Rivers release ancient carbon dioxide into the atmosphere – reshaping climate science

Rivers have long played a key role in the planet’s carbon cycle. They move and release gases like carbon dioxide (CO₂) and methane (CH₄), connecting land, air, and ocean systems. Until recently, scientists believed the carbon gases released from rivers mostly came from decaying plant material that had entered the ecosystem within the past few decades. That view is now changing.

A major international study led by scientists at the University of Bristol and published in Nature reveals something unexpected. It turns out that more than half of river CO₂ emissions come from carbon that is far older—stored for centuries, millennia, or even longer before finding its way back into the atmosphere.

This discovery points to a previously hidden pathway through which ancient carbon leaves long-term storage in soils, sediments, and rocks, only to reenter the atmosphere via rivers and streams. The amount of this older carbon now thought to be released each year—about 1.2 petagrams, or 1.2 billion metric tons—is roughly equal to the amount of CO₂ that land ecosystems remove from the atmosphere annually.

Dr. Josh Dean, Associate Professor in Biogeochemistry at the University of Bristol and the study’s lead author, was surprised by the results. “It turns out that old carbon stores are leaking out much more into the atmosphere than previous estimates suggested,” he said. “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.”

This shift in understanding forces scientists to rethink how stable these old carbon stores really are—and how they may respond to future environmental change.

To determine the age of carbon in rivers, the research team gathered over 1,100 published measurements and added 54 new ones. These samples came from more than 700 river sites across 26 countries, including rivers in North and South America, Europe, China, East Africa, Australia, and even Antarctica.

By analyzing the radiocarbon content, also known as F¹⁴C (fraction modern), researchers could estimate how long ago the carbon had been removed from the atmosphere. Carbon with higher F¹⁴C values is newer, while lower values suggest much older origins.

Dr. Gemma Coxon, co-author and hydrology expert at Bristol, explained the significance: “Rivers globally release about two gigatonnes of carbon each year. 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.”

In simpler terms, while part of the river carbon comes from recent photosynthesis and decomposition, a large portion is not part of the current ecosystem cycle. Instead, it is being reintroduced after being locked away for hundreds or even thousands of years.

Scientists break down the sources of river carbon into three main types:

Dr. Dean’s team used isotopic mass balance calculations to determine the contributions of each source. They found that 59% ± 17% of river CO₂ emissions come from millennial and petrogenic sources combined.

Professor Bob Hilton, a sedimentary geographer at the University of Oxford and co-author of the study, said, “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.”

These findings carry weighty implications for climate science. Carbon that has been long stored in the Earth’s crust or soils was previously thought to be mostly stable. Now it appears some of it is being flushed into rivers and re-emitted into the atmosphere.

This matters because climate models often treat soil carbon as a secure sink, assuming it stays locked away unless disturbed by major human activity. If older carbon is leaking back into the air on its own through rivers, then models could be underestimating the true amount of carbon being added to the atmosphere.

Additionally, this unexpected flux could mean that the Earth’s natural systems—especially plants and soils—are working harder than thought to balance rising CO₂ levels. Dr. Dean emphasized this when he noted that plants and trees must be absorbing an extra gigatonne of CO₂ each year to make up for this newly recognized release.

The researchers believe this new understanding will require adjustments to global carbon cycle budgets and climate prediction models. Knowing the source of river CO₂ helps refine projections and improve strategies for managing carbon.

With the release of the study, researchers are now turning their focus to regions that weren’t covered in the current dataset. They plan to explore how river carbon emissions vary by climate, landscape, and human activity. Another goal is to understand whether these ancient emissions have increased over time, particularly in response to deforestation, agriculture, or warming.

The work received support from the UKRI Natural Environment Research Council. Though the study already spans several continents and includes diverse river systems, much remains to be discovered about this global carbon leak.

In the meantime, the research has added an important piece to the puzzle of how carbon moves through the Earth’s systems. Rivers, long seen as transport routes for water and nutrients, are now recognized as critical players in returning ancient carbon to the sky.

As the planet continues to warm and landscapes change, tracking this hidden carbon flow will be essential to understanding how Earth’s carbon budget really works—and how to protect it.

Note: The article above provided above by The Brighter Side of News.

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