An Introduction to Greenhouse Gas Emissions from Hydroelectric Reservoirs: The What, the Where and the How

By Emily Cleare

The purpose of this paper is to discuss the basic principles concerning greenhouse gas emissions from hydroelectric reservoirs. Reservoirs have been shown to produce carbon dioxide and methane as a result of the decay of biomass at the reservoir bottom. These gases are then released into the atmosphere via a variety of mechanisms, such as bubbling and turbine diffusion. In addition, there are a variety of factors, such as climate and reservoir structure, that influence the amount of methane and carbon dioxide that is produced and emitted into the atmosphere. However, although much is know about the what, where and how of greenhouse gas emissions from reservoirs, the global impact of reservoir greenhouse gases is still under contest. This paper will discuss some of the factors that must be taken into account when measuring and assessing the global warming impact of reservoir emissions. This includes the concept of net versus gross emissions and the importance of the global warming potential (GWP) in understanding methane emissions. However, the reader should keep in mind while reading this paper that more research needs to be done before any conclusions can be reaching concerning the impact of hydroelectric dams on global climate change.

Hydropower has long been considered a clean source of energy not responsible for global climate change. However, recent research has shown that hydropower is a significant contributor to greenhouse gas emissions and “is not as climate-friendly as its proponents have assumed” (International Rivers Network, 2002). In fact, the reservoirs created for large hydropower dams emit substantial quantities of the greenhouse gases (GHG’s) carbon dioxide and methane into the atmosphere. There is still much controversy, however, over the global impact of these emissions. Some scientists feel that the emissions do contribute significantly to global climate change. Other disagree and feel “It’s baloney and it’s much overblown…Methane is produced quite substantially in the rain forest and no one suggests cutting down the rain forest” (a quote from the US National Hydropower Association, 1995 cited from International Rivers Association, 2002). This debate is evident in the literature – scientists have still not reached an agreement concerning the long term effects and substantiality of reservoir GHG emissions.

However, even though there is still much debate over the impact of these emissions, it is certain that reservoirs do emit methane and carbon dioxide into the atmosphere. This paper is an introduction to these reservoir emissions and is directed to the reader who is interested in the environmental impacts of hydroelectric dams and/or issues related to global climate change. The controversies in the literature are recognized, but the majority of information presented in this paper is the facts that most scientists agree on in terms of the basic principles behind reservoir emissions. The first part of the paper will describe the various sources of methane and carbon dioxide in the reservoir. This includes how the gases are produced and the distinct ways in which they are released into the atmosphere. Furthermore, the factors that influence the amount of methane and carbon dioxide that a particular reservoir produces – such as climate and reservoir structure – will be discussed. Then, once this basic foundation has been laid out, the difficulties with measuring these emissions and understanding their global warming impact will be discussed. This includes the concept of net versus gross emissions and the importance of the global warming potential (GWP) in understanding the global warming impact of methane and carbon dioxide.

It is important to keep in mind, while reading this paper or other literature on the subject, that this is a very new area of research. Reservoir emissions are slowly becoming a hot topic in evaluating hydroelectric dams as an energy source. However, much more research needs to be done before any major conclusions can be drawn.

The Production of Carbon Dioxide and Methane in the Reservoir Environment

Methane and Carbon dioxide are the two major greenhouse gases (GHG’s) associated with hydroelectric dams. Both affect the global climate, but methane is significantly more powerful than carbon dioxide in terms of its efficiency in trapping heat (International Rivers Network, 2002). However, methane is shorter lived in the atmosphere, possibly decreasing its influence on global warming (International Rivers Network, 2002). (Understanding the overall global warming impact of these two gases will be discussed later in the paper).

