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.

Lead author Dr Josh Dean, Associate Professor in Biogeochemistry and UKRI Future Leaders Fellow 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.

“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.”

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.

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.

Co-author Prof Bob Hilton, Professor of Sedimentary Geography at the University of Oxford, explained: “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.”

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

Co-author Dr Gemma Coxon, Associate Professor in Hydrology and UKRI Future Leaders Fellow at 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. 

Paper

‘Old carbon routed from land to the atmosphere by global river systems’ by Joshua F. Dean et al. in Nature [open access]