Hydropower May Be Huge Source of Methane Emissions
Oct 29, 2014 02:30 PM ET // by Bobby Magill, Climate Central
Imagine nearly 6,000 dairy cows doing what cows do, belching and being
flatulent for a full year. That's how much methane was emitted from one
Ohio reservoir in 2012.
Reservoirs and hydropower are often thought of as climate friendly
because they don't burn fossil fuels to produce electricity. But what if
reservoirs that store water and produce electricity were among some of
the world's largest contributors of greenhouse gas emissions?
Scientists are searching for answers to that question, as they study how
much methane is emitted into the atmosphere from man-made reservoirs
built for hydropower and other purposes. Until recently, it was believed
that about 20 percent of all man-made methane emissions come from the
surface of reservoirs.
New research suggests that figure may be much higher than 20 percent,
but it's unclear how much higher because too little data is available to
estimate. Methane is about 35 times as potent a greenhouse gas as carbon
dioxide over the span of a century.
Think about man-made lakes in terms of cows passing gas: Harsha Lake, a
large reservoir near Cincinnati, Ohio, emitted as much methane in 2012
as roughly 5,800 dairy cows would have emitted over an entire year,
University of Cincinnati biogeochemist Amy Townsend-Small told Climate
Methane emissions from livestock are the second-largest source of
methane emissions in the U.S., behind crude oil and natural gas,
according to the U.S. Environmental Protection Agency. But the EPA's
greenhouse gas emissions estimates do not yet account for methane
emissions coming from man-made reservoirs.
Part of the reason is that, generally, very little is known about
reservoirs and their emissions, especially in temperate regions, such as
in the U.S., where few studies have been conducted.
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In 2012 study, researchers in Singapore found that greenhouse gas
emissions from hydropower reservoirs globally are likely greater than
previously estimated, warning that "rapid hydropower development and
increasing carbon emissions from hydroelectric reservoirs to the
atmosphere should not be downplayed."
Those researchers suggest all large reservoirs globally could emit up to
104 teragrams of methane annually. By comparison, NASA estimates that
global methane emissions associated with burning fossil fuels totals
between 80 and 120 teragrams annually.
But how much reservoirs contribute to global greenhouse gas emissions is
"still a big question mark," because the issue remains relatively
unstudied and emission rates are highly uncertain, said John Harrison,
an associate professor in the School of the Environment at the
Washington State University-Vancouver whose research focuses on how
reservoirs can be managed to reduce greenhouse gas emissions.
"So I don't think we really know what the relative greenhouse gas effect
of reservoirs is compared to other sources of energy in the U.S.," he said.
Research at Harsha Lake may help scientists better understand how
reservoirs contribute to climate change.
In a study published in August, Townsend-Small and researchers from the
EPA found that Harsha Lake emitted more methane into the atmosphere in
2012 than had ever been recorded at any other reservoir in the U.S.
"When you compare the annual scale of the methane emission rate of this
reservoir (Harsha Lake) to other studies, it's really much higher than
people would predict," EPA research associate and Harsha Lake study lead
author Jake Beaulieu told Climate Central.
Scientists have long thought reservoirs in warmer climates in the
tropics emitted more methane than reservoirs in cooler climates, but the
research at Harsha Lake shows that may not be the case, Townsend-Small said.
"We think this is because our reservoir is located in an agricultural
area," she said.
Methane is generated in reservoirs from bacteria living in
"These microbes eat organic carbon from plants for energy, just like
people and other animals, but instead of breathing out carbon dioxide,
they breathe out methane," Townsend-Small said. "These same types of
microbes live in the stomachs of cows and in landfills, which are other
sources of methane to the atmosphere."
Runoff from farmland around Harsha Lake provides more nutrients in the
water, allowing algae to grow, just like numerous other reservoirs
surrounded by agricultural land across the country.
Methane-generating microbes feed on decaying algae, which means that
lakes catching a lot of nutrient-rich agricultural runoff generate a lot
"There are a very large number of these reservoirs in highly
agricultural areas around the U.S.," Townsend-Small said. "It could be
that these agricultural reservoirs are a larger source of atmospheric
methane than we had thought in the past."
Emissions from reservoirs in all climates could be underestimated
because of a discovery Beaulieu's team found at Harsha Lake: The area
where a river enters a man-made lake emits more methane than the rest of
the lake overall.
Nobody has measured that before, Beaulieu said.
Most other research studying reservoir methane emissions doesn't account
for how emissions may vary across the surface of a lake, he said.
The EPA is about to begin a more comprehensive study measuring methane
emissions from 25 reservoirs in a region stretching from northern
Indiana to northern Georgia, with sampling beginning next year, Beaulieu
That study will help the EPA eventually include reservoir methane
emissions in its total estimates of human-caused methane emissions.
Until that and other studies are complete, scientists can only speculate
on the impact hydropower is having on the climate.
"We're still in the very early days here of understanding how these
systems work with respect to greenhouse gas production," Harrison said.
This article originally appeared on Climate Central, all rights reserved.
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