A study by researchers at the University of Oxford in the United Kingdom challenges the notion that natural rock erosion solely acts as a CO2 sink. It shows that CO2 can also be released by rocks, potentially matching the scale of emissions from volcanoes. The findings were published in Nature and could have meaningful implications for climate policy in North America and beyond.
Rocks hold vast carbon stores that trace back to ancient plant and animal matter millions of years old. This means the geological carbon cycle helps regulate the Earth’s temperature, acting like a thermostat over geologic timescales.
During chemical weathering, rainwater acidity interacts with minerals in rocks, absorbing CO2. This natural process offsets ongoing CO2 emissions from volcanoes and keeps life supported on a habitable Earth for billions of years.
For the first time, evidence shows that rock-to-atmosphere CO2 release is a natural process
New research reveals that CO2 release from rocks into the atmosphere is a natural occurrence. The study finds that this rock-derived CO2 emission can be as significant as volcanic CO2 emissions globally. Because this process is newly established, many climate models do not yet include it in simulations of the natural carbon cycle.
Weathered rocks in France provide a visible example of CO2 emissions into the atmosphere
Rocks formed in ancient seabeds, where organic matter was buried, are exposed when mountain ranges such as the Himalayas or the Andes push the crust upward. This brings organic carbon into contact with air and water, triggering reactions that release CO2. In this view, eroded rocks may contribute CO2 rather than trapping it as once assumed.
Measuring this CO2 release has been challenging because sampling all the world’s rivers is impractical. The new study used rhenium, a trace element released when organic carbon in rocks reacts with oxygen, as a proxy to quantify CO2 emissions in water samples. Even so, scaling this approach to a global river network remains a major task.
To build a global picture, the researchers first mapped surface rock carbon content and then identified regions most exposed to erosion, particularly tall, steep mountain belts.
Rivers and landscapes across Canada were included in the analysis to build a broader understanding, though the exact global river system measurements pose logistical hurdles.
Mackenzie River in Canada provides a real-world example of these processes
Lead investigator Jesse Zondervan of Oxford’s Department of Earth Sciences explained that the team faced the challenge of integrating diverse global maps with river data while accounting for uncertainties. Data were processed on a high-performance computer to simulate how physical, chemical, and hydrological processes interact—producing a robust estimate of the carbon dioxide released as rocks erode and expose old carbon to air and water.
The findings suggest that rock-derived CO2 emissions can rival natural weathering outputs of silicate minerals, challenging traditional views on how weathering drives the carbon cycle. Several regions emerged as hotspots for CO2 release, especially mountainous areas exposing sedimentary rocks at high elevations.
Regions with strong CO2 release from erosion
Eastern Himalayas, the Rocky Mountains, and the Andes were identified as key areas where CO2 is emitted during rock erosion. The study estimates global CO2 emissions from this process at roughly 68 megatons of carbon per year, a figure that helps balance understandings of natural carbon fluxes against human-caused emissions.
Robert Hilton, a co-leader of the ROC-CO2 project, notes that these natural emissions are about 100 times smaller than current human CO2 emissions from fossil fuels, and resemble the amounts produced by volcanic activity. This places rock erosion as a meaningful, though comparatively modest, element of the Earth’s natural carbon cycle.
Past climates likely saw different dynamics. When many carbon-rich rocks were exposed during mountain-building periods, CO2 releases could have shaped global climate. Ongoing studies aim to determine how erosion responds to human activity and warming temperatures, and whether natural CO2 emissions will rise over the next century. The team emphasizes that forecasts depend on the methods used and data quality, with uncertainty remaining about future trajectories.
Source reference: Nature article on global rock-derived CO2 emissions. Marked citation: Nature. 2023.