Humans Adding ‘Fossil’ Carbon to Rivers

Though soil has often been considered a reliable long-term carbon sink, new research suggests that the effects of human land-use choices—from urbanization to agricultural intensification and deforestation—are reducing how much carbon is actually stored in the ground, says Professor David Butman, lead author on a paper just published in Nature Geoscience, “Increased mobilization of aged carbon to rivers by human disturbance.”

Professor David Butman

Professor David Butman

Professor Butman is a new faculty member with the School of Environmental and Forest Sciences (SEFS) who holds a joint appointment with Civil and Environmental Engineering. He began this research in 2011 as an offshoot of his doctoral work at Yale University involving 13 major river basins in the United States. Starting from a trend he discovered in that initial data, Butman and his co-authors expanded the scope with direct sampling of aquatic carbon at a number of field sites around the world, and also combed the literature for other relevant studies, tracking down researchers whenever possible to verify data. The resulting study range covers 84 degrees of latitude from the Arctic to tropical ecosystems, providing a comprehensive, global data set of radiocarbon ages of riverine dissolved organic carbon, coupled with spatial data on land cover, population and environmental variables.

From exploring this data, Butman and his co-authors were able to determine how carbon isotopes of organic matter in rivers can show the impact of land cover disturbances—specifically, the release of ‘old’ carbon into the modern carbon cycle, analogous to the burning of fossil fuels. Most dissolved organic carbon in rivers originates from young organic carbon from soils and vegetation, but the results of this study suggest that 3.2 to 8.9 percent of that dissolved organic carbon is actually aged carbon that human disturbances have churned back into the system.

What that means, says Butman, is that the release of carbon through land use and land cover change has been undercounted in previous estimates of anthropogenic carbon emissions. The full impact of this increase on the global carbon cycle is not entirely clear yet, but it definitely means we’re reducing how much carbon is being stored in the land purely through how we manipulate and change the physical surface of the planet.

Check out the full results and conclusions in the paper, and contact Professor Butman if you have any questions about this research or his other projects!

Photo © David Butman.