Freshwater Methane Emissions Offset the Continental Carbon Sink

D. Bastviken et al.'s reply (1) to S. Li and X. Lu (2) on the question of methane emissions from reservoirs ignores the major points raised, indicating that Bastviken et al.'s Brevia ("Freshwater methane emissions offset the continental carbon sink," 7 January 2011, p. 50) greatly underestimated the impact of hydropower development, such as ignoring emissions by degassing from water passing through the turbines.

A large pulse of methane invariably occurs in the first few years after a reservoir is formed because easily decomposed carbon stocks in leaves and soil are converted to methane under anoxic conditions at the bottom of the reservoir. After the pulse of emission from initial carbon stocks, a lower but still-significant emission can continue indefinitely from renewable carbon sources such as periodic flooding of soft vegetation that grows in the drawdown zone.(3–6) The resulting total methane emission is not small, as indicated by calculations for Amazonian dams (7–9).

Bastviken et al. conclude their E-Letter (1) by calling for evaluating hydropower projects "based on their long-term effects on net carbon and GHG [greenhouse gas] emissions and savings." This is a formula for downplaying the impact of hydropower. Hydropower creates a very large emission in the first few years, followed by an indefinitely sustained emission at a lower level, whereas electrical generation from fossil fuels emits gases at a constant rate over time. This makes the time horizon and any weighting given to the value of time the key elements in comparing these two energy sources. A long-term horizon, say, of 100 years, without any discounting for time over this period, will inevitably favor fossil fuels, even if any benefit for climate would emerge 40 years or more in the future (9). Major impacts from global warming on a much shorter time scale than this makes such logic untenable as a representation of the interests of human society.

Philip M. Fearnside

National Institute for Research in Amazonia (INPA), C.P. 478, 69.011-970 Manaus, Amazonas, Brazil.

References

1. D. Bastviken et al., Response to S. Li and X. X. Lu's E-Letter, Science (E-Letter, 28 June 2011), www.sciencemag.org/content/331/6013/50.short/reply#sci_el_14254.

2. S. Li, X. X. Lu, Greenhouse gas emissions from reservoirs could double within 40 years, Science (E-Letter 28 June 2011), www.sciencemag.org/content/331/6013/50.short/reply#sci_el_14254.

3. P. M. Fearnside, Water Air Soil Poll. 133, 69 (2002).

4. P. M. Fearnside, Oecologia Brasiliensis 12, 100 (2008).

5. P. M. Fearnside, Climatic Change 66, 1 (2004).

6. P. M. Fearnside, Climatic Change 75, 103 (2006).

7. P. M. Fearnside, Mitig. Adapt. Strat. Glob. Change 10, 675 (2005).

8. P. M. Fearnside, Environ. Manage. 35, 1 (2005).

9. P. M. Fearnside, Novos Cadernos NAEA 12(2), 5 (2009).

Conflict of Interest:

None declared

S. Li and X. X. Lu use our Brevia data ("Freshwater methane emissions offset the continental carbon sink," D. Bastviken et al., 7 January 2011, p. 50) to predict dramatically increased greenhouse gas (GHG) and carbon dioxide equivalent (CO₂eq) emissions from hydroelectric reservoirs by 2050. Although increased reservoir construction may increase reservoir CH₄ emissions, it is unclear by how much, and we feel that their interpretations of our data may be inaccurate.

Li and Lu state that "Bastviken et al. estimated the GHG emissions 0.41 Pg of C (CO₂eq) year^-1 from reservoirs using a total reservoir area of 0.5 M km²…". We estimated CO₂eq from CH₄ emissions only—not the total GHG emissions. Furthermore, our estimate for reservoirs was 20.1 Tg CH₄ year^-1, which corresponds to 0.14 Pg of C (CO₂eq) year^-1. Nevertheless, reservoirs contribute substantial anthropogenic emissions.

Care is needed in the temporal extrapolation of our reservoir data. Although we made every effort to include data on reservoirs, our small sample size (35) yielded a broad coefficient of variation (87 to 176%). Emission from each reservoir may be dependent on local conditions and reservoir design, such as the size of the reservoir, characteristics of the flooded terrain, how that terrain is managed before flooding, bathymetry, the age of the reservoir, organic and nutrient inputs, water residence time, and the depth of the generator intake. The procedure of extrapolation into the future applied by Li and Lu assumes that future reservoirs will be similar to current ones. As Li and Lu point out, the development of new reservoirs, particularly at low latitudes, may result in higher emissions than they predicted. On the other hand, if experiences from current reservoirs are used to minimize future CO₂eq emissions, and considering the possibility of reduced emissions as reservoirs age (1), simple extrapolation from the present situation may lead to overestimation.

It is important to note that our Brevia and Li and Lu's comments focus on CH₄ or GHG emissions of reservoirs and other inland waters, and do not address other aspects important for assessing the net CO₂eq balances of reservoirs. Complete representation of the total reservoir net CO₂eq balance would require consideration of the CO₂eq exchange of the reservoir and its construction and operation, corrected for the CO₂eq exchange of the affected area before dam construction (2). Few such net assessments are available and we agree that more data are needed. An initial set of proposed guidelines for making such net reservoir assessments has recently been published (2). In addition, the net CO₂eq emission per kW produced would also be useful for comparing hydroelectric reservoirs with other means of electrical production, and the comparison should perhaps also include the cost of dependency on limited resources such as fossil fuels and uranium. Our analysis had different objectives than to make interpretations about future emissions or present-day net CO₂eq exchanges of reservoirs. Although it may contribute to informed consideration of GHG balances, predictions of future reservoir contributions from limited data regarding only CH₄ and CO₂ may lead to inaccurate forecasts.

