Diffusive + ebullitive methane (top), carbon dioxide (middle), and nitrous oxide (bottom) emissions from reservoirs on a CO2-equivalent basis (100-year horizon). Few reservoirs had measurements for all three gases. Graphic: Deemer, et al., 2016 / BioScience

By Claire Salisbury
14 February 2017

(Mongabay) – From the Amazon Basin to boreal forests, and from the Mekong to the Himalayan foothills, rivers worldwide are being targeted for major new dams in a global hydropower boom that also aims to supply drinking water to exploding human populations and to facilitate navigation on the planet’s rivers; 3,700 new dams — 847 of them larger than 100 MW — are slated for construction.

But one strong argument in favor of hydropower is now looking far weaker. Scientists have compiled the most comprehensive assessment yet of the global impact that dam reservoirs have on the world’s atmosphere and greenhouse emissions. And it isn’t good news.

Globally, the researchers estimate that reservoirs — long considered “zero emitters” by the United Nations climate program — contribute 1.3 percent of man-made greenhouse gas emissions. Emissions on this scale are comparable to those from rice paddy cultivation or biomass burning, the study authors write.

But despite their magnitude, these reservoir emissions are not currently counted within United Nations Intergovernmental Panel on Climate Change (UN IPCC) assessments. In fact, countries are currently eligible under the UN’s Clean Development Mechanism to receive carbon credits for their newly built dams. The study raises the question as to whether hydropower should continue to be counted as green power.

The study, published in BioScience, looked at the carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) emitted from 267 reservoirs across six continents. In total, the reservoirs studied have a surface area of more than 77,287 square kilometers (29,841 square miles). That’s equivalent to about a quarter of the surface area of all reservoirs in the world, which together cover 305,723 square kilometers (118,040 square miles) — roughly the combined size of the United Kingdom and Ireland.

“The new study confirms that reservoirs are major emitters of methane, a particularly aggressive greenhouse gas,” said Kate Horner, Executive Director of International Rivers, adding that hydropower dams “can no longer be considered a clean and green source of electricity.” [more]

Counterintuitive: Global hydropower boom will add to climate change


ABSTRACT: Collectively, reservoirs created by dams are thought to be an important source of greenhouse gases (GHGs) to the atmosphere. So far, efforts to quantify, model, and manage these emissions have been limited by data availability and inconsistencies in methodological approach. Here, we synthesize reservoir CH4, CO2, and N2O emission data with three main objectives: (1) to generate a global estimate of GHG emissions from reservoirs, (2) to identify the best predictors of these emissions, and (3) to consider the effect of methodology on emission estimates. We estimate that GHG emissions from reservoir water surfaces account for 0.8 (0.5–1.2) Pg CO2 equivalents per year, with the majority of this forcing due to CH4. We then discuss the potential for several alternative pathways such as dam degassing and downstream emissions to contribute significantly to overall emissions. Although prior studies have linked reservoir GHG emissions to reservoir age and latitude, we find that factors related to reservoir productivity are better predictors of emission.

Greenhouse Gas Emissions from Reservoir Water Surfaces: A New Global Synthesis

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