Uhe Furnas Dam, Brazil. Located on the middle of the Rio Grande, the UHE FURNAS reservoir is one of the largest in Brazil. It impounds the Grande River and has a capacity of 22,590,000,000 m3 and a surface area of 1,473 km2. |
An
international team of scientists has amassed the largest data set to
date on greenhouse gas emissions from hydroelectric reservoirs. Their
analysis, published today in the online version of Nature Geoscience, posits that these human-made systems emit about 1/6 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% of the total carbon dioxide
and methane emissions from all types of human-made reservoirs combined.
“Our
analysis indicates that hydroelectric reservoirs are not major
contributors to the greenhouse gas problem,” comments Dr. 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% 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 MSc. 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.”
APM Manso Dam, Brazil. Located in the State of Mato Grosso, the APM Manso Dam impounds the Manso River and has a capacity of 5,600,000,000 m3 and a surface area of 357 km2. |
Hydroelectricity
supplies an estimated 20% of the world’s electricity and accounts for
more than 85% of electricity from renewable sources. Future development
is expected globally. 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,” Cole notes.
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.
Other
contributors to the paper included Drs. Lars J. Tranvik, Yves T.
Prairie, David Bastviken, Vera L. M. Huszar, Paul del Giorgio, and Fábio
Roland.
The
work was supported by grants from FURNAS Centrais Elétricas and from
the Swedish Foundation for International Cooperation in Research and
Higher Education (STINT).
Carbon emission from hydroelectric reservoirs linked to reservoir age and latitude