The source of the carbon dioxide and methane in reservoirs is rotting and decaying vegetation (St. Louis et. al., 2000). When land is flooded to create a reservoir (Figure 1), the vegetation dies and is no longer able to absorb carbon dioxide from the atmosphere via photosynthesis (St. Louis et. al., 2000). Instead, those plants decay and the stored organic carbon is converted into methane and carbon dioxide (St. Louis et. al., 2000). Fearnside explains this process:

“…reservoirs become virtual methane factories, with the rise and fall of the water

level in the reservoir alternately flooding and submerging large areas of land around the shore; soft green vegetation quickly grows on the exposed mud, only to decompose under aerobic conditions at the bottom of the reservoir when the water rises again. This converts atmospheric carbon dioxide into methane, with a much higher impact on global warming than the CO₂ that was removed from the atmosphere when the plants grew” (Fearnside, 2004).

Figure 1

http://www.namtheun2.com/gallery/dia_veget/dia_veget6.htm

As this vegetation is flooded, the plant matter decays and ultimately results in the production of methane and CO2.

This conversion of the stored organic carbon into methane and carbon dioxide occurs by two distinct mechanisms depending on the oxygen level of the water (Rosa et. al, 2004). In an oxic environment, more toward the surface of the reservoir, aerobic decomposition of organic matter produces carbon dioxide and methane (Rosa et. al., 2004). In an anoxic environment, methanogenesis produces carbon dioxide and methane (Rosa et. al., 2004). Methanogenesis is the pathway in which the products carbon dioxide and methane are produced by a biological process involving methanogen bacteria (United States Geological Society, 2005). In either case, methane and carbon dioxide are the product of the decomposition of vegetation in the reservoir environment.

How Methane and Carbon Dioxide are Released into the Atmosphere: Diffusion, Bubbling, and Above Water Decomposition

Once methane and carbon dioxide are produced, they are not immediately released into the atmosphere. The gases are soluble in the water of the reservoir until a chemical event occurs that causes the gases to be released (Fearnside, 2002). There are several ways by which this release can occur, depending on the characteristics of the reservoir. The gases can be released from the surface of the reservoir by rapid diffusion directly into the atmosphere (Fearnside, 2002). Or, the gas can form bubbles at the bottom of the reservoir and rise to the surface (Fearnside, 2002). These events are random and unpredictable, however, making them very difficult to predict and quantify (Fearnside, 2002). In addition, there is no consistent proportionality between the amount of bubbling and the amount of diffusion from a particular reservoir, making it even more difficult to measure (Fearnside, 2002). Nevertheless, diffusion and bubbling are two important mechanisms by which carbon dioxide and methane are emitted into the atmosphere (Fearnside, 2002).

Another way by which methane and carbon dioxide can be released into the atmosphere is the aerobic decomposition of biomass (Fearnside, 2002). As Figure 2 shows, when the land is not properly cleared prior to flooding a reservoir, a large number of trees (and other forms of biomass) are left projecting from the water surface. The exposed biomass then decomposes and releases a lot of carbon dioxide and/or methane directly into the atmosphere (Fearnside, 2002). For example, it is estimated that in the year 1990, the dams of Brazil released 10 million tons of carbon dioxide as a result of above water decomposition (Fearnside, 2004).

Figure 2

http://www.dfid-kar-water.net/w5outputs/images/slides/irrig13.jpg

When a dam is not cleared prior to flooding, this is the situation that

is left behind – a large number of trees and other forms of biomass are left projecting from the water. Decomposition of exposed trees such as these

releases a large amount of GHG’s.

How Methane and Carbon Dioxide are Released into the Atmosphere: Turbine Diffusion

Bubbling, diffusion and above water decomposition are not the only ways by which GHG’s are emitted from reservoirs. New research shows that a major pathway for GHG emissions is diffusion as a result of reservoir water moving through the turbines of a dam (Figure 3) (Fearnside, 2002). This occurs because the turbines create a sudden change in water pressure and temperature, which reduces the solubility of gaseous methane and carbon dioxide in water (Fearnside, 2004). Consequently, the majority of the methane and carbon dioxide present in the water, prior to moving through the dam, is released into the atmosphere within a few seconds of the water emerging from the turbines (Fearnside, 2004). Scientists have illustrated this principle when bringing water samples to the reservoir surface (Fearnside, 2004). When the flask containing the water sample is opened, the gases fizz out – in the same way they do when a bottle of coke is opened (Fearnside, 2004).

