KIT Researchers Show How Climate Change and Land Use Turn Rivers into Greenhouse Gas Sources

Rivers worldwide are under severe stress: They are warming, losing oxygen and as a result emitting increasing amounts of greenhouse gases. Researchers at Karlsruhe Institute of Technology (KIT) have now quantified these global trends over a period of more than two decades. Their results show that rising temperatures and anthropogenic land use are fundamentally transforming river systems, with serious consequences for the climate. The findings have been published in Global Change Biology. (DOI: 10.1111/gcb.70828)

Rivers are habitats, sources of water, and shapers of entire cultural landscapes. Accordingly, the local impacts are severe when agriculture and industry place pressure on river systems. “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.

Machine Learning Complements Missing Data

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.

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.”

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