Thawing permafrost may trigger overlooked carbon sink in rivers

A new study published in Nature study shows that rock weathering increasingly counteracts river CO₂ emissions as permafrost degrades. The study has been carried out by a collaborative team of researchers from Umeå University, Sweden, and East China Normal University.

Thawing permafrost is often viewed as a growing source of greenhouse gases as climate warming releases ancient carbon stored in frozen soils. But a new study published in the journal Nature reveals a more complex picture. As permafrost thaws, rivers may also develop an overlooked capacity to remove carbon dioxide (CO₂) through intensified rock weathering. Researchers found that warming and permafrost degradation expose reactive minerals and increase water–rock interactions, accelerating chemical weathering processes that consume CO₂. In some river catchments, this geological carbon uptake partially or even fully offset river CO₂ emissions.

The international research team investigated 50 rivers across the Qinghai–Tibet Plateau, Earth’s largest high-altitude cryosphere outside the polar regions, to understand how thawing permafrost reshapes carbon cycling. By combining measurements of river CO₂ emissions, dissolved carbon, isotopic tracers and geochemical modelling, the researchers found evidence that thawing landscapes intensify chemical weathering, transferring carbon into dissolved inorganic forms while consuming atmospheric CO₂.

Carbon uptake can even exceed emissions

“We found that river CO₂ emissions decline while carbon uptake through rock weathering increases as permafrost cover decreases,” said Liwei Zhang, biogeochemist at East China Normal University. “In some catchments where permafrost has become patchier, weathering-driven carbon uptake was large enough to offset or even exceed river CO₂ emissions.”

Across the study region, the team estimated that carbon uptake from rock weathering offsets roughly 35 percent of river CO₂ emissions on average. In regions with continuous permafrost, the offset was relatively small. However, in landscapes with discontinuous or isolated permafrost, weathering-driven carbon uptake sometimes exceeded 100 percent of river CO₂ emissions, suggesting that geological carbon uptake can rival biological carbon release.

Biological and geological carbon cycles linked

The findings challenge a simplified view of thawing permafrost as only a carbon source. As frozen soils thaw, rivers receive large inputs of ancient organic carbon that microbes convert into greenhouse gases released to the atmosphere. The new study suggests that geological processes operating alongside biological ones may partly counterbalance these emissions.

Still, the researchers caution that rock weathering is not a simple or permanent climate solution. Carbon cycling in thawing landscapes remains complex, and some weathering reactions can also release CO₂ depending on mineral composition. The study instead highlights a mechanism that remains poorly represented in many climate and carbon-cycle models.

“Our findings show that biological and geological carbon cycles are tightly linked,” says Jan Karlsson, professor at Department of Ecology, Environment and Geoscience at Umeå University. “To understand whether thawing permafrost ultimately amplifies or dampens climate warming, we need to consider both the carbon released from ancient soils and the carbon consumed through rock weathering.”

The researchers argue that future climate assessments should move beyond a sole focus on biological driven carbon emissions and incorporate geological carbon sources and sinks that emerge as frozen landscapes thaw.