Figure 3

http://www.uaf.edu/energyin/webpage/Pictures/dam1.jpg

This is a simplified illustration of a hydroelectric dam.

As the water passes through the turbines, the pressure

drops and carbon dioxide and methane are released

Into the atmosphere.

Although both carbon dioxide and methane are released as the reservoir water passes through the turbines, it is methane emissions in particular that are most affected by this process. The reason for this it that the concentration of methane increases as one goes further down in the water column (Graph 1) (Fearnside, 2002). This means that the water that passes through the turbines, which is usually from deep in the water column, has a high concentration of methane (Fearnside, 2004). This is troublesome to many scientists because it is the actual structure of the dam that is causing large quantities of methane to be released into the atmosphere, as opposed to the more natural methods of diffusion and bubbling from the reservoir surface (Fearnside, 2002). And, as described earlier, methane is more efficient at trapping heat in the atmosphere and contributing to global warming.

Graph 1

http://www.springerlink.com/media/788Y6NRXQJ1KP6RVMEWP/Contributions/L/8/3/8/L8381013770378V5.pdf

This graph illustrates the increase in methane concentration as the depth of the reservoir increases. As the graph illustrates, the concentration is fairly high when the water goes through the turbines.

Factors that Influence the Amount of Methane and Carbon Dioxide Emitted

There are a variety of factors that influence the amount of methane and carbon dioxide emitted from a reservoir. The most important factor seems to be the climate of the surrounding environment (Dirty Reservoirs, 2005). Reservoirs in tropical environments have been found to have significantly larger emissions than reservoirs in boreal and temperate climates (Table 1) (Dirty Reservoirs, 2005). One possible reason for this is that the annual water temperature is much higher in tropical climates (St. Louis et. al., 2000). This means that the rate of decomposition is faster, leading to higher carbon dioxide and methane flux in the water (St. Louis et. al., 2000).

Table 1

Image from: St. Louis, V., Kelly, C.A., Duchemin, E., Rudd, J.W.M, and Rosenberg, D.M., 2000, Reservoir Surfaces as Sources of Greenhouse Gases to the Atmosphere: A Global Estimate: Bioscience, v. 50, no. 9, pp. 766-775.

This table illustrates that the GHG emissions from tropical

reservoirs are higher than those from temperate reservoirs.

The age of a reservoir is also thought to play an important role in the amount of GHG emissions over time (St. Louis et. al., 2000). Initially, it was thought that the total emissions decreased over time (Dirty Reservoirs, 2005). There was thought to be a large pulse of emissions immediately after filling the reservoir, but that once the majority of the vegetation had decayed, the emissions would “quickly decline to insignificant levels” (Dirty Reservoirs, 2005). However, recent research shows that the emissions do not decline much after the initial pulse, but instead are fairly constant for the lifetime of the reservoir (Dirty Reservoirs, 2005). One reason for this is that some trees can take up to 1000 years to decay, and aquatic plants that grow and die in the reservoir continue to emit GHG’s for an extended period of time (International Rivers Network, 8). Furthermore, organic matter that is washed into the reservoir from upstream can generate GHG’s and may continue to do so for the lifetime of the reservoir (Pearce, 2000). Nevertheless, it is still uncertain as to how great of an effect the age of a reservoir has on the amount of emissions. Many things need to be taken into account, such as the type of ecosystem flooded and the quantity of biomass, before an accurate conclusion can be reached concerning the reservoir flux-age relationship (St. Louis et. al., 2000).

In addition to these factors, there are a variety of variables that can affect the amount of GHG’s emitted from different areas of the same reservoir (International Rivers Network, 3). For example, the amount of vegetation and growth of aquatic plants in certain areas of a reservoir will greatly affect the amount of methane and carbon dioxide that is produced (International Rivers Network, 3). This, in turn, depends on the amount of sunlight the reservoir is exposed to and therefore the climate and season of the ecosystem (International Rivers Network, 3). Furthermore, changes in depth within the reservoir affect the ability of the GHG’s to escape to the surface and be released into the atmosphere (International Rivers Network, 3). These variables contribute to different emissions levels from different areas of a reservoir, as well as at different times of the year (International Rivers Network, 3).

