When you think about sources of planet-heating greenhouse gases, dams and reservoirs probably aren’t some of the first things that come to mind.

But scientific research has shown that reservoirs emit significant amounts of methane, a potent greenhouse gas. It’s produced by decomposing plants and other organic matter collecting near the bottom of reservoirs. Methane bubbles up to the surface of reservoirs, and also passes through dams and bubbles up downstream.

Scientists call these processes ebullition and degassing.

And there is a growing debate about how much of these gases would be emitted by California’s planned Sites Reservoir, which is slated to be built in a valley north of Sacramento to store water for agriculture and cities.

In a 2021 study, scientists estimated that the world’s reservoirs are annually giving off greenhouse gases equivalent to 1.07 gigatons of CO2 — a relatively smaller piece of the picture if compared the more than 36 gigatons of emissions from the burning of fossil fuels and other industrial sources, but still significant.

John Harrison.

John Harrison, the study’s lead author and a professor at Washington State University, read the findings by the environmental groups and said the per-area methane emissions rates they used in their analysis are within the rate reported for reservoirs in temperate regions, “albeit toward the high end of the distribution and quite a bit higher than emissions from temperate zone reservoirs of comparable size.”

Harrison said in an email that he thinks it’s important to work toward the kind of estimates the groups have attempted, but “due to a lack of supporting data and relevant studies, many of the flux estimates put forth in this report are necessarily quite uncertain.”

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Los Angeles Times