A new study has revealed for the first time that ancient carbon, stored in landscapes for thousands of years or more, can find its way back to the atmosphere as CO₂ released from the surfaces of rivers.
The findings, led by Bristol University and co-authored by Professor Ed Tipper of Cambridge Earth Sciences, mean plants and shallow soil layers are likely removing around one gigatonne more CO₂ each year from the atmosphere to counteract this, emphasising their pivotal and greater part in combating climate change.
The study is the cover story of the journal Nature. Lead author Dr Josh Dean at the University of Bristol, said: “The results took us by surprise because it turns out that old carbon stores are leaking out much more into the atmosphere then previous estimates suggested. The implications are potentially huge for our understanding of global carbon emissions. Plants and trees take up CO2 from the atmosphere and can then lock this carbon away in soils for thousands of years."
Rivers transport and release methane and carbon dioxide as part of the global carbon cycle. Until now, scientists believed the majority of this was a quick turnover derived from the recycling of recent plant growth – organic material broken down and carried into the river system in the past 70 years or so. This new study indicates the opposite, with more than half – some 60% – of emissions being attributed to long-term carbon stores accumulated over hundreds to thousands of years ago, or even longer.
“Our findings show some of this old carbon, as well as ancient carbon from rocks, is leaking sideways into rivers and making its way back to the atmosphere. We don’t yet know how humans are affecting this flow of ancient carbon, but we do know plants and trees must be taking up more carbon from the atmosphere today to account for this unrecognised release of old carbon,” said Dean.
The international research team studied more than 700 river reaches from 26 different countries across the world. They took detailed radiocarbon measurements of carbon dioxide and methane from the rivers. By comparing the levels of carbon-14 in the river samples with a standard reference for modern atmospheric CO2, the team was able to date the river carbon.
The global dataset was supplemented by field data collected by Prof Ed Tipper’s research group on the Mekong River system, who have been doing extensive fieldwork in this area over the past decade.
Ed Tipper's research group (Jotis Baronas, Katy Relph and Linshu Feng) pictured sampling river geochemistry along the Mekong.
“Rivers provide one of the major transfers of carbon from the land mass to the oceans. In doing so, there is a large release of carbon to the atmosphere. This study measured the age of the carbon lost from the rivers, demonstrating that around 60% of the carbon is older than 1000 years,” said Tipper.
Co-author Dr Gemma Coxon from the University of Bristol, said: “Rivers globally release about two gigatonnes of carbon each year, compared to human activity that results in between 10-15 gigatonnes of carbon emissions. These river emissions are significant at a global scale, and we’re showing that over half of these emissions may be coming from carbon stores we considered relatively stable. This means we need to re-evaluate these crucial parts of the global carbon cycle.”
Further building on these findings, the researchers plan to explore how the age of river carbon emissions varies across rivers the study was not able to capture, as well as investigating how the age of these emissions may have changed through time.
Reference: Dean, Joshua F., et al. "Old carbon routed from land to the atmosphere by global river systems." Nature 642.8066 (2025): 105-111.
Adapted from an original press release by the University of Bristol and Vrije Universiteit Amsterdam.
Feature image: The Khone Waterfall, Mekong River. Credit Ed Tipper.