All of the factors discussed above need to be taken into account when assessing reservoir emissions. This is especially true in the planning and designing of a new hydropower facility. Although the global impact of these emissions is still being debated, it does not hurt to plan efficiently to make GHG production as minimal as possible.

Measuring Emissions: Net vs. Gross Emissions

Once scientists learned how methane and carbon dioxide were being produced and emitted into the atmosphere, they had to determine an accurate way of measuring these emissions. One thing they discovered that was extremely important in emissions assessments was to differentiate between net and gross emissions. Gross emissions are only those emissions that are measured at the reservoir surface (or from the turbines) (McCully, 2004). Net emissions take into account the gases emitted from the reservoir as well as the amount of gases that would have been absorbed by any sinks in the land, and the amount of gases that would have been released from the land before it was flooded (International Rivers Network, 8). Figure 4 below gives a visual representation of the formula for calculating net emissions.

Figure 4

http://www.irn.org/programs/greenhouse/2002ghreport.pdf

The formula for calculating net GHG emissions

from a reservoir.

The concept of net emissions relies on knowledge of the ecosystem to be flooded and its role in the carbon cycle. For example, prior to the creation of a reservoir, the land to be flooded most likely contains both sinks and sources of GHG’s, including methane and CO2 (International Rivers Network, 11). (Table 2 on the next page illustrates some common ecosystems and their roles in the carbon cycle). A carbon sink is an area that takes in or “sequesters” more carbon than it releases (EPA, 2005). A carbon source is an area that releases more carbon into the atmosphere than it absorbs (International Rivers Network, 9). Sources and sinks of carbon dioxide are crucial to maintaining the carbon cycle (Figure 3) of the global environment.

Figure 3

http://earthobservatory.nasa.gov/Library/CarbonCycle/Images/carbon_cycle_diagram.jpg

This is an illustration of the Carbon Cycle – certain ecosystems

are sources or sinks of carbon

Table 2

http://www.irn.org/programs/greenhouse/2002ghreport.pdf

This table represents methane and CO2 emissions from various ecosystems.

Downward arrows indicate a carbon sink and upward arrows indicate a carbon source.

With this in mind, a reservoirs overall contribution to global climate change could increase or decrease depending on the type of ecosystem flooded (International Rivers Network, 9). Unfortunately, much of the literature is only able to supply gross emissions values because GHG emissions are not usually measured prior to flooding (Rosa et. al., 2004). Thus, one thing that is agreed upon by scientists worldwide is that emissions assessments must be undertaken before the construction of a dam, in order to fully understand the global warming impact (Rosa et. al., 2004).

Measuring Emissions: Methane vs. Carbon Dioxide

So, it is clear that reservoirs release methane and carbon dioxide into the atmosphere. However, what is not clear is the overall global warming impact of each gas. Methane is a stronger GHG than carbon dioxide but how much stronger? And how does one compensate for that difference in emissions data? Scientists have dealt with this problem by establishing a conversion factor between methane and carbon dioxide known as the 100 year Global Warming Potential or GWP (International Rivers Network, 9). The GWP converts “the impact of methane into ‘carbon dioxide equivalent’ units…and represents the cumulative warming impact after 100 years of a one-time pulse into the atmosphere of a ton of methane compared to a ton of carbon dioxide” (International Rivers Network, 9). The value of GWP has varied over the years. The most recent GWP was 23, as established in the IPCC’s 2001 Third Assessment Report (International Rivers Network, 9). However, the older figure of 21 is commonly used in data (International Rivers Network, 9).

