Study Downgrades Hydroelectric
Reservoirsí Impact On Greenhouse Gas Emissions
Bulletin August 05, 2011
An international team of scientists has amassed
the largest data set to date on greenhouse gas
emissions from hydroelectric reservoirs.
Their analysis, published this week in the
online version of Nature Geoscience, posits that
these human-made systems emit about one-sixth of
the carbon dioxide and methane previously
attributed to them.
Prior studies based on more limited data
cautioned that hydroelectric reservoirs could be
a significant and large source of both carbon
dioxide and methane to the atmosphere.
Through an analysis of 85 globally-distributed
hydroelectric reservoirs, the authors revealed
that these systems emit 48 million metric tons
of carbon annually, a downgrade from earlier
estimates of 321 million metric tons. Further
putting things in perspective, hydroelectric
reservoirs are responsible for less than 16
percent of the total carbon dioxide and methane
emissions from all types of human-made
"Our analysis indicates that hydroelectric
reservoirs are not major contributors to the
greenhouse gas problem," said Jonathan Cole, a
limnologist at the Cary Institute of Ecosystem
Studies and one of the paper's authors. "But
there are some caveats. To date, only 17 percent
of potential hydroelectric reservoir sites have
been exploited, and impacts vary based on
reservoir age, size, and location."
Carbon dioxide and methane are two of the main
greenhouse gases created by human activities.
Carbon dioxide is produced during the combustion
of nearly any organic material; methane has a
variety of industrial sources. Both gases are
also produced naturally, particularly in
wetlands and lakes.
When rivers are dammed to make the reservoirs
needed for hydroelectricity, flooding creates
lake-like conditions that generate carbon
dioxide and methane. Emissions are the highest
following reservoir construction, due to
decomposing vegetation and soil organic matter.
As reservoirs age, emissions decline, with
cold-water systems stabilizing more rapidly than
their warm-water counterparts.
Lead author Nathan Barros, of the Federal
University of Juiz de Fora further explains,
"The bottom line is that per unit of energy,
hydroelectric generation produces much less
carbon dioxide and methane emissions than
previously thought, but impacts are not equal
across all landscapes."
The amount of greenhouse gases generated by
hydroelectric reservoirs depends on where they
are built, with the team's analysis indicating
that emissions are correlated with latitude and
the amount of biomass in the watershed. With
Barros adding, "Reservoirs in tropical
locations, such as the Amazon, emit more methane
and carbon throughout their lifecycles."
Hydroelectricity supplies an estimated 20
percent of the world's electricity and accounts
for more than 85 percent of electricity from
renewable sources. Future development is
The paper's authors urge careful consideration
of site location and design. "During the
environmental impact phase, it should be a goal
to minimize the amount of carbon dioxide and
methane emitted per unit of energy generated,"
To truly tease apart the emissions generated by
hydroelectricity, the authors also call for a
study that assesses a site's carbon budget
before and after reservoir construction. Pre-
and post flooding analysis would clarify the net
carbon impact of hydroelectric reservoirs.