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Nature study reveals rivers release millennial carbon, potentially altering the global climate cycle and accelerating warming.
On June 4, 2025, a study published in the scientific journal Nature revealed a profound shift in how science understands the global carbon cycle. Led by researchers from the University of Bristol, with participation from the University of Oxford and the UK Centre for Ecology and Hydrology, the work showed that 59 ± 17% of CO₂ emitted by rivers worldwide may come from ancient carbon, millennia-old or older, stored for long periods in soils, sediments, and geological formations.
This result challenges one of the most important premises of climate models: the idea that rivers primarily release recent carbon, linked to biomass produced by plants in recent decades. In practice, the data indicate that river systems also function as a leakage route for ancient pre-industrial carbon, transferring approximately 1.2 ± 0.3 petagrams of carbon per year to the atmosphere from terrestrial stocks that were considered more stable in the planet’s cycle.
Continue reading below to understand why this phenomenon concerns scientists, how this carbon is being released, and why it could alter the global climate balance.
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Study shows rivers are releasing millennial carbon that was trapped in soil and rocks
The central discovery of the study is based on radiocarbon analyses, which allow identifying the age of carbon present in river water. By compiling a global database with hundreds of measurements, researchers identified that a significant portion of the CO₂ emitted by river systems originates from very ancient carbon.
This carbon may have been stored for centuries, millennia, or even millions of years, in deep soils, rocks, and geological deposits.
When this material is mobilized by natural processes such as erosion, water runoff, and mineral dissolution, it is transported by rivers and released into the atmosphere in the form of carbon dioxide.
This means that part of the carbon that was outside the active climate cycle is returning to circulation, something that was not fully considered in previous climate models.
Volume released can reach 1.2 petagrams of carbon per year on a global scale
The study also quantified the impact of this phenomenon. The data indicate that rivers release approximately 1.2 ± 0.3 petagrams of carbon per year from ancient sources, a volume comparable to important fluxes in the global carbon cycle.
For scale, this value approaches the magnitude of the net carbon exchange between terrestrial vegetation and the atmosphere, one of the main natural processes that regulate climate.
This positions rivers as a much more active element in the climate system than previously imagined, not only transporting carbon but also releasing ancient stocks.
Discovery challenges climate models that underestimated river emissions
Before this study, most climate models considered that the CO₂ emitted by rivers primarily came from the decomposition of recent organic matter, such as leaves and plants.
This view implied that the released carbon was already part of the active cycle, being reabsorbed relatively quickly by vegetation.
The new result changes this understanding. It shows that a significant portion of the released carbon was outside this cycle and may represent a real loss of stored carbon on the planet.
This creates an important gap in current climate models, which may be underestimating the amount of carbon released into the atmosphere.
Rivers function as “invisible corridors” connecting deep soil and atmosphere
Researchers highlight that rivers act as a lateral transport system, connecting deep regions of the soil and Earth’s crust to the atmosphere.
Instead of carbon remaining stored, it can be mobilized and released through water flow. This process occurs continuously and globally, going unnoticed in most traditional climate analyses.
This is why scientists describe this phenomenon as an “invisible leak”, as it does not involve direct emissions like industries or wildfires, but still contributes to the increase of CO₂ in the atmosphere.
Origin of ancient carbon involves deep soils, rocks, and geological processes
The study identified two main sources of this ancient carbon. One is deep organic carbon, accumulated in the soil over thousands of years. The other is petrogenic carbon, originating from the decomposition of carbon-rich rocks.
These materials can be mobilized by processes such as:
- soil erosion,
- chemical weathering,
- groundwater runoff,
- fluvial transport.
When they reach rivers, these compounds can be transformed into CO₂ and released into the atmosphere.
Global warming may accelerate the release of this ancient carbon
Although the study does not point to a single cause, there are indications that environmental changes can intensify this process.
Increased temperature, changes in rainfall patterns, and changes in land use can accelerate the erosion and mobilization of ancient carbon. Furthermore, extreme events such as floods and droughts can increase the transport of material to rivers.
This creates the possibility of a feedback loop, where global warming contributes to releasing more carbon, which in turn intensifies warming.
Impact may alter the balance between carbon sources and sinks
Climate balance depends on the relationship between carbon sources and sinks. Forests, soils, and oceans act as absorption systems, while human activities and natural processes release CO₂.
The discovery that rivers release large quantities of ancient carbon suggests that this balance may be more fragile than previously thought.
If a significant portion of this carbon is not offset by absorption in other systems, the result could be a net increase in atmospheric concentrations.
Global phenomenon identified in rivers on all continents
The data used in the study covers rivers from different regions of the planet, including tropical, temperate, and cold areas.
This indicates that the phenomenon is not localized, but global. The presence of ancient carbon was observed in systems with different geological and environmental characteristics, reinforcing the idea that this process is part of the general functioning of the planet.
The main consequence of the discovery is the need to revise the role of rivers in the carbon cycle. They are no longer seen merely as transporters of recent carbon but are recognized as active sources of ancient carbon.
This may require adjustments in climate models, environmental policies, and mitigation strategies. What seemed like a hydrological detail is revealed to be a structural component of the global climate.
Now the direct question remains: if rivers around the world are releasing carbon that has been trapped for thousands of years, to what extent is the planet prepared to deal with this “invisible leak” that could alter the climate balance without most people even noticing?
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