For years hydroelectric dams have shown up on lists of energy sources that are renewable and non-polluting. Environmental complaints about dams have been over more local considerations such as the fact that dams can disrupt fish spawning and that the dams contribution to water evaporation by increasing the surface area over which water can evaporate. Well, Philip Fearnside of Brazil's National Institute for Research in the Amazon says that dams increase the amount of plant matter that decompose in anaerobic condtions and produce methane which is 21 times more potent as a greenhouse gas than carbon dioxide.
In a study to be published in Mitigation and Adaptation Strategies for Global Change, Fearnside estimates that in 1990 the greenhouse effect of emissions from the Curuá-Una dam in Pará, Brazil, was more than three-and-a-half times what would have been produced by generating the same amount of electricity from oil.
This is because large amounts of carbon tied up in trees and other plants are released when the reservoir is initially flooded and the plants rot. Then after this first pulse of decay, plant matter settling on the reservoir's bottom decomposes without oxygen, resulting in a build-up of dissolved methane. This is released into the atmosphere when water passes through the dam's turbines.
Note that a dam in Brazil which is right on the equator is probably going to receive a lot more plant matter from a river that fills its reservoir than would a dam on a river further from the equator. Even if some hydroelectric dams turn out to be net producers of greenhouse gasses we can't assume that all hydroelectric dams cause more in warming effects from methane production than they prevent in avoided carbon dioxide release.
Methane is a valuable gas to capture in situations where it can be captured because methane can be burned for the energy. Also, the burning of methane turns the carbon in it into a compound (carbon dioxide) that is far less potent as a greenhouse gas.
Parenthetically, James Hansen of NASA has been arguing for several years that reduction in methane emissions would reduce global warming effects more cheaply than lowering carbon dioxide emissions (and see more on this here and here). I especially like his argument that lowering methane emissions would both increase air quality down at ground level where we live and decrease greenhouse warming effects. Hansen still thinks carbon dioxide emissions restrictions will be necessary. But why not first implement the far cheaper option of decreasing methane emissions and also get better ground level air quality in the bargain? Just the increase in ground level air quality alone would, in my opinion, justify the costs. Efforts to capture methane would be at least partially paid back because the captured methane could be burned for energy.
Update: Dave Schuler mentioned methane production from agriculture in the comments. I can't answer his question about the relative contribution agriculture makes to methane emissions. But this reminds me of recent research at the University of California at Davis which showed that most methane from cows comes from cow belching.
California dairy cows produce only half the amount of certain air pollutants as had been believed and, perhaps more important, most of a dairy cow's contribution to smog comes not from her fresh manure, but from her belching, according to preliminary findings by a UC Davis scientist.
Those unexpected results may affect the thinking and practices of California regulators and dairy operators trying to reduce air pollution.
"We have to re-think the idea that the only good solutions are engineering solutions, and consider biological avenues such as animal feeding and management to reduce emissions," said Frank Mitloehner, the UC Davis air quality specialist who is conducting the study.
For three months, Mitloehner and his co-workers have studied dairy cows in sealed environmental chambers to simulate emissions from one type of cow housing, known as freestall conditions. Under these controlled conditions -- the first study of its kind -- the researchers were able to collect precise measurements of the volatile organic compound (VOC) emissions that cows and their fresh waste produce.
"For the first time we can tell dairy farmers the source of VOCs from the cow-housing part of their dairy," Mitloehner said. "For the most tightly regulated pollutant, the 700 ozone-forming gases collectively called volatile organic compounds, that source is not the cows' fresh waste. It's the cows."
This result makes methane emissions reduction easier to do than was previously thought. Supplements of bacteria types or compounds that inhibit methane production could be placed in cow feed to reduce methane production. But the idea of changing feedstocks as a way to reduce methane emissions from cows is nothing new and I've come across mentions of research along these lines in Switzerland, France, Australia, and New Zealand. In NZ agricultural scientists are experimenting with different types of grasses to lower methane production in grazing animals. They found substantial differences in methane production depending on which grass was fed to animals. (and the CH4 mentioned below is methane)
Improving the quality of the diet of ruminants tends to result in higher feed intakes, which in turn tends to increase productivity and CH4 output per animal. However, if CH4 is expressed per unit of product, then using a smaller number of high-producing animals to produce a given amount of product emits less CH4 than using a larger number of lower producing animals. This is because a smaller proportion of the feed eaten is required to maintain the animal and because high feed intakes tend to reduce CH4 yield per unit of feed eaten. Concentrate diets produce less CH4 than forage diets but are too expensive for extensive use in New Zealand. Research undertaken by AgResearch and Dexcel indicates that certain forage species e.g. white clover, lotus and sulla, improve animal performance and produce less CH4 per unit of feed eaten. Experiments are currently underway to look at whether ryegrass cultivars selected for improved animal performance also result in lower CH4 yields per unit of product.
One can easily imagine a great reduction in agricultural methane production by seeding pastures and farm fields with grasses that are found to reduce methane production of cows. Countries willing to genetically engineer grasses to add factors that reduce methane production will be able to achieve the greatest reduction in agricultural methane emissions. This will probably turn out be a fairly cheap and easy way to reduce methane emissions.
Agricultural scientists Garry Waghorn and Michael Tavendale of AgResearch Grasslands in New Zealand have found that higher levels of condensed tannins in grasses reduce methane production.
Methane is either burped or expelled out in breath, and is a by-product of the fermentation of feed in the rumen. Dr Waghorn and Dr Tavendale say about 90 percent of all methane emissions come from ruminants. Greenhouse gases affect everyone, because the Government is committed to ratifying the Kyoto Protocol. Once the agreement is signed, New Zealand will face financial penalties if it exceeds the emissions it recorded in 1990. But, and this is the dilemma for the country, if agricultural production expands, so will gas emissions. Condensed tannins are a naturally occurring compound found in red wine, apple skins and cocoa, as well as some pasture grasses, including lotus and sulla. They can also be found in docks, white clover flowers and some seeds. Besides reducing methane emissions, condensed tannins have other animal-related benefits, including improved milk yields, increased liveweight gain, decreased internal parasite burden and reduced bloat, dags and fly strike. Dr Waghorn said tannins had in the past been considered "evil" because some plants, especially tropical ones, contained them in high concentrations, which were bad for animals. But in a temperate climate such as New Zealand's, condensed tannins werefound in "weedy species", or less common plants, he said. "It's unusual to find it in grasses, which is a problem because animals eat grass," Dr Waghorn said.
But in some reports I came across condensed tannins reduced methane emissions by only 15%. Tannins alone might not be a panacea.
Whereas reportedly only 2 percent of greenhouse gas effects in the United States come from agriculture New Zealand has only 4 million people but 10 million cattle and 45 million sheep. Therefore most of New Zealand's greenhouse gas emissions come from agriculture.
"New Zealand is unique in that over 50 percent of its greenhouse gas emissions arise from methane released by enteric fermentation," said Katharine Hayhoe, an atmospheric scientist at the University of Illinois, Urbana-Champaign, Illinois.
Therefore it is not surprising that New Zealand agricultural scientists are especially interested in reducing methane emissions from cows and sheep.
|Share |||Randall Parker, 2005 February 28 12:58 AM Energy Tech|