The Hidden Costs of Hydropower: How Hydropower Drives the Climate Crisis

Hydropower and CO2 Emissions

When constructing a hydropower plant, large amounts of concrete and steel are required. However, the production of these materials is highly energy-intensive and requires the combustion of fossil fuels, especially coal and natural gas, leading to significant CO2 emissions. For a complete life cycle assessment of hydropower plants, the emissions resulting from reservoir construction must be considered, as well as the increase in emissions due to land inundation. Often, these emissions are either not considered or only partially accounted for, portraying hydropower as more environmentally friendly than it actually is.¹.

The fact is that the flooding of land for reservoirs alters the CO2 balance of the flooded ecosystem. Clearing forests and flooding valleys not only changes the landscape but also converts a landscape that stores CO2 (called a sink) into a source of greenhouse gases. The flooded landscapes typically consist of a mosaic of different habitats such as forests, meadows, marshes, or fields. These terrestrial landscapes are usually a sink for CO2 because the plants in forests and meadows store CO2 in the soil through photosynthesis. When these landscapes are flooded, not only can no more CO2 be stored, but the organic material stored in the soil is also decomposed underwater and released into the atmosphere as CO2 or CH4. Since this organic material, without flooding, would have been stored in the soil for several decades, these emissions are a direct result of hydropower.² Scientists have shown that as a direct result of flooding, several tons of stored CO2 are mobilized and released into the atmosphere³, significantly increasing reservoir emissions for about 100 years.² Particularly in the first 15-20 years after flooding, greenhouse gas emissions from reservoirs are particularly high². This means that older reservoirs already cause fewer emissions, but the creation of new ones is particularly environmentally harmful.

Scientific studies show that when these emissions are taken into account, certain hydropower plants produce similar amounts of CO2 per kilowatt-hour of electricity produced as gas and coal-fired power plants.¹

In addition to the direct emissions during reservoir construction and the increase in emissions due to land inundation, indirect effects must also be considered. The creation of hydropower reservoirs alters the hydrology of an area and can influence the environment in various ways. For example, the altered water flow can disrupt downstream ecosystems, leading to further CO2 emissions if forests or wetlands are destroyed or drained.

Reservoirs as Methane Hot-Spots

Dams are also emission hotspots for methane.⁴ Methane is 30 times more potent as a greenhouse gas compared to CO2, making its release particularly harmful for the climate.⁵
Scientists have recently identified reservoirs as significant sources of greenhouse gases and have attributed 5.2% of the world’s anthropogenic methane emissions to outgassing from reservoirs.⁶

The methane production in reservoirs is a complex process influenced by various factors. The type of organic materials, temperature, and sediment composition all play a role. Researchers have found that the impoundment of rivers by dams and weirs leads to the accumulation of large amounts of organic material. Under anaerobic conditions (i.e., in the absence of oxygen), this organic material begins to decompose in deep, near-bottom water layers, releasing methane.^4,7 The methane generated in the sediment rises in gas bubbles, thus entering the atmosphere unfiltered.

A previous assumption that only reservoirs in tropical regions release significant amounts of methane has been disproven by studies conducted at an alpine reservoir in Switzerland. The authors of the study were able to demonstrate the highest methane emissions ever measured from reservoirs at the Wohlen reservoir near Bern.⁸ Hence, it is expected that reservoirs in the Alps also release significant amounts of methane. However, studies on this matter are still pending.

Outgassing at the turbines

An outgassing method unique to reservoirs used for energy production is outgassing at the turbines. For energy production, water is mainly extracted from deep water layers. Methane is usually dissolved in these deep waters at very high concentrations. This is due to the high water pressure, which allows more methane to be dissolved than in surface waters, and also because methane production is particularly high in these deep water layers near the bottom. Under normal, natural circumstances, methane dissolved in deep water layers does not reach the atmosphere. However, when these water layers are directed into the turbines, the pressure on the water phase decreases. The solubility drops suddenly, and the methane escapes into the atmosphere.²^{, 9}

