Big Hydro’s role in global warming

[Auggie]

San Francisco Chronicle, Friday, November 17, 2006

FIZZY SCIENCE -- Big Hydro’s role in global warming
by Patrick McCullym

It comes as a surprise to most people, but the reservoirs behind the world's dams are likely a major source of global warming pollution. In the case of big reservoirs in the tropics -- where most new dams are proposed -- hydropower can actually emit more greenhouse gases per kilowatt-hour than fossil fuels, including dirty coal.

Climate change scientist Philip Fearnside estimates that hydro projects in the Brazilian Amazon emit at least twice as much greenhouse gas as coal plants. The worst example studied, Balbina Dam, had a climate impact in 1990 equal to an astonishing 54 natural gas plants generating the same amount of power, according to Fearnside.

How is this possible? When a big dam is built, its reservoir floods vast amounts of carbon in vegetation and soils. This organic matter rots underwater, creating carbon dioxide, methane and, in at least some cases, the extremely potent warming gas, nitrous oxide. While emissions are particularly high in the first few years after a reservoir is filled, they can remain significant for many decades. This is because the river that feeds the reservoir, and the plants and plankton that grow in it, will continue to provide more organic matter to fuel greenhouse gas production.

Some of the emissions bubble up from the reservoir's surface. The rest occur at the dam: When methane-rich water jets out from turbines and spillways, it suddenly releases most of its methane, just like the fizz from a newly opened bottle of Coke. While the scientists working in the field agree on the emissions from reservoir surfaces, there is a heated dispute between industry-backed and independent researchers on the amount of gases released at dams. Accounting for these "fizz" emissions greatly increases estimates of the global-warming impact of hydropower.

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http://sfgate.com/cgi-bin/article.cgi?file=/chronicle/archive/2006/11/17/EDG6ELJ3U01.DTL

[porphyrian]

1. Couldn't this be prevented with some form of aeration?

If the methane is from anaerobic decomposers, which I'm assuming is the case, I think it can be eliminated or reduced by aerating the water behind the dam, allowing oxygen-breathing organisms to eliminate the food supply of the ones creating the methane.

[phantom power]

2. It pretty much has to come out eventually, either as methane or CO2.

Of course, CO2 is preferable to methane, by a factor of 20. I hate hydro, but if you're going to do it, it's probably optimal to harvest the timber and at least get some use out of it. And ensure that it ultimately shows itself as CO2, as the least-bad scenario.

[porphyrian]

3. I'm quite sure energy companies can afford to do it right. - n/t

[Auggie]

4. I find this highly alarming. For years we were all told hydro was a great

alternative to burning fossil fuels. It produced a reasonably decent return in value in terms of energy versus the cost of constructing new facilities. Solar and wind are great, but as I understand it they do not provide the same "bang for the buck." True, we don't get the pollution and carcinogenic effects of burning coal, nor the radioactive waste of nuclear with hydro, but still...

Any energy experts out there who can shed light on this?

[lapfog_1]

5. There are a number of issues I have with this

OK, I'll buy into the stmt that for a few years after a reservoir is created, there is a release of CO2 from decaying plants, but after that I wouldn't expect any additional CO2 or methane to be produced simply because the river that comes into the reservoir has plankton in it. No more so than the river had before the dam was built. And as for the CO2 released for the first years of operation, are there not solutions to this by planting acreage equivalent to the acreage to be covered with trees and plants? Also, this is talking about dams in the tropics, where the vegetation levels are high. What about dams in more temperate climates?

It just seems a bit counter-intuitive, especially the stmt that for the same megawatts, dirty coal electric plants would be better than a hydro electric plant as far as greenhouse gases emitted.

[nealmhughes]

6. This needs to be addressed in a longitudinal manner: it may be true for an intiial

damming, when the vegetation is under water and decaying, but once that has stopped, then the decay is arrested. Flow through the turbines and spillways should also tend to arrest the decay gas buildup, I would think.

From personal observation of the lakes of the TVA on the River itself, there is no noticeable debris or mud buildup because the flow is maintained towards the Ohio/Mississippi/Gulf. Lake levels are regularly raised and lowered as a method of mosquito control and to prepare for spring rains/thaws in the head waters.

For example, the spillways may be opened for several days when there is a hurricane that will get trapped by the Appalachians is on its way in order to lower the level of the Tennessee and its main feeder creeks and rivers such as Shoal Creek, the Buffalo and Elk River, etc. This raises the level of the next lake, which is then lowered, in a cascade until it reaches the Ohio and the Mississippi system takes over.

The Tennessee is not a tropical, but a temperate system, so the research in the tropics may be very different from a river that rises in Virginia and flows through Tenn, Al, and KY to the Mississippi. Therefore the analysis of tropical systems may differ widely from temperate zones.

[AndyTiedye]

7. This Assumes That You Are Going to Flood a Rainforest to Build a Dam

Why would anyone do that? The wood is too valuable for that.