Understanding the carbon cycle

Rivers release ancient carbon dioxide into the atmosphere, uncovering a greater role for plants and soil in the carbon cycle

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Published: June 4, 2025
This Article was written by: Katie Hill - My Green Pod
carbon, emissions, plants, rivers, soil, trees

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Main image: A scenic spot on the Lymington River in the New Forest National Park

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 scientists at the University of Bristol and the cover story of the journal Nature, 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 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.

‘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.’

DR JOSH DEAN
Lead author, Associate Professor in Biogeochemistry and UKRI Future Leaders Fellow at the University of Bristol

Young and old carbon stores

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.

‘We discovered that around half of the emissions are young, while the other half are much older, released from deep soil layers and rock weathering that were formed thousands and even millions of years ago.’

PROF BOB HILTON
Co-author, Professor of Sedimentary Geography at the University of Oxford

Emissions from rivers

The international research team, led by scientists at the University of Bristol, University of Oxford and the UK Centre for Ecology and Hydrology, 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.

‘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.’

DR GEMMA COXON
Co-author, Associate Professor in Hydrology and UKRI Future Leaders Fellow at the University of Bristol

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.

The research was supported by funding from UK Research and Innovation (UKRI) Natural Environment Research Council (NERC).