As rivers carry organic matter from the land to the oceans, bacteria turn it into greenhouse gases. While previous analyses had quantified emissions of these gases from rivers in Brazil, Europe and North America, the study in the journal Nature Geoscience last week (20 July) largely fills the African gap.
It used measurements of dissolved carbon dioxide (CO2), methane and nitrous oxide taken from 12 rivers between 2006 and 2014 to estimate carbon emissions from inland waters across Sub-Saharan Africa.
“There needs to be much more attention [paid] to preserving river basins and wetlands as important providers of fresh water, fisheries, biodiversity, ecotourism.”
Christopher Martius, Center for International Forestry Research in Indonesia
The study found that river channels alone emit about 400 million tonnes of carbon a year, which is equivalent to two-thirds of the net amount previously reported as being captured and stored on the land in Africa. If emissions from the wetlands of the Congo River are also included, these emissions rise to 900 million tonnes of carbon a year — equivalent to about a quarter of the global carbon reservoir for both land and the oceans, it says.
“These results can be considered as the reference number for natural emissions for African rivers,” says Frédéric Guérin, one of the study’s coauthors, from France’s Institute of Research for Development.
He adds that rapid population growth, deforestation for agriculture and a rising demand for energy will probably cause a surge in emissions from inland waters in coming years. For instance, he says, the rise of the volume of sewage and a growing use of hydroelectric reservoirs is known to increase emissions from drowned organic matter. This study will be a baseline to measure human-caused changes.
Guérin, together with colleagues from the University of Liège and the Catholic University of Leuven in Belgium, as well as Kenya’s Kenyatta University, sampled 12 rivers spread over Africa from their sources to their mouths. They covered a broad range of climates and vegetation, and included the Congo River, the Tana River in Kenya and the Rianila River in Madagascar.
Christopher Martius, a scientist at the Center for International Forestry Research in Indonesia, says the contribution of African rivers to greenhouse gas emissions had previously been overlooked and commends the researchers for gathering a large dataset. “There needs to be much more attention [paid] to preserving river basins and wetlands as important providers of fresh water, fisheries, biodiversity, ecotourism” and also to their contribution to greenhouse gas emissions, Martius says.
Water management in Africa and other parts of the world seeks to maximise water productivity for drinking, agriculture and industrial use, and environmental concerns are often an afterthought, Martius tells SciDev.Net. Because rivers often flow through several countries, management efforts must be aligned across borders, he says.