Dams and water reservoirs are new global warming culprits

Researchers at Washington State University-Vancouver have documented the emission of greenhouse gases like methane, as water levels go up and down in dams and reservoirs.

Methane is 25 times more effective than carbon dioxide at trapping heat in the atmosphere. And while dams and the water behind them cover only a small portion of the earth's surface, they harbour biological activity that can produce large amounts of greenhouse gases.

"Reservoirs have typically been looked at as a green energy source. But their role in greenhouse gas emissions has been overlooked," researcher at Washington State University Bridget Deemer said.

Researchers measured dissolved gases in the water column of Lacamas Lake in Clark County and found methane emissions jumped 20-fold when the water level was drawn down. They also sampled bubbles rising from the lake mud and measured a 36-fold increase in methane during a drawdown.

There are nearly 80,000 dams in the United States alone, according to the US Army Corps of Engineers National Inventory of Dams.

A previous study estimated that the ability of terrestrial ecosystems to act as carbon sinks, storing greenhouse gases, could be one-fourth less than estimated once emissions from reservoirs are considered.

This is the first study to demonstrate and quantify the relationship between water-level drawdowns and greenhouse gas releases

The research could lead to different ways of managing drawdowns as emissions may be higher in summer months, when warmer temperatures and low oxygen conditions in bottom waters stimulate the microbial activity that produces greenhouse gases.

"We have the ability to manage the timing, magnitude and speed of reservoir drawdowns, which all could play a role in how much methane gets released to the atmosphere," advisor and an assistant professor of Earth and Environmental Sciences John Harrison said.