospheric data show that in 2024 the terrestrial carbon sink offset almost zero of humanity’s emissions, while atmospheric CO₂ leaped by a record 3.73 parts per million, eclipsing the 2015-16 El Niño spike.

The finding shocked researchers who expected a rebound after the fire-scorched year of 2023.

Land carbon sink weakens faster

“Everybody I’ve talked with who is working on this subject is very surprised,” remarked Guido van der Werf of Wageningen University (WUR) in the Netherlands. His surprise reflects how quickly a system long seen as resilient has moved into the danger zone.

Under typical conditions terrestrial photosynthesis, measured as gross primary production, outweighs the carbon returned by decay and fire, locking away billions of tons of CO₂ each year.

The balance varies, but even during strong drought years the land sink has historically removed about 30 percent of fossil fuel emissions.

Plants absorb faster when CO₂ is abundant, a phenomenon sometimes called fertilization, and global restoration projects have added new leaf area across entire continents. Those gains lulled planners into assuming the sink would remain at least modestly helpful well past mid-century.

Climate models do predict a gradual slowdown, yet most place an abrupt collapse several decades away. The back-to-back weak years of 2023 and 2024 hint that tipping points may lurk closer than advertised.

Hot-wet conditions weaken carbon sink

The 2024 pattern differs from the fire-driven slump of the previous year. Grasslands and shrublands across the tropics turned extraordinarily green, but microbes and plant tissues respired even faster, releasing roughly 2.6 billion extra tons of CO₂, a deficit equal to the annual emissions of India.

“The large decline in land sink is caused by the respiration. It’s two quite different underlying mechanisms,” explained Guanyu Dong of Nanjing University, who led the new analysis. 

Heat speeds biochemical reactions, and moisture provides the dissolved oxygen microbes need to break down leaf litter. When both arrive together, ecosystem respiration roars, overpowering photosynthetic gains.

Such compound hot-pluvial extremes are becoming more frequent as global mean temperature now sits about 2.7°F above the late nineteenth-century level. Models that include these extremes project stronger positive feedbacks between the biosphere and climate.

Plants and microbes exhale carbon

Autotrophic respiration, the oxygen-using metabolism of living plant cells, rises naturally with bigger harvests of solar energy.

In 2024 satellite instruments recorded record greenness for several months, meaning plants were working overtime and burning more carbohydrates even before microbes joined the party.

Heterotrophic respiration, run by bacteria and fungi, jumped when swollen rivers spread organic debris across floodplains and when nighttime lows stayed unusually high, preventing cool-season recovery.

The combined effect raised total ecosystem respiration to its highest level since global monitoring began in the late 1990s.

Microbial respiration is invisible

Unlike forest fires, which release carbon in dramatic plumes, microbial respiration operates invisibly, so its influence often hides in the background noise of year-to-year variability.

The new study isolates the signature by comparing satellite estimates of productivity with atmospheric inversions of CO₂ fluxes.

Van der Werf noted that even small errors in decomposition rates can swing the balance sheet by billions of tons. That uncertainty highlights the need for more field-based measurements of soil carbon dynamics in warm, wet regions that have historically been understudied.

Past El Niño hit carbon sink differently

The infamous 1997-98 and 2015-16 El Niño events also slashed the land sink, but their main culprit was drought-induced fire in Southeast Asian peatlands.

Those episodes added visible smoke to the sky and left charred scars that satellites tracked for months.

By contrast, 2024 featured a relatively modest burned area, down about 35 percent from 2023’s Canadian-dominated fire season. That difference matters because a policy focused purely on fire prevention would have missed this year’s stealthy respiration surge.

Carbon sink with two switches

Scientists sometimes speak of the land sink as having two switches, one controlled by precipitation and another by temperature.

El Niño often flips one, but climate-change warming is now pushing the temperature switch toward the red zone even in neutral-Pacific years.

Low-latency carbon budget analyses suggest the tropics are becoming the swing region, modulating global CO₂ growth more than high-latitude forests once thought to dominate.

That insight shifts attention to grasslands, which cover almost 40 percent of Earth’s land and have received far less research funding than rainforests.

Preventing carbon sink collapse

A weakened land sink implies the carbon budget for keeping warming below 2°C is thinner than official tallies suggest. If ecosystems cease to remove that traditional 30 percent slice, governments must cut emissions roughly one-third faster to stay on track.

The 2024 global carbon budget estimated fossil-fuel emissions at 37.4 billion tons, up 0.8 percent from 2023. Subtract a missing sink and the atmospheric load rises as though humanity had burnt several extra countries’ worth of coal.

Ocean uptake once cushioned the blow, absorbing about 90 percent of excess heat, but recent work shows that buffer is eroding as surface layers warm and stratify. Longer marine heat waves also threaten to reduce the oceanic carbon sink by stressing kelp forests and phytoplankton.

Carbon-dioxide removal technologies, from afforestation to direct air capture, look more like necessities than insurance. Yet their deployment timelines are tight, and none can replace the vast free service that natural ecosystems once provided.

Research gaps and cautious optimism

“You need to be cautious interpreting even two years of growth as a persistent collapse,” said Scott Denning of Colorado State University. He urged patience before declaring the land sink permanently broken. 

Statistically, back-to-back anomalies can occur by chance, and some Pacific forecasts hint at a mild La Niña that could cool and dry parts of the tropics next year. If that happens, microbial respiration would likely slow, offering a natural experiment to test the new hypothesis.

Still, the episode highlights blind spots in Earth-system monitoring: scant flux towers in Africa, limited soil moisture sensors, and patchy airborne sampling. Closing those gaps will help scientists distinguish a wobble from a genuine tipping point.

Whether the 2024 surprise becomes a new normal depends on choices made during the remainder of this crucial decade. The next few annual updates will reveal whether the biosphere stages a comeback or leaves humanity to shoulder the full weight of its carbon bill.

The study is published in the journal Research Square.

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