One of the touted pluses of hydropower is the fact that it is a renewable, carbon-neutral energy source. Dams depend on the self-renewing water cycle, and, after the initial carbon expenditures in constructing the dam in the first place, do not burn any fossil fuels or add any carbon dioxide to the atmosphere. That’s a good thing. Let it be said, though, that carbon dioxide is not the only greenhouse gas out there. In fact, as greenhouse gases go, it isn’t even all that potent, it’s just found in higher concentrations than gases like nitrous oxide or hydrofluorocarbon, which are capable of exerting a much stronger warming effect. Carbon dioxide does most of the damage because there is so much more of it - it’s measured in parts per million, everything else is measured in parts per billion. For the purposes of this post, I am choosing to ignore the role water vapor plays in global warming, because that is worthy of its own separate post. Now, one of those more potent but lower concentration greenhouse gases is methane.
Methane comes from a number of sources, some hilarious (cow burps), some less so (wildfires). One source is decomposition of organic matter in low or zero oxygen environments - environments with minimal oxygen (aka anaerobic environments) favor the growth of certain bacteria which can live and consume matter without oxygen, and produce methane as a byproduct. Where do you find such decomposition? Well, wetlands are a common source - while wetlands are extremely important to a functioning biosphere, they do tend to produce methane.
Where besides a wetland would you expect to find organic matter decomposing in a low-oxygen environment? Actually, there are a lot of answers to that question, but one option is at the bottom of a reservoir. When a dam is put in and a reservoir fills up behind the dam, there is typically a lot of organic matter left below the reservoir.
They think differently about boating hazards in Europe
Think they removed all the trees, plants and soil organic matter before that tower was submerged in the Reschen Reservoir? Unlikely. Vegetation is usually left in place prior to flooding - vegetation dies underwater, and then decomposes in a low oxygen environment - oxygen does percolate through water, but very slowly, and the lower oxygen concentrations at depth in a reservoir are quickly exhausted.
The methane production is greatest in tropical reservoirs, as a result of the higher temperatures and typically greater quantities of vegetation present prior to flooding. One rather infamous reservoir in French Guiana had the effect of bubbling out significant quantities of methane - up to 800 metric tons per day of methane. A dam system in Brazil was found to release methane equivalent to 6% of total emissions for the entire regional floodplain Some tropical reservoirs have even been considered as sources of methane for energy production. However, methane emissions have also been measured from temperate reservoirs - Switzerland may get very cold, but they do have summer there, and those warmer temperatures can raise the methane emissions of reservoirs.
The methane production is greatest in tropical reservoirs, as a result of the higher temperatures and typically greater quantities of vegetation present prior to flooding. One rather infamous reservoir in French Guiana had the effect of bubbling out significant quantities of methane - up to 800 metric tons per day of methane. A dam system in Brazil was found to release methane equivalent to 6% of total emissions for the entire regional floodplain Some tropical reservoirs have even been considered as sources of methane for energy production. However, methane emissions have also been measured from temperate reservoirs - Switzerland may get very cold, but they do have summer there, and those warmer temperatures can raise the methane emissions of reservoirs.
Different though they may be, Petit Saut (Fr. Guiana) and Lake Wohlen (Switzerland) both emit their fair share of methane.
The emission effect, fortunately, tends to be short-lived. Particularly in temperate or arid tropical regions, the drowned vegetation decomposes within a decade or so of dam construction - comparing 10 years of elevated methane emissions and then 20 years of essentially carbon neutral power generation with the 30 years of consistent carbon dioxide and nitrous oxide emissions to be expected from running a coal power plant for that long still makes hydropower the more appealing choice. However, damns sited in humid, tropical regions take longer to pass the elevated methane emission stage, and emit considerably more methane to the atmosphere. You might say it’s a matter of location, location, location.
A recent study found dams to be, in regard of greenhoues gas emission, superior to fossil fuel combustion for energy generation, but that the effect varied with both the latitude of the dam site (tropics/not), and with how much biomass was in the area to be flooded (makes sense, right?). Reservoirs that cover more dead vegetation and experience higher average temperatures emit more methane, simple as that. However, even in the tropics, if a site is selected with sufficiently low biomass, the really elevated greenhouse gas emissions can be avoided - a more favorable environment for deposition can only do so much if there isn’t that much material to decompose. Simply adding how much biomass is going to be flooded to the existing criteria for selecting a dam location can eliminate much of the problem. The reservoir methane problem is real, but it’s also avoidable. Now there’s an encouraging thought.
A recent study found dams to be, in regard of greenhoues gas emission, superior to fossil fuel combustion for energy generation, but that the effect varied with both the latitude of the dam site (tropics/not), and with how much biomass was in the area to be flooded (makes sense, right?). Reservoirs that cover more dead vegetation and experience higher average temperatures emit more methane, simple as that. However, even in the tropics, if a site is selected with sufficiently low biomass, the really elevated greenhouse gas emissions can be avoided - a more favorable environment for deposition can only do so much if there isn’t that much material to decompose. Simply adding how much biomass is going to be flooded to the existing criteria for selecting a dam location can eliminate much of the problem. The reservoir methane problem is real, but it’s also avoidable. Now there’s an encouraging thought.
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