In the context of an ever-changing climate, every action towards reducing greenhouse gas (GHG) emissions counts. While dams have long been associated with clean energy production, growing scientific evidence reveals that many freshwater reservoirs and ponds are, on the contrary, significant sources of greenhouse gas emissions.
The methane problem
Methane is the second most important greenhouse gas contributor to climate change after carbon dioxide. The Intergovernmental Panel on Climate Change (IPCC) recognises methane as having a global warming potential up to 86 times greater than carbon dioxide over a 20-year timeframe.
Freshwater reservoirs, particularly in tropical climates – but also in European rivers – create ideal conditions for organic matter to decompose, turning them into “methane hotspots”. Globally, reservoirs contribute 5.7% of all methane emissions.
Dam removal: an overlooked opportunity to reduce GHG emissions
Removing river barriers that no longer fulfil any function can restore natural river flows, reduce stagnant water areas, and eliminate the primary conditions under which methane is produced in the ponds or reservoirs formed upstream of a dam.
A project led by CEAB-CSIC is studying how the removal of river barriers affects the balance of greenhouse gases. We spoke with Rafael Marcé, Scientist at CEAB and Lead Researcher of the UNDAMMED project, who explained
“When a dam is built, a new ecosystem is created and sediment accumulates at the bottom, including organic matter carried by the flow. On one hand, this organic matter represents stored carbon – a CO₂ sink. However, the flip side is that microorganisms proliferate in these sediments, breaking down the organic matter and generating methane, a gas with a much greater greenhouse effect than CO₂.”
Although still in its early stage, preliminary figures within the project have revealed that
“Carbon emissions from the dam’s impoudment were 1 mole of CO₂ equivalent per square metre per day, which dropped to 0.1 mole per square metre per day after dam removal.”
Monistrol de Calders before and after removal © Catalan Water Agency (ACA). Source.
Figures also showed that removing Monistrol de Calders Dam (Barcelona, Spain) reduced methane emissions by up to 10 times.
These are not final results, but they strongly suggest that dam removal could significantly support decarbonisation efforts. Marcé continues to explain:
“For example, if we consider the Monistrol de Calders Dam in Barcelona, we estimate it emitted 125 tonnes of CO₂ equivalent per year. Given that there are approximately 170,000 barriers in Spain’s watercourses, if just 5% of them were similar to Monistrol, we could be looking at a potential reduction of 1 million tonnes of CO₂ equivalent emissions annually.”
Prioritising the removal of high-emission dams – particularly those no longer serving essential energy or water management functions – represents an effective climate mitigation strategy. By targeting the most methane-emitting reservoirs, we can make measurable progress in reducing GHG emissions while simultaneously revitalising ecosystems and reconnecting riverine habitats and communities.
