How black carbon causes global warming

09.07.2018 17:00

Black carbon ages in soils and rivers before being transported to the sea

Rita Ziegler communication
University of Zurich

In forest fires and the burning of fossil fuels, most of the carbon is released into the atmosphere as carbon dioxide. Just under a third remains as black carbon. UZH researchers are now showing that it can age over millennia on land and in rivers before it flows into the sea and is deposited in the sediments.

Most of the carbon that arises from the incomplete combustion of plant material in forest fires and from the burning of fossil fuels is released into the atmosphere as carbon dioxide within a short period of time. The increase in this greenhouse gas is one of the key factors behind global warming. However, almost a third of the organic carbon burned in forest fires remains as black carbon - especially in the form of charcoal. Black carbon is initially stored in soils and lakes, over time erodes on the river banks and is then transported into the sea. However, little is known about its role in the global carbon cycle, as there is little knowledge about rivers, stocks and dwell times of black carbon in the environment. So far, it has not been taken into account in simulations of global warming.

Worldwide transportation of black carbon in rivers

“In our study, we examined the transport of black carbon in river sediments on a global scale for the first time. A surprisingly large amount of black carbon (up to 32 percent) is exported into the sea in this way, ”says first author Alysha Coppola, postdoc at the Institute of Geography at the University of Zurich. The study covers several of the world's largest rivers such as the Amazon, the Congo, the Brahmaputra and the major rivers of the Arctic. The age and amount of the black carbon transported as particles were examined. The researchers show that the more sediments are transported by rivers to the coasts, the more black carbon migrates with it and is ultimately buried in the ocean sediments. This creates an important long-term sink for atmospheric carbon dioxide.

Black carbon can age in intermediate storage

In order to get an overview of the processes in the world's rivers, the UZH researchers worked with colleagues from ETH Zurich, the American Woods Hole Oceanographic Institution and Research Center, as part of the international research project “Global Rivers Observatory”. They found that it is primarily the erosion of sediment particles and thus of black carbon in river catchment areas that controls the path from land to sea. Unexpectedly, black carbon could be stored for thousands of years before being carried away by rivers. Until now, it was assumed that the black carbon remaining after a fire would be quickly removed by wind and water.

According to the authors, black carbon does not always come from the most recent forest fires, but can live up to 17,000 years, especially in the Arctic. “We have thus solved the mystery of why black carbon is always present in river water - regardless of the time of past forest fires. It can age in caches before it is exported to the sea, ”explains Alysha Coppola.

Alysha I. Coppola, et. al. Global scale evidence for the refractory nature of riverine black carbon. Nature Geoscience. July 9, 2018. DOI: 10.1038 / s41561-018-0159-8

Prof. Dr. Michael W.I. Schmidt
Geographic Institute
University of Zurich
Tel. +41 44 635 51 40
[email protected]

Dr. Alysha I. Coppola
Geographic Institute
University of Zurich
Tel. +41 44 635 52 28
[email protected]

Features of this press release:
Biology, geosciences, sea / climate, animals / land / forest, environment / ecology
Research results, research projects