Local warming due to the “Albedo climate penalty” of reservoirs

When large land areas are flooded, the reflectivity of these areas changes. A heat-reflecting landmass is replaced by a heat-absorbing body of water. This is because the newly created water surface has a lower albedo than the previously dominant terrestrial surface. Albedo, a measure of the reflective properties of a surface, determines how much a surface heats up from incoming solar energy. Bright surfaces, such as snow, have a high albedo and reflect much of the incoming sunlight, while dark surfaces, such as asphalt, have a low albedo and absorb sunlight, storing it as heat. When the albedo is reduced due to the flooding of landmasses, more sunlight is absorbed, leading to local warming effects that negatively impact the local climate. Scientists use the term “albedo climate penalty” in reference to reservoirs to describe the negative impacts of flooding on the climate.¹⁰ Despite scientists emphasizing the importance of considering albedo effects in the planning and evaluation of hydropower projects, it has not been adequately taken into account when assessing the impacts of hydropower projects on the climate.¹⁰

Conclusion

In summary, it can be said that despite their role as a renewable energy source, hydropower plants can have significant environmental impacts, especially concerning CO2 emissions and climate change. The construction and operation of such facilities lead to direct and indirect emissions of greenhouse gases such as CO2 and methane. The flooding of land by reservoirs alters ecosystems, releases stored CO2, and promotes methane production. Additionally, local warming effects are intensified by the “albedo climate penalty.”

It is crucial to fully understand and consider these impacts to make an informed assessment of the environmental compatibility of hydropower projects. This requires a comprehensive life cycle assessment that includes both the direct and indirect effects of hydropower use on climate and the environment. Unfortunately, comprehensive life cycle assessments are not typically required by policy, and hydropower operators often fail to account for all the impacts of hydropower on the climate. This leads to the mistaken belief that electricity from hydropower is climate-neutral. Since hydropower operators are unlikely to voluntarily provide a comprehensive life cycle assessment, it is up to policymakers to demand this. Only by considering all emissions can future energy supply truly be made sustainable.

References & Links

1. Levasseur, A., Mercier-Blais, S., Prairie, Y. T., Tremblay, A. & Turpin, C. Improving the accuracy of electricity carbon footprint: Estimation of hydroelectric reservoir greenhouse gas emissions. Renew. Sustain. Energy Rev. 136, 110433 (2021).

2. Prairie, Y. T. et al. Greenhouse Gas Emissions from Freshwater Reservoirs: What Does the Atmosphere See? Ecosystems 21, 1058–1071 (2018).

3. Prairie, Y. T. et al. A new modelling framework to assess biogenic GHG emissions from reservoirs: The G-res tool. Environ. Model. Softw. 143, 105117 (2021).

4. Maeck, A. et al. Sediment trapping by dams creates methane emission hot spots. Environ. Sci. Technol. 47, 8130–8137 (2013).

5. Saunois, M. et al. The Global Methane Budget 2000-2017. Earth Syst. Sci. Data Discuss. 1–138 (2019) doi:10.5194/essd-2019-128.

6. Soued, C., Harrison, J. A., Mercier-Blais, S. & Prairie, Y. T. Reservoir CO2 and CH4 emissions and their climate impact over the period 1900–2060. Nat. Geosci. 15, 700–705 (2022).

7. Bednařík, A., Blaser, M., Matoušů, A., Hekera, P. & Rulík, M. Effect of weir impoundments on methane dynamics in a river. Sci. Total Environ. 584–585, 164–175 (2017).

8. Delsontro, T., Mcginnis, D. F., Sobek, S., Ostrovsky, I. & Wehrli, B. Extreme methane emissions from a swiss hydropower Reservoir: Contribution from bubbling sediments. Environ. Sci. Technol. 44, 2419–2425 (2010).

9. Deemer, B. R. et al. Greenhouse gas emissions from reservoir water surfaces: A new global synthesis. BioScience vol. 66 949–964 (2016).

10. Wohlfahrt, G., Tomelleri, E. & Hammerle, A. The albedo–climate penalty of hydropower reservoirs. Nat. Energy 6, 372–377 (2021).