River Restoration as a Tool for Climate Mitigation by Reducing River Methane Emissions

Gerard Rocher-Ros 
Swedish University of Agricultural Sciences and Umeå University, Sweden

Winning article: Global methane emissions from rivers and streams (Nature, 2023)

“Rivers emit methane to the atmosphere, by restoring rivers we can contribute to climate mitigation beyond improving water quality”

River networks are the veins and arteries of the planet, acting as recipients of large amounts of matter and energy that are transported to the ocean. As a result, rivers are thriving ecosystems, often with ample resources, harbouring some of the most biodiverse communities on Earth. Rivers have also been the backbone of human civilization, providing means of transport, fertile land, and water resources. Yet, the collective activities of humanity have caused large impacts on the health of river ecosystems, which currently are among the most degraded ecosystems. We have released waste into rivers, extracted large fractions of water, constructed artificial channels and ditches, built dams and reservoirs, and over-exploited fish populations. As vital as rivers are for the planet and our civilization, their health status is now critical.

Rivers are also massive biogeochemical reactors, where organic matter and nutrients are consumed to produce carbon dioxide and methane, two powerful greenhouse gases. Despite their small size, rivers are disproportionately large sources of carbon dioxide and methane to the atmosphere and thus contribute to global warming. Our research is focused on unravelling the magnitude and patterns of global methane emissions from rivers. We show that methane in rivers largely comes from the lands they drain, highlighting that land cover and land use by humans potentially exerts important controls on river methane emissions. Furthermore, our research also shows that highly impacted rivers by humans - for example impounded rivers, canals, or rivers affected by sewage - have even higher methane emissions to the atmosphere.

Restoring rivers is a major priority to restore and maintain our civilization within planetary boundaries. At the crossroads between land and sea, rivers are key elements in the planetary boundaries of biogeochemical flows, freshwater change, biosphere integrity, land-system change and climate change, all of which have already been transgressed and under urgent need to recover. River restoration can consist of a wide range of measures, for example removing unnecessary dams, reducing inputs of nutrients from land or restoring river habitats and adjacent vegetation. River restoration is currently a focus point of environmental legislation, for example the EU Biodiversity strategy aims to restore up to 25 000 km of European rivers before 2030, a massive and expensive undertaking (https://environment.ec.europa.eu/strategy/biodiversity-strategy-2030_en). A currently overlooked effect of river restoration is the reduction in river methane emissions, which are higher for degraded rivers than natural ones. This means that besides the benefits of improving freshwater status, protecting biosphere integrity and a better regulation of biogeochemical flows, river restoration can be a measure of climate mitigation by reducing atmospheric emissions of methane.

The reduction of methane emissions derived from river restoration can represent an added opportunity, by certifying it and exchanging it as carbon credits. By this mechanism, it could provide a financial incentive to river restoration that could expand the extent of current restoration projects. Yet, before this, careful assessments of changes in river methane emissions during restoration projects have to be performed, and currently research in this area is missing. Our future research will establish baselines and accurate quantifications of river methane emissions of rivers before, during and after restoration projects, to develop a framework that links river restoration, methane emissions and carbon credits commercialization. We will partner with ongoing restoration projects in Europe to create important synergies with public administrations and private actors, and jointly develop effective restoration techniques that maximize river restoration and river methane emissions. Once potential reductions in river methane emissions are well understood and linked to specific interventions, we will be able to incorporate actions into our river restoration workflows that target this environmental outcome.

A final goal will be to expand and lower the cost of river restoration worldwide, as an effective way to contribute to the sustainability of our society within the planetary boundaries. To do this, we will use our high-resolution global maps of river methane emissions, to identify areas with the highest potential to reduce methane emissions as a result of river restoration. As a consequence, restoration in those locations will have a high potential to produce substantial amounts of carbon credits, which can aid in financing the actual river restoration project. This can be particularly important in developing countries, where financial support for river restoration is limited, river contamination is high, and is likely the location where solutions will have the largest impact. As a whole, the link between river restoration and climate change mitigation, mediated by river methane emissions, can unlock new resources for a more expansive role of river restoration, as a tool to maintain our society operating in a safe space within the planetary boundaries.

Photo 1. A mountain wetland stream (photo by: Gerard Rocher-Ros).

Photo 2. A stream with a small dam in Spain (photo by: Gerard Rocher-Ros).