What Greenhouse Gas Do Dams Emit?

Dams provide many benefits such as hydroelectric power, flood control, irrigation, and water storage (Advantages of Dams, Dams 101, Flood Control Benefits). However, dams also emit greenhouse gases like methane, carbon dioxide, and nitrous oxide. There has been rising concern recently over the level of greenhouse gas emissions from dams and their contribution to climate change. This article will provide an overview of the main greenhouse gases emitted by dams and discuss the factors impacting emissions and mitigation strategies.

Methane Emissions

Methane is produced in reservoirs through the decomposition of organic material that sinks to the bottom. This occurs through a process called methanogenesis, where microbes break down organic matter in oxygen-low environments like reservoir sediment and produce methane as a byproduct. Reservoirs contain a large amount of terrestrial organic carbon that provides fuel for microbial methane production.

Methane is estimated to account for over half of the total greenhouse gas emissions from reservoirs globally. Though short-lived in the atmosphere compared to carbon dioxide, methane has a global warming potential 28-34 times greater over a 100 year period. This means methane can trap much more heat in the atmosphere and contribute significantly to climate change impacts in the near-term compared to an equivalent amount of carbon dioxide. Reducing methane emissions from reservoirs is therefore an important climate change mitigation strategy.

According to research by the EPA, methane emissions from reservoirs represent about 1.5% of total global anthropogenic methane emissions. These emissions are increasing over time as more organic material accumulates on reservoir floors after flooding.

Carbon Dioxide

Dams emit carbon dioxide (CO2) primarily from the decomposition of organic matter like vegetation and carbon inflows from the upstream watershed. When the area is flooded to create the reservoir, terrestrial vegetation that was living there dies. This dead plant material sinks to the bottom of the reservoir and decomposes. The decomposition process converts the carbon stored in the plants into carbon dioxide, which bubbles up and is emitted into the atmosphere.

Reservoirs also receive a continuous inflow of organic matter like leaves, branches, and soil carbon from upstream areas. This material also undergoes decomposition and emits CO2. According to research by the EPA, carbon dioxide emissions are highest in the first few years after dam construction since there is a large amount of dead vegetation that decomposes initially. Emissions stabilize over time but continue as long as the dam operates.

One study by the EPA found that carbon dioxide emissions from reservoirs accounted for around 65% of total greenhouse gas emissions from dams in the United States. The amount of emissions depends on the size of the reservoir, the climate, and how much terrestrial vegetation was flooded.

Nitrous Oxide

Reservoirs and other water bodies can be significant sources of nitrous oxide (N2O) emissions. According to Li et al. (2021) from Nature, global lakes emitted 64.6 ± 12.1 Gg N2O-N per year in the 2010s, an increase of 126% since the 1850s. This is attributed to nutrient loading from human activities like agriculture and wastewater discharge. Reservoirs can facilitate N2O production through microbial processes like nitrification and denitrification.¹

A study by Wang et al. (2021) published in ScienceDirect found that the Xiaowan Reservoir in China had an N2O emission flux of 15.48 ± 2.87 μmol m−2·d−1 in 2019.² N2O is produced in reservoirs by microbes converting ammonium and nitrate in the water. The production of N2O depends on factors like water temperature, dissolved oxygen levels, and organic matter content. Proper management of nutrient loads and water parameters can help control N2O emissions.

Total Emissions

Dams and their associated reservoirs are a significant global source of greenhouse gas emissions. According to the EPA, reservoirs account for around 4% of all man-made methane emissions globally. The EPA estimates that methane makes up 80-90% of greenhouse gas emissions from reservoirs, with carbon dioxide and nitrous oxide making up the remainder.

In 2016, hydroelectric reservoirs emitted over 70 million metric tons of methane and 260 million metric tons of CO2 – comparable to about 1.5% of total global greenhouse gas emissions that year. Reservoirs emit particularly high levels of methane compared to other sources. The EPA notes reservoir methane emissions are over 4 times higher than ruminant livestock methane emissions globally.

While dams and reservoirs provide renewable energy, their greenhouse gas footprint can be significant compared to other energy sources. Some studies estimate reservoirs emit over 100 times more methane per kWh generated compared to natural gas plants. Strategies like optimizing turbine usage and managing organic matter can help reduce emissions from existing and future dams.

