Reservoirs emit significant greenhouse gases planet-wide, study finds; researchers urge that new hydropower projects not be christened with green energy label.
Fom 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 megawatts (MW) – are slated for construction.
Global distribution of future hydropower dams, either under construction (blue dots 17 %) or planned (red dots 83 %). Image: Zarfl et al (2014)
Number of future hydropower dams per major river basin. Image Zarfl et al (2014)
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% 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.
Construction at the São Manoel dam site on the Teles Pires River, Brazil, where three other dams are now nearing completion. These new hydropower dams also form part of the controversial Tapajos Complex of dams and reservoirs intended to support a vast industrial waterway for transporting soy from Brazil’s interior downriver to the Amazon River, to the coast, and on to China.
Dams not “emission free”
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 sq km – 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.”
Importantly, the study teased apart the relative emission contributions of each of the three gases – a crucial consideration as these gases have varying degrees of impact on global temperature.
Methane and nitrous oxide are many times more potent than carbon dioxide, and they also behave differently over time once released into the atmosphere, and both of these factors are relevant in the context of short and long-term policies on emission targets. Over a 100-year timeframe, methane’s effect on global warming is more than 30 times, and nitrous oxide’s effect is almost 300 times, greater than CO2.
But the study authors argue that the next 100 years are not nearly so relevant as the next 20 years for determining climate change policy aimed at quickly curbing global warming and meeting global emission targets set out in the Paris Agreement. And because methane “is relatively short-lived in the atmosphere (atmospheric lifetime on the order of a decade) relative to CO2 (atmospheric lifetime on the order of centuries),” they write, CH4 “has a higher global warming potential over the shorter 20-year time horizon.”
In fact, methane’s effect is 86 times greater than that of CO2 when considered on this two-decade timescale. Importantly, the study found that methane is responsible for 90% of the global warming impact of reservoir emissions over 20 years.
The trouble with bubbles
Around half of the methane emitted from reservoirs is released in bubbles, which rise from sediment and travel through the water column to the reservoir’s surface. The gas trapped inside these bubbles in the water column, “is the most direct route for methane to reach the atmosphere without being turned into carbon dioxide via [interaction with] oxygen,” said Tonya DelSontro, one of the study’s co-authors, of the University of Quebec, Montreal.
Accounting for the contribution of methane bubbles is therefore a vital component of accurate reservoir emission estimates, but measuring them is challenging. Bubbles are hard to locate, explained DelSontro, who has studied methane emissions from lakes and reservoirs in Switzerland, Zambia and Canada.