How Rivers Regulate Global Carbon Cycle

Humans concerned about climate change are working to find ways of capturing excess carbon dioxide (CO2) from the atmosphere and sequestering it in the Earth. But Nature has its own methods for the removal and long-term storage of carbon, including the world’s river systems, which transport decaying organic material and eroded rock from land to the ocean.
While river transport of carbon to the ocean is not on a scale that will bail humans out of our CO2 problem, we don’t actually know how much carbon the world’s rivers routinely flush into the ocean – an important piece of the global carbon cycle.
But in a study published May 14 in the journal Nature, scientists from Woods Hole Oceanographic Institution (WHOI) calculated the first direct estimate of how much and in what form organic carbon is exported to the ocean by rivers. The estimate will help modelers predict how the carbon export from global rivers may shift as Earth’s climate changes.

“The world’s rivers act as Earth’s circulatory system, flushing carbon from land to the ocean and helping reduce the amount that returns to the atmosphere in the form of heat-trapping carbon dioxide,” said lead author and geochemist Valier Galy.  “Some of that carbon—‘new’ carbon—is from decomposed plant and soil material that is washed into the river and then out to sea. But some of it comes from carbon that has long been stored in the environment in the form of rocks— ‘old’ carbon—that have been eroded by weather and the force of the river.”
The scientists, who included Bernhard Peucker-Ehrenbrink, and Timothy Eglinton (now at ETH Zürich), amassed data on sediments flowing out of 43 river systems all over the world, which cumulatively account for 20 percent of the total sediments discharged by rivers. The representative rivers also encompassed a broad range of climates, vegetation, geological conditions, and levels of disturbance by people.
From these river sediment flow measurements, the research team calculated amounts of particles of carbon-containing plant and rock debris that each river exported.  They estimated that the world’s rivers annually transport 200 megatons (200 million tons) of carbon to the ocean. The total equals about .02 percent of the total mass of carbon in the atmosphere. That may not seem like a lot, but over 1000 to 10,000 years, it continues to add up to significant amounts of carbon (20 and 200 percent) extracted from the atmosphere.