In the face of future global warming, a critical question remains unresolved: will land ecosystems absorb more carbon dioxide (CO2) and help slow climate change, or will they turn into carbon sources that accelerate it?
In a study published in Communications Earth & Environment, researchers from Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences and their collaborators revealed that during one of the most intense global warming events in Earth’s history, the terrestrial biosphere(forests, soils, and other land ecosystems) likely amplified the warming, rather than mitigating it.
The researchers primarily investigated the vulnerability of terrestrial carbon reservoirs and their response to extreme warming during the Paleocene-Eocene Thermal Maximum (PETM) which occurred 56 million years ago. By integrating a dynamic global vegetation model (LPJ-LMfire) with a carbon isotope mass balance framework, the study systematically quantified feedback mechanisms within the terrestrial carbon cycle during the PETM.
During the PETM, land ecosystems did not act as a strong carbon sink. Instead, as warming progressed and peaked, carbon stored on land dropped significantly, meaning the land biosphere likely released large amounts of carbon into the atmosphere.
Under moderate warming, the model showed that while vegetation absorbed more carbon, this gain could not offset the carbon released from soils. There was a net release of about 66 petagrams of carbon (Pg C).
Under stronger warming, a tipping point was crossed. Both vegetation and soil carbon storage degraded sharply, leading to a catastrophic collapse of the land carbon pool, with losses up to roughly 900 Pg C.
Using isotopic mass-balance modeling, the researchers tested various carbon release scenarios and found that the PETM was likely driven by multiple, interacting carbon sources, rather than a single trigger.
The findings carry an urgent message for humanity. Under extreme warming, terrestrial ecosystems cannot be relied upon to provide a sustained carbon sink. Once global temperatures cross a critical threshold, land carbon stocks may rapidly destabilize and release vast amounts of CO₂ back into the atmosphere, triggering a dangerous climate–carbon cycle positive feedback.
“This study provides essential quantitative evidence for assessing future terrestrial carbon cycle feedbacks and climate risks. It highlights that relying on land ecosystems to absorb our emissions is a risky bet in a rapidly warming world,” said LI Shufeng of XTBG.
Published: 29 May 2026