The development of hydropower depends on a range of conditions, including trade-offs and conflicts concerning conservation and alternative forms of land use, the development of robust power generation, and climate effects. Some reservoirs emit large amounts of CO₂eq kW^-1, whereas emissions from others are low compared to alternative energy sources (3). It seems most reasonable to evaluate reservoir projects based on their long-term effects on net carbon and GHG emissions or savings. Therefore, there is an urgent global need for representative data of net CO₂eq exchange from reservoirs.

David Bastviken

Department of Thematic Studies—Water and Environmental Studies, Linköping University, 58183 Linköping, Sweden.

Lars J. Tranvik

Department of Limnology, Evolutionary Biology Centre, Uppsala University, Uppsala, Sweden.

John A. Downing

Department of Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, Iowa 50011, USA.

Patrick M. Crill

Department of Geological Sciences, Stockholm University, SE-106 91 Stockholm, Sweden.

Alex Enrich-Prast

Department of Ecology, University Federal of Rio de Janeiro, 68020 Rio de Janeiro, Brazil.

References

1. G. Abril et al., Global Biogeochem. Cycles 19, GB4007 (2005).

2. J. A. Goldenfum, Ed., GHG Measurement Guidelines for Freshwater Reservoirs, The Unesco/IHA Greenhouse Gas Emission from Freshwater Reservoirs Research Project (International Hydropower Association, London, UK, 2010); www.hydropower.org/climate_initiatives.html.

3. A. Tremblay et al., Net greenhouse gas emissions at Estmain 1 Reservoir, Quebec, Canada (Proceedings of the World Energy Congress, Montréal, September 12 to 16, 2010).

Conflict of Interest:

None declared

In the Brevia "Freshwater methane emissions offset the continental carbon sink" (7 January 2011, p. 50), D. Bastviken et al. estimated the greenhouse gas (GHG) emissions to be 0.41 Pg of C (CO₂eq) year^-1 from reservoirs given a total reservoir area of 0.5 M km². Hydro-electric reservoirs are considered a clean energy and are constructed to meet energy needs and reduce the GHG emissions. For instance, China intends to increase its hydropower capacity from the current 654×10⁹ kilowatt-hour (kWh) year^-1 (in 2010) to 1200×10⁹ kWh year^-1 in 2020 (1–3). Current world reservoirs have a capacity of 3045×10⁹ kWh year^-1, and will surpass 7000 ×10⁹ kWh year^-1 in 2050—50% of the total technically hydropower potential (1, 4). The hydropower capacity of a particular reservoir is affected by terrain, dam height, reservoir storage, water flow, and operation, among other factors. If a simple linear relation between hydropower capacity and reservoir water surface area is employed (5), the GHG emissions from the combined reservoirs are projected to reach 0.90 Pg of C (CO₂ eq) year^-1 in 2050. Considering the estimate of the reservoirs' area ranging from 0.26 M km² (6) to 1.5 M km² (7), and the technically exploitable hydropower capacity of 14653×10⁹ kWh year^-1 (1, 4), the GHG emissions from the total artificial reservoirs could reach 2.7 to 5.4 Pg of C (CO₂ eq) year^-1, much higher than the 0.98 Pg of C (CO₂ eq) year^-1 from lakes [in the Report, and (8)] [0.53 Pg of C year^-1 as CO₂, 0.45 Pg of C (CO₂ eq) year^-1 as CH₄].

Bastviken et al.'s estimates of the GHG emissions from reservoirs in most cases ignored the downstream degassing of water releasing through turbines and spillways (7). Studies on the GHG emissions from reservoirs were mainly conducted in the Americas and Northern Europe, but much fewer in tropical Asia and Africa (4) where more reservoirs will be constructed. The GHG emissions from reservoirs in these low-latitude areas were as much as 15-fold higher than those in other regions, and the potential hydropower capacity in Asia, Africa, and South America accounts for about 80% of the world's total emissions (1, 4, 9). Therefore, our estimates of the GHG emissions from reservoirs in 2050 are probably conservative, given that more reservoirs will be built in warm climatic zones. Further revisions based on more case studies, particularly in low-latitude regions, are likely to substantially increase the estimate of the total amount of carbon emissions from reservoirs.

Siyue Li

Lee Kuan Yew School of Public Policy, National University of Singapore, 259772, Singapore, and Key Laboratory of Aquatic Botany and Watershed Ecology, Wuhan Botanical Garden, The Chinese Academy of Sciences, Wuhan 430074, China.

Xi Xi Lu

Department of Geography, National University of Singapore, 117570, Singapore.

References

1. Chinese National Committee on Large Dams (Chincold).

2. C. Peng, G.. L. Qian, Water Power 32, 6 (2006) [in Chinese].

3. X. L. Changet al., Energy 35, 4400 (2010).

4. P. Lako et al., "Hydropower development with a focus on Asia and Western Europe, Overview in the framework of VLEEM 2. project" (Project Number 7.7372, ECN Policy Studies and Verbundplan, Austria, 2003).

5. Worldwide Rivers Database, http://www.cws.net.cn/riverdata/Search.asp [in Chinese].

6. J. A. Downing et al., Limnol. Oceanogr. 51, 2388 (2007).

7. V. L. St. Louis et al., Bioscience 50, 766 (2000).

8. L. J. Tranvik et al., Limnol. Oceanogr. 54, 2298 (2009).

9. D. Altinbilek, C. Cakmak, "The role of dams in development" (DSI Third International Symposium, Ossiac, Austria, 2001).

Conflict of Interest:

None declared