Ancient Carbon Emissions from Rivers Challenge Climate Change Models

A recent international study, published in the prestigious journal *Nature* on June 15, 2025, has significantly altered the understanding of carbon emissions and their sources within the global carbon cycle. The research, led by Dr. Josh Dean, a climate scientist at the University of Bristol, reveals that rivers serve not only as channels for the rapid cycling of carbon from contemporary sources but also as major conduits for the release of ancient carbon stores that have been sequestered for millennia.

The study indicates that more than half of the carbon dioxide (CO₂) and methane (CH₄) emissions from rivers globally originate from long-term carbon reservoirs—such as deep soils and weathered rocks—rather than from the decomposition of recent plant material, a perspective that has dominated climate science for decades. Specifically, the research found that approximately 60% of river carbon emissions are traced back to ancient carbon sources, some of which date back thousands or even millions of years. This revelation necessitates a reevaluation of existing climate models and carbon budgets.

To reach these conclusions, the research team compiled a comprehensive global database of over 1,100 radiocarbon measurements taken from more than 700 river sites across 26 countries. By analyzing the carbon-14 isotopic ratios in dissolved inorganic carbon, CO₂, and CH₄ in river waters and comparing these to atmospheric levels, the researchers could effectively determine the age of the carbon being emitted. Their calculations revealed that, on average, roughly 59% (±17%) of river CO₂ emissions stem from ancient carbon sources, while only around 41% is derived from recently fixed carbon.

The findings of this study are significant: it is estimated that rivers emit approximately 2 gigatons (2 billion metric tons) of carbon each year as CO₂ and CH₄, with about 1.2 gigatons sourced from these ancient reservoirs. This volume is comparable to the total net carbon uptake by all of the world's terrestrial ecosystems annually, indicating that the emissions from rivers may be a critical factor in understanding global carbon dynamics.

Dr. Dean emphasized the implications of the study, stating, "The results took us by surprise because it turns out that old carbon stores are leaking out much more into the atmosphere than previous estimates suggested. The implications are potentially huge for our understanding of global carbon emissions."

The study's findings also prompt a reconsideration of the role that terrestrial ecosystems play in the global carbon balance. If rivers are indeed acting as significant pathways for the release of ancient carbon into the atmosphere, then it follows that land ecosystems must be absorbing at least one gigaton more CO₂ annually than previously estimated to maintain equilibrium in atmospheric carbon levels. This indicates that the carbon sink capacity of forests and soils, which is vital for mitigating climate change, may be more critical and potentially more susceptible to disruption than previously thought.

Moreover, the study highlights the potential challenges posed by human activities, such as land-use changes, deforestation, and agriculture, which could exacerbate the mobilization of ancient carbon from deep soils and geological formations, further increasing the emissions of ancient carbon into the atmosphere.

Moving forward, the research team intends to investigate how the age of riverine carbon varies across different environments and how these dynamics may change over time. The current study marks a significant shift in the understanding of Earth's carbon balance, unveiling a substantial leak of ancient carbon from terrestrial sources to the atmosphere, thereby underscoring the urgent need for revised climate science and policy considerations.

This study generates critical questions for future research, particularly regarding the interplay between ancient carbon emissions and the broader implications for climate policy, ecosystem management, and global carbon accounting. As researchers continue to explore these new dynamics, the findings will likely inform climate strategies aimed at mitigating the impacts of climate change globally.