Hydropower, which generates electricity through flowing water, is often categorized as a clean or zero-emission energy source because no fuel is burned during operation. However, a complete life-cycle assessment reveals that hydropower facilities do release greenhouse gases (GHGs), primarily from the reservoirs they create. The measurement of these emissions is complex, involving various factors that lead to a wide range of outcomes, making it difficult to assign a single, definitive carbon footprint value.
The Source of Greenhouse Gas Emissions in Reservoirs
The emissions associated with hydropower stem from biological processes occurring within the flooded reservoir area, not the turbine itself. When a dam is constructed and the reservoir is filled, submerged terrestrial organic matter (trees, vegetation, and soil carbon) decomposes. This decomposition by aquatic microbes is the primary mechanism for producing GHGs.
In the upper, oxygen-rich layers, aerobic decomposition generates carbon dioxide (\(\text{CO}_2\)), which diffuses across the water surface. In the deeper, oxygen-deprived bottom layers and sediments, anaerobic decomposition occurs. This anoxic environment favors the production of methane (\(\text{CH}_4\)), a potent greenhouse gas with a significantly higher global warming potential than \(\text{CO}_2\).
Methane is released through diffusion across the water surface and ebullition (gas bubbles from the sediment). Water passing through the dam’s turbines and spillways also causes dissolved gases, especially deep-water methane, to be released into the atmosphere, a process known as degassing. These biogenic gases—\(\text{CO}_2\), \(\text{CH}_4\), and trace amounts of nitrous oxide (\(\text{N}_2\text{O}\))—constitute the majority of the facility’s greenhouse gas footprint.
Quantifying Emissions: Average Carbon Footprint Data
To compare the climate impact of hydropower with other sources, emissions are measured using a life-cycle assessment, expressed in grams of carbon dioxide equivalent per kilowatt-hour (\(\text{gCO}_2\text{e/kWh}\)). This metric accounts for all GHGs released during the construction, operation, and decommissioning of a facility, converting the warming effect of methane and other gases into a comparable \(\text{CO}_2\) measure.
The median life-cycle greenhouse gas emission intensity for hydropower is cited by international bodies like the Intergovernmental Panel on Climate Change (IPCC) as approximately \(24\ \text{gCO}_2\text{e/kWh}\). A recent analysis of nearly 500 global reservoirs confirmed this low carbon footprint, finding a median value of \(23\ \text{gCO}_2\text{e/kWh}\).
The figures for hydropower show a wide range, spanning from as low as \(1\ \text{gCO}_2\text{e/kWh}\) for certain run-of-river projects to over \(200\ \text{gCO}_2\text{e/kWh}\) in high-emission cases. This significant variability highlights the complexity of quantifying the true carbon footprint of any single hydropower facility.
Key Factors Influencing Emission Variability
The wide range of reported emission intensities is linked directly to the specific environmental and design characteristics of each project. Location and climate are primary factors, with tropical reservoirs typically exhibiting higher emission rates than temperate ones because warmer water accelerates the microbial decomposition of organic matter.
Reservoir age is another determining factor. Emissions are usually highest in the first few years following flooding, peaking within the first 10 to 20 years as the newly submerged biomass decomposes. Over decades, emissions generally decline toward a level comparable to natural lakes and rivers.
Reservoir design, including depth and surface area, also influences gas production and release. Deeper reservoirs tend to develop thermal stratification, creating oxygen-deprived bottom waters that favor anaerobic decomposition and methane production. Shallow areas exposed to sunlight can lead to higher methane ebullition rates.
The pathways of gas release also contribute to variability, particularly the ratio of inflow to outflow and the type of release structure. High water flow through the turbines can cause substantial degassing of deep-water methane downstream of the dam. The volume of carbon stored in the flooded soil and the organic material washed into the reservoir further influence the long-term emissions profile.
Emissions Context: Comparing Hydropower to Other Energy Sources
High-emission fossil fuel sources demonstrate vastly greater carbon intensities: coal-fired power plants have median emissions around \(820\ \text{gCO}_2\text{e/kWh}\) and natural gas plants around \(490\ \text{gCO}_2\text{e/kWh}\). These figures are orders of magnitude higher than the median for hydropower.
Among low-carbon alternatives, hydropower generally ranks slightly higher than the lowest-emission sources. Nuclear power and onshore wind power have median life-cycle emissions around \(12\ \text{gCO}_2\text{e/kWh}\) and \(11\ \text{gCO}_2\text{e/kWh}\), respectively. Solar photovoltaic (PV) technology has a median carbon intensity of approximately \(41\ \text{gCO}_2\text{e/kWh}\).
Hydropower’s median emission intensity of \(24\ \text{gCO}_2\text{e/kWh}\) positions it within the low-carbon energy category. While not zero-emission, its climate impact is far below that of fossil fuels, allowing it to serve a significant role in reducing global emissions when replacing fossil fuel generation.