Rivers are releasing a massive amount of carbon dioxide into the air

A new study has revealed that carbon stored deep in Earth’s landscapes for thousands – or even millions – of years is making its way back into the atmosphere as carbon dioxide released from river surfaces.

This discovery changes our understanding of the global carbon cycle and suggests that plants and soils are playing an even bigger role in removing carbon from the air than previously believed.

Led by scientists at the University of Bristol, the international research team found that rivers across the world are releasing carbon not just from recent organic material but also from much older sources. This ancient carbon comes from deep soil layers and even weathered rock.

“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,” said lead author Josh Dean, an associate professor of biogeochemistry at Bristol.

Deep carbon emerges unexpectedly

Previously, scientists assumed that the gases released from rivers – methane and carbon dioxide – mostly came from recent plant matter that had decayed and washed into the water within the last 70 years.

But this study found something different. More than half – around 60% – of the CO2 released by rivers actually comes from carbon that has been buried underground for centuries to millennia.

“The implications are potentially huge for our understanding of global carbon emissions,” Dean said. “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.”

Although it’s unclear how human activity influences the release of ancient carbon, researchers found that plants and trees are currently absorbing more atmospheric carbon to offset this unrecognized release.

A global river investigation

The research team examined over 700 river reaches from 26 countries across every continent except Antarctica.

Using advanced radiocarbon dating techniques, they measured the age of carbon dioxide and methane released from rivers.

By analyzing the amount of carbon-14, an isotope that helps scientists determine how old a sample is, they were able to distinguish between “young” carbon and ancient sources.

“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,” said co-author Bob Hilton, a professor of sedimentary geography at the University of Oxford.

Hidden carbon changes climate math

Rivers are often overlooked in climate discussions, but they play a critical role in the carbon cycle.

Globally, rivers release about two gigatons of carbon each year. In comparison, human activity emits around 10 to 15 gigatons annually.

This makes river emissions a significant part of the natural carbon flow – one that scientists are now realizing is more dynamic than previously believed.

“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,” said co-author Gemma Coxon, an associate professor of hydrology at Bristol. “This means we need to re-evaluate these crucial parts of the global carbon cycle.”

Old carbon emissions need review

The discovery opens up several new questions. If ancient carbon is routinely leaking back into the atmosphere through rivers, how might that flow change over time – especially as the climate warms and human activity reshapes the land?

The team plans to continue its research by investigating how the age of river carbon emissions varies in areas not captured in the current study, and whether those patterns have shifted historically.

By reshaping our understanding of where carbon emissions originate, this study also underscores the vital role of natural ecosystems in counterbalancing those emissions.

According to the researchers, plants and soils may already be compensating for this unexpected source by pulling roughly one extra gigaton of CO2 out of the atmosphere every year.

The findings highlight the need for more accurate carbon budgets as policymakers and scientists work to address climate change.

Understanding the hidden sources and sinks of carbon in the environment isn’t just a scientific activity – it’s essential for building effective strategies to reduce greenhouse gases and protect our planet’s future.

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