Rivers worldwide emitting increasing amounts of greenhouse gases
Rivers, major sources of water worldwide, are under severe stress and getting transformed due to rising temperatures and anthropogenic land use. This trend has serious consequences for the climate as rivers may be losing oxygen up to 2.5 times faster than lakes and oceans globally, a new research study has warned.
Rivers are emitting increasing amounts of greenhouse gases but the combined effects of these pressures on river greenhouse gas (GHG) supersaturation and deoxygenation has remained poorly understood so far. Now, researchers at Karlsruhe Institute of Technology (KIT), Germany have quantified these global trends over a period of more than two decades.
The research study modeled past global trends (2002–2022) in river GHG saturation, dissolved oxygen (DO) levels, water temperature, and eight other water quality parameters using machine learning models powered by satellite observations.
Rivers are habitats and shapers of entire cultural landscapes. Accordingly, the local impacts are severe when agriculture and industry place pressure on river systems, the study found.
“Rivers also play a key role in the global climate system”, said Dr Ralf Kiese of the Institute of Meteorology and Climate Research—Atmospheric Environmental Research (IMKIFU) at KIT’s Campus Alpin in Garmisch-Partenkirchen. “We are increasingly observing that rivers are becoming a significant source of greenhouse gases.”
This is mainly due to biogeochemical decomposition processes involving microorganisms: Organic carbon and nutrients entering rivers from farming or wastewater are converted into carbon dioxide, nitrous oxide, and methane—greenhouse gases with an adverse effect on the atmosphere.
For a first-time global quantification of these trends, the researchers combined measurement data with satellite maps and machine learning. Their study is based on water parameter measurements from more than 1,000 river monitoring sites. They linked these measurements with globally available satellite information on vegetation, radiation, and topography.
Based on this combined data, computations using machine learning models revealed how these environmental factors affect water temperatures, oxygen levels, and increasing greenhouse gas concentrations. The researchers then applied the resulting relationship data to more than 5,000 additional catchments worldwide to reconstruct, for the first time, consistent time series from 2002 to 2022, even for regions where no measurement data was available, according to an official statement.
The evaluations revealed definite global trends: rivers are warming, losing oxygen, and becoming increasingly saturated with greenhouse gases. “On average, the oxygen content is decreasing by 0.058 milligrams per liter and decade, much faster than in lakes and oceans. At the same time, the emissions of carbon dioxide, methane, and nitrous oxide are rising,” said Dr Ricky Mwanake of IMKIFU, who was mainly responsible for the computations.
“Overall, we estimate that the additional anthropogenic emissions from rivers during the study period from 2002 to 2022 amounted to approximately 1.5 billion metric tons of CO₂ equivalent. These additional emissions weren’t accounted for in the existing global greenhouse gas budgets.”
Rapid changes are particularly evident in regions with expanding agricultural land use and urbanization, where rising water temperatures coincide with increased inputs of nutrients and organic carbon. This accelerates microbial processes and creates hotspots in which the adverse factors reinforce each other, leading to the accumulation of greenhouse gases in the water. As a result, rivers can become major emitters of greenhouse gases.
“If we succeed in protecting rivers better by reducing inputs of harmful substances, this effect can be reversed,” said Mwanake. “This means that protecting rivers is nothing less than active climate protection.”
