A new study of one of the most intense global warming events in Earth's history suggests that, under extreme warming, terrestrial ecosystems may amplify climate change, rather than mitigate it.

Researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences and their collaborators discovered that, during the Paleocene-Eocene Thermal Maximum (PETM), a rapid warming event that occurred about 56 million years ago, the terrestrial biosphere, (including forests, soils, and other land ecosystems) did not function as an effective carbon sink. As warming progressed and peaked, however, they found that carbon stored on land dropped significantly. This suggests that the land biosphere likely released large amounts of carbon into the atmosphere.

Their findings were published in Communications Earth & Environment on May 29.

The study addresses the critical question of whether, in the face of future global warming, land ecosystems will absorb more carbon dioxide (CO₂) and help slow climate change or turn into carbon sources that accelerate it.

To address this question, the researchers primarily investigated the vulnerability of terrestrial carbon reservoirs and their response to extreme warming during the PETM. They systematically quantified feedback mechanisms within the terrestrial carbon cycle during the PETM by integrating a dynamic global vegetation model (LPJ-LMfire) with a carbon isotope mass balance framework.

Under moderate warming, the model showed that although 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).

As warming intensified, a tipping point was crossed. Vegetation and soil carbon storage degraded sharply, leading to the catastrophic collapse of the land carbon pool, with losses of 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.

Their 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.