A groundbreaking new study published in Nature has revealed that rivers around the world are quietly releasing large amounts of ancient carbon into the atmosphere — carbon that had been locked away in soil, sediment, and even rocks for centuries to millennia. This discovery could change how scientists understand the global carbon cycle and how we account for the sources of increasing carbon dioxide (CO₂) levels in the atmosphere.
For a long time, scientists believed that the carbon dioxide (CO₂) released from rivers mostly came from recently decayed plants and organic material — in other words, part of the natural, short-term carbon cycle. But this new study, led by Dr. Joshua Dean from the University of Bristol and Dr. Robert Hilton from the University of Oxford, shows that nearly 60% of the carbon released by rivers is actually “old” carbon — thousands of years old in many cases.
Using radiocarbon analysis, which can determine the age of carbon molecules, the research team examined over 1,100 samples of river water from around the world. These samples included dissolved carbon dioxide (CO₂), methane (CH₄), and inorganic carbon. Their analysis showed that much of the carbon being emitted by rivers today is not just recycled plant matter but comes from deeper soil layers and even from the breakdown of ancient rocks.
In total, rivers release about 2 billion tons of carbon into the air each year — a number almost equal to the amount of carbon absorbed by land ecosystems through plant growth. This means that while forests and soils are pulling carbon out of the atmosphere, rivers are quietly putting much of it back in — and now we know a lot of it is coming from very old carbon stores.
In short, the research reveals a hidden leak in Earth’s carbon system.
So why does this matter?
The idea that rivers release long-sequestered carbon has direct and timely relevance to dam removal projects like the most recent one on the Klamath River, where three large dams have been dismantled. As these reservoirs drained, sediments and underlying bedrock that have been submerged — and relatively undisturbed — for decades have now been exposed. This exposure can unlock a significant flux of “old” carbon, much like the process described in the Nature study.
When dams are removed, the sediments that built up behind them over decades — often rich in organic matter and fine particulates — become exposed to air and flowing water for the first time in years. These sediments can include millennial-aged soil carbon and even petrogenic (rock-derived) carbon, which had previously been locked away from the active carbon cycle.
As erosion washes these materials into the newly free-flowing river, carbon once safely stored is now vulnerable to decomposition and oxidation, potentially releasing CO₂ and methane (CH₄) into the atmosphere.
With restored river flow comes more energy to move and disturb sediments. Increased turbulence, oxygen exposure, and microbial activity in these reshaped channels can stimulate the breakdown of old organic material, especially as the river carves through freshly exposed floodplain soils and reservoir beds.
This dynamic matches the mechanism described in the Nature paper: hydrological routing (the way water moves across land) is key in liberating old carbon and transporting it into river systems — and ultimately, into the atmosphere.
The Klamath River basin is geologically diverse, with volcanic, sedimentary, and metamorphic formations. Some of these rocks and soils contain fossil organic carbon or ancient carbonates. As dam removal alters erosion patterns and exposes deeper soil and rock layers, the weathering of these materials could contribute to the release of even older, petrogenic carbon, which is generally resistant but can oxidize under certain conditions.
The study in Nature emphasizes that riverine carbon emissions are often excluded from carbon budgets. If dam removal unintentionally boosts emissions of old carbon, restoration projects need to account for this potential feedback — especially when they’re promoted as climate-beneficial due to improved ecosystem function or fish passage.
In summary, dam removal may unintentionally mobilize long-sequestered carbon by exposing sediments and ancient rocks to erosion, oxygen, and microbial breakdown. This links directly to the findings of the Nature study, which shows that rivers are significant sources of old carbon emissions — a process likely intensified when major hydrological changes like dam removals occur.
This connection highlights the need to consider potential sequestered carbon releases into the atmosphere before dam removals are approved and weigh this against any other potential ecological benefits.
