Do Hydro Reservoirs Produce Greenhouse Gases?
Are hydro reservoirs sources or sinks of greenhouse gases? The simple answer is they are both and neither. Here's why:
All growing plants take carbon dioxide from the air and water around them, and by the wonder of photosynthesis, they use the sun's energy to bind that carbon dioxide into much larger molecules of carbohydrates. The carbohydrates form the structural material of the plant. The efficiency of conversion of the sun's energy into the chemical energy contained in the hydrocarbons varies considerably, but is typically around 1% or less:
https://www.britannica.com/science/photosynthesis/Energy-efficiency-of-photosynthesis
So during a plant's lifetime, it is a sink for carbon dioxide.
When a plant dies, it can be ingested (providing bioenergy for animals), burned (by combustion to carbon dioxide), or left to decay. Whatever the path, the plant is then a source of carbon dioxide, which ends up back in the atmosphere. If by decay on the ground, the plants' carbon is mostly released directly into the atmosphere as carbon dioxide. This is aerobic decomposition. If under water, oxygen is less available and the decomposition is anaerobic, so that the initial product is methane. Eventually that bubbles up into the atmosphere, where it slowly oxidizes to carbon dioxide. The lifetime of free methane in the atmosphere is typically about a decade (more or less, depending on how much is being released):
http://climatemodels.uchicago.edu/methane/methane.doc.html
Although methane is a much more potent greenhouse gas than CO2, its lifetime in air is short. The result is that it doesn't make much difference whether the decay occurs aerobically or anaerobically: it's all part of the carbon cycle. The cycle is diverted only if the hydrocarbons are buried into the deep, hot crust of Earth to slowly convert into fossil fuels; this process takes place over geologic time scales, that is to say hundreds of millions of years. A second path of withdrawing CO2 from the natural cycle can occur if material is buried into the cold, shallow crust found under permafrost, or at great ocean depths. There it can be locked up for shorter geological periods of time in a loose chemical bond as methane hydrates.
More about Carbon Transformations and Organic Matter
So what does all this have to do with hydro reservoirs? Some have made a cottage industry of claiming that hydro reservoirs “create greenhouse gases, especially methane”. What hydro reservoirs do is flood land with water: where plants were exposed to air, they now decay anaerobically, releasing methane. But the end result differs little, and it's all part of a natural cycle. So the question of hydro reservoirs related to greenhouse gases is really a false issue.
The hysteria surrounding methane and reservoirs is sadly reminiscent of the confusion that tobacco companies were able to generate over the dangers of their product. By fabricating confusing reports, scientific efforts to recognize and mitigate the lung cancer epidemic were delayed by decades, and many lives lost to cancer. Today the fossil fuel companies are majorly creating confusion over efforts to create non-fossil energy. Unfortunately many neophytes are willing to sing for them.
To put it crudely, that supper you ate the night before last created a lot of methane. By you won't solve the problem by banning toilets. Toilets, like dams and reservoirs, are merely conduits in the carbon cycle.
To create a more proper perspective, let's look at the most important diagram in our understanding of carbon and climate, the Keeling Curve. Charles David Keeling (1928-2005) was the first person to systematically and accurately monitor atmospheric carbon dioxide (CO2) levels, beginning in 1958 from Mauna Loa, Hawaii and from the south pole.
Keeling discovered the annual fluctuations of carbon dioxide concentrations of about + or – 3 parts per million, which result from the annual cycle of plant growth and dormancy in the Northern Hemisphere (where most of the Earth's land surface and land plants are located). Carbon dioxide in the air is taken up by plants during spring and summer, then returned to the air in the fall and winter when leaves wilt and decay, and fuels are combusted. The Earth literally "breathes" on its own annual cycle. The Earth uptakes a certain amount of CO2 into biomass each year, and then releases it again. In the big scheme of things, it really doesn't make much difference if that carbon spends time as plant material in a lake or reservoir, or if it passes as cow farts: the carbon comes from the atmosphere as CO2, and largely returns there as CO2. Actually of course, there are many pathways; much of the CO2 is absorbed by the oceans and processed there. But the cycle isn't particularly sensitive to whether we practice agriculture or forestry, or whether we are vegans or carnivores. Nature does her thing.
But the dramatic part of the Keeling Curve is the upward trend. This has averaged about 1.5 ppm per year over the span of observations, but is now rising faster at around 2 ppm per year. This amount represents the hydrocarbons which are extracted from the Earth's crust as gas, petroleum and coal and combusted into the atmosphere. The CO2 so created is beyond what nature can incorporate into the natural carbon cycle.
Nature had the CO2 cycle in balance most of the time, although cataclysms such as asteroid impacts, volcanoes and ice ages have all upset that balance every so often. After each major cataclysm that upset the CO2 level, catastrophic climate change would follow. Then along came humans, who created their own cataclysm. We started the industrial revolution. We decided to dig up nature's fossil energy of the past 300 million years, and put all that carbon back into the atmosphere within 300 years. Initially this had benefits: the next ice age has been postponed indefinitely. But now it's obviously becoming a Very Bad Idea. And so we are now risking a Sixth Extinction:
So let's just stop this whole carbon extraction thing now. It isn't that hard, but we need to keep our eyes of the goal of replacing fossil energy as expeditiously as possible.
It won't be that hard, because the Sun sends to Earth 173,000 terawatts of energy, continuously. We need to capture only 0.01% of it amount to power all human activity on the planet. As of this point in time, photoelectric panels are cheap and efficient enough to capture all the energy we can use, from a small fraction of the Earth's surface. Solar photovoltaic cells create electricity about 100 times more efficiently than (for example) burning biomass to run a thermal generating station. In the daytime, we can have all the energy we want, directly from the sun, as solarPV. But half the time the sun has set, and we are in darkness. At night we can get still get energy efficiently and reliably (but not as cheaply) as stored hydroelectricity, from reservoir-to-reservoir. This too is a form of solar energy, since it is the Sun's heat that drives the water cycle, depositing rain and snow at altitude. With dams and generators, we capture the energy that the Sun has imparted to each and every raindrop as gravitational energy, and transform it into electricity.
...And that is why Peace River's Site C is so important. It is a vital key to solving the global climate crisis.
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caikman (at) telus.net
