Over time the water collected behind dams will release greater amounts of methane, a greenhouse gas with even worse effects than carbon dioxide, according to a recent study.

While previous research established reservoirs as significant sources of greenhouse gases including methane, this study, published in Nature Geoscience, is the first global-scale analysis of trends in those emissions. Researchers analyzed data and projections from 9,000 reservoirs on five continents to assess their climate impact from 1900 to 2060. They found that while carbon dioxide emissions are declining, methane emissions have increased and without any mitigation measures, will likely continue to rise.

John Harrison.

“On a per mass basis, methane has a much stronger impact on climate than carbon dioxide does,” said John Harrison, professor in WSU’s School of the Environment in Vancouver and a co-author on the study. “The balance of greenhouse gases emitted from those systems is shifting from carbon dioxide to methane. That means that although the total amount of carbon-based gas is decreasing, the reservoirs’ net climate impact is increasing.”

While methane stays in the atmosphere for a shorter time than carbon dioxide, it holds on to infrared radiation more effectively. Over a 100-year timescale, methane is roughly 34 times more effective as carbon dioxide in warming the atmosphere, Harrison said.

The study showed that total greenhouse gas emissions generated by the reservoirs reached a peak in 1987 in connection with the period of construction of the large dams. Those emissions though were dominated by carbon dioxide, and despite that overall decline, the rise of methane poses a big problem.

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