There are several scientists, however, that greatly disapprove of the GWP of 21 or 23 in comparing methane emissions with carbon dioxide emissions. According to the World Commission on Dams, the 100 year GWP value of 23 “can significantly underestimate the climate change impact of reservoirs over the first several decades” (International Rivers Network, 11). They feel that methane emissions cannot be considered as a one time event or “pulse” from the surface of the reservoir, but that the emissions are fairly continuous for the lifespan of the reservoir (as was discussed earlier) (International Rivers Network, 9). Instead, a new conversion method, that takes continuous emissions into account, should be considered (International Rivers Network, 11). Philip Raphals and Stuart Gaffin developed this new conversion factor and determined that “the cumulative global warming effect after 100 years of a constant methane emitter is 39.4 times greater than that of a constant emitter of an equivalent quantity of carbon dioxide” (International Rivers Network, 11). This number is significantly higher than the GWP of 21 or 23, and consequently has a great effect on emissions data presented using this figure.

The GWP value, or other conversion factor that is used, is just one more thing that must be taken into account when evaluating GHG emissions from reservoirs. Methane emissions could be greatly overestimated or underestimated depending on the value that is used.

Conclusion

This paper has provided a brief overview to the what, where and how of reservoir greenhouse gas emissions. However, much more research must be done before scientists understand the full global impact of reservoir emissions. But, one thing that is agreed upon by most scientists is that hydroelectric reservoir emissions should be included in the global emissions inventory. In addition, the location of future dams should be evaluated for emissions prior to construction and flooding. This will enable a more accurate calculation of net emissions and assessment of the global warming impact.

I found this paper very interesting but also very challenging to write. And I feel I should give the reader some advice before they close the paper and draw their own conclusions. I know that hydropower has many environmental consequences including massive damage to riverine ecosystems. However, do not be sucked in to the idea that reservoir GHG emissions is just one more thing that makes hydropower a poor energy source. We need to consider the other energy options and hold back on any decisions until more research has been conducted. Perhaps hydropower can be adjusted in the future to prevent such serious environmental damage.

References

Dirty Reservoirs: Greenhouse Gas Emitting Dams. Accessed on April 4, 2005 at

http://www.irn.org/programs/greenhouse/index.asp?id=intro.sr2001.html

Fearnside, P.M, 2002. Greenhouse Gas Emissions from a Hydroelectric Reservoir

(Brazil’s Tucurui Dam) and the Energy Policy Implications: Water, Air and Soil Pollution, v. 133, pp. 69-96.

Fearnside, P.M., 2004. Greenhouse Gas Emissions from Hydroelectric Dams:

Controversies Provide a Springboard for Rethinking a Supposedly ‘Clean’ Energy Source: An Editorial Comment: Climatic Change, v. 66, no. 1 - 2, pp. 1 – 8.

International Rivers Network, 2002. Flooding the Land, Warming the Earth:

Greenhouse Gas Emissions from Dams. Accessed on April 4, 2005 at

http://www.irn.org/programs/greenhouse/index.asp?id=frontpage.html

McCully, Patrick, 2004. Tropical Hydropower is a Signifigant Source of Greenhouse

Gas Emissions: A Response to the International Hydropower Association. Accessed on April 4, 2005 at

http://www.irn.org/basics/conferences/cop10/pdf/TropicalHydro.12.08.04.pdf

Pearce, F, 2000. Raising a Stink: New Scientist, v. 166, pp. 4.

Rosa, L.P., Dos Santos, M.A., Matvienko, B., Dos Santos, E.O., and Sikar, E, 2004.

Greenhouse Gas Emissions from Hydroelectric Reservoirs in Tropical Regions: Climatic Change, v. 66, pp. 9-21.

St. Louis, V., Kelly, C.A., Duchemin, E., Rudd, J.W.M, and Rosenberg, D.M., 2000,

Reservoir Surfaces as Sources of Greenhouse Gases to the Atmosphere: A Global Estimate: Bioscience, v. 50, no. 9, pp. 766-775.

The United States Geological Society, 2005. Toxic Substances Hydrology Program:

Accessed on April 7, 2005 at http://toxics.usgs.gov/definitions/methanogenesis.html

U.S Envrionmental Protection Agency (EPA), 2005. Global Warming – Actions:

Accessed on April 7, 2005 at http://yosemite.epa.gov/OAR/globalwarming.nsf/content/ActionsInternationalLandUse&Forestry.html