Typically, at moderate sizes, power generated by dams and reservoirs is considered “green.” However, a new study from Washington State University has found that during times of drawdown — a period in which the water level behind a dam is rapidly lowered — temperate reservoirs can produce up 20 times more methane than normal.
Methane is a greenhouse gas 25 times more effective than CO2 at trapping heat in the atmosphere over 100-year period, and is a hundred times more potent over 20 years. It is produced naturally in reservoirs thanks to biological activity.
“Bridget Deemer, a doctoral student at Washington State University-Vancouver, 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. A fellow WSU-Vancouver student, Maria Glavin, sampled bubbles rising from the lake mud and measured a 36-fold increase in methane during a drawdown.”
Though researchers have long known that methane levels spike in reservoirs during drawdown, this study was the first to show the relationship and put a number on the actual methane emissions.
A 2011 study published in the science journal Science found that the “ability of terrestrial ecosystems to act as carbon sinks,” which contain greenhouse gasses and keep them out of the atmosphere, could be up to one quarter less than previously thought when the greenhouse gas release from reservoirs is taken into consideration.
Clearly, the problem is not negligible — particularly when we consider the number of mega-dams being constructed around the world. International Rivers explains:
“Drawdown emissions have been studied and modeled in the tropical context (see Fearnside, 2009 and 2005) and to a limited extent at the Three Gorges Dam(see Chen, H. et al., 2009). In the case of Three Gorges, for instance, one-third of the reservoir is a drawdown region and given its massive size (its surface area is the size of Hong Kong), that is no insignificant source of methane. While dam reservoirs cover a small portion of the earth’s surface, as Harrison notes, they harbor biological activity that can produce large amounts of greenhouse gases. When you think of the number of large dams in the world – more than 54,000 that are over 15 meters – and the countless others that are being proposed or are under construction, continuing to overlook reservoirs as a carbon source and treating dams as a ‘carbon neutral’ energy source is no longer a viable option.”
The immediate effects of the WSU study may be a change in the way dams are operated, with an eye toward minimizing greenhouse gas emissions. For instance, summer months — when warm temperatures and low oxygen levels are perfect for the microbes that produce methane — might be the worst time for a drawdown:
“‘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,’ Harrison says.
Managers can also consider the optimal time to take out a dam, Deemer says. While a dam removal may lead to some greenhouse gas emissions initially, she says it will be a one-time occurrence, while emissions can recur with regular drawdowns. The ability of soils and plants to store greenhouse gases could also make reservoir decommissioning a net sink, she says, but researchers ‘simply don’t know at this point.'”
Over the longer term, the research could help establish a tracking mechanism to inventory natural greenhouse gas emissions internationally in order to better understand the magnitude of the problem.
— Max Frankel