Impact Factors

The level of greenhouse gas emissions from reservoirs are affected by several key factors including climate, vegetation, and water depth¹. Reservoirs in warmer climates tend to have higher emissions due to increased microbial activity at higher temperatures. The type and amount of vegetation flooded by the reservoir also plays a role, as vegetation provides organic matter that decays and releases gases. Shallow reservoirs with more vegetation tend to have higher emissions per unit area than deep reservoirs with less vegetation. However, deep stratified reservoirs can also have high emissions from deep water layers depleted in oxygen. Overall, emissions are complex and site-specific based on the unique characteristics of each reservoir.

Mitigation Strategies

Reservoir hydrodynamics can be managed to reduce GHG emissions from reservoirs. One approach is to draw down water levels in the colder months to expose vegetation and allow it to decompose aerobically, producing less CH4 than anaerobic decomposition (1). Removing vegetation from the area to be flooded prior to dam construction can also reduce organic material available for decomposition. Adding oxygen to bottom waters through aeration and maintaining aerobic conditions in the hypolimnion can also reduce CH4 production (2).

Turbine design can help as well. Upgrading to more efficient turbine technologies like Kaplan turbines can enable power generation even during lower water flow periods, reducing spillage over the dam and consequently reducing degassing of supersaturated GHGs in the water (3).

Sources:

(1) https://www.hydropower.org/factsheets/greenhouse-gas-emissions

(2) https://therevelator.org/dam-removal-climate/

(3) https://www.nature.com/articles/s41467-019-12179-5

Hydropower vs. Alternatives

When comparing the greenhouse gas emissions from hydropower to other energy sources, it’s important to consider the full lifecycle emissions. This includes emissions from constructing the dams and reservoirs, methane emitted from the reservoirs, and emissions from operating the power plants over their lifetime.

According to the International Hydropower Association (IHA), hydropower’s lifecycle emissions are estimated to be 24g CO2e/kWh on average. This is far below the lifecycle emissions from fossil fuels like coal (820g CO2e/kWh) and natural gas (490g CO2e/kWh). It’s also lower than nuclear (60g CO2e/kWh) and on par with wind (21g CO2e/kWh) and solar PV (41g CO2e/kWh) (IHA, 2022).

One study estimated the net emissions from Canada’s hydroelectric reservoirs to be 35 million tonnes CO2 equivalent per year. In contrast, if the same amount of electricity was generated from a natural gas plant, emissions would be around 64 million tonnes CO2e/year (Impactful, 2022).

While hydropower does result in some greenhouse gas emissions from reservoirs, especially in tropical regions, overall it has a much lower carbon footprint compared to fossil fuel alternatives. Developing hydropower allows countries to reduce their reliance on coal, oil and natural gas for electricity generation.

Future Outlook

The future of dam construction and associated greenhouse gas emissions remains uncertain. On one hand, dams provide renewable hydropower and water storage benefits that support development and agriculture. However, climate change concerns and environmental impacts may limit new large dam projects. One estimate predicts a 50-70% slowdown in new dams globally through 2030.

Developing countries still plan major dam projects to meet economic growth needs. However, financing and environmental restrictions may slow construction. Developed countries aim to limit new large dams and prioritize alternative renewable energy sources like solar and wind. Overall, global dam construction could plateau or decline slightly in the coming decades.

Slower dam building provides time to improve greenhouse gas monitoring and mitigation strategies. New dams may increasingly utilize green design principles and emissions reduction practices. However, existing dams present a challenge, as reducing emissions from current reservoirs is difficult and expensive. Abandoning or removing older dams has its own environmental consequences.

Beyond hydropower, growth in non-hydro renewables can also limit emissions by reducing need for future dams. Solar, wind, geothermal and other alternatives may play a larger role in a low-carbon future. But hydropower remains essential to many national energy strategies. With careful planning and mitigation, dams can continue providing clean energy while minimizing their emissions impact.

Conclusions

In this article we explored the three main types of greenhouse gases emitted from dams – methane, carbon dioxide and nitrous oxide. Dams emit more greenhouse gases than other electricity sources like wind or solar, but less than fossil fuel power plants. Total emissions vary widely between dams based on the climate, vegetation and age. Tropical regions tend to have higher emissions than temperate regions. Methane emissions in particular can spike after flooding and decrease over time.

Considering dam greenhouse gas emissions is important when evaluating energy options and climate impacts. Though hydropower is renewable, dams may emit substantial greenhouse gases that can reduce their climate benefit compared to alternatives. Strategies like turbines and vegetation removal can help mitigate emissions. Overall, a nuanced view accounting for all impacts is needed when assessing dams and planning energy systems.