There are dams, and then there are bad dams.
The Three Gorges Project in China, for example, was a very bad dam, having displaced 1.2 million people. Even the Chinese government admits that it threatens "environmental catastrophe" for a range of reasons.
A quick objection to that criticism is that "there is also something to be said, environmentally, for anything that provides China with lots of electricity and isn't coal," notes Kenneth Pomeranz, environmental historian and critic of the Three Gorges project.
Hydropower is traditionally held to be considerably less carbon-intense than coal or many other energy sources—even "emissions-free."
But what if that's wrong? What if dams are frequently bad dams? What if "emissions-neutral" mega-dams are actually major methane or NO2 or CO2 producers?
It would follow that building dams to produce clean energy is a pretty bad idea.
Here's the science: When engineers create dams, they flood massive tracts of land. The vegetation on that land starts to immediately rot and decompose, while organic matter from further up-stream joins it. In tropical regions, the cycle of exposure-then-flooding, inducing the growth then decay of fast-growing vegetation, adds more bio-mass to the mix. All of that bio-mass produces greenhouse gases as it decomposes. Some of those gases slowly bubble up to the surface, and then into the atmosphere.
But some of the methane, under intense pressure at the bottom of the reservoir, doesn't migrate to the surface—until it shoots through the dam's turbines.
The water that goes though a dam's turbines can be, depending on the dam's construction, the coldest, most pressurized water, the water with the highest concentration of methane. As the pressure on the water decreases, the volume of gas it can hold decreases too. So it goes into the atmosphere.
Simultaneously, tropical soils, heavy emitters of NO2, a major greenhouse gas, are covered by the reservoir waters, and so emit far less gas than they would if they were left uncovered. There are also major differences in the gas production of arboreal, temperate, and tropical dams, with the latter likely producing the most GHG per-unit of energy produced, while controlling for other factors.
This much isn't heavily disputed.
Starting in the mid-1990s, Canadian scientists advanced the claim that reservoirs might be significant sources of greenhouse gas emissions.
Brazilian scientists, most notably Philip Fearnside, published their initial findings on emissions from Brazilian reservoirs, focusing on Tucuru´ı, in the eastern Amazon, stating a case that they have continued to strengthen to this day: that greenhouse gas emissions from dams are of more than passing concern.
As the editors of the scientific journal Climactic Change comment,
"The issue at hand is not whether dams produce greenhouse gases, but whether the magnitude of their emissions is comparable to (or even greater than) traditional thermal power technologies, such as natural gas turbines or coal boilers. Comparing the emissions from gas and coal-fired power plants with those of hydroelectricity, Rosa et al. conclude that in most cases 'hydro power plants are better' ... Fearnside is more equivocal and points to striking examples where hydro dams appear worse."
Some of this also has to do with how much vegetation is submerged relative to a dam's power-generation capabilities. Some dams, with shallow but wide basins, will do badly when evaluated in this way. Others, like the Itaipu mega-dam bordering Paraguay and Brazil, do better.
Still, the question of "relative" emissions is, to some extent, a red herring. Mega-dams represent massive capital outlays, and will produce power for many decades. By then if fossil-fuel plants are even still in use, they're likely to have some sort of carbon-capture technology. Gathering the methane, NO2, and CO2 from huge reservoirs and whirling turbines, on the other hand, would be a technically difficult task—although some think it will be feasible.
This has both ecological and policy implications. Ecologically, if dam-building is no panacea, and indeed could be far from emissions-neutral, dam-based hydropower can't be included among the Clean Development Mechanisms (CDM).
The environmental NGO International Rivers points out that currently, carbon dioxide fluxes and methane emissions don't have to be measured, while future national carbon inventories will have to account for the vegetation submerged by the creation of mega-dams. This effectively understates the greenhouse gas emissions from dam construction and could lead to the granting of CDM credits for projects that aren't ecologically clean.
Does this mean that large dams are ecologically bankrupt? Probably.
"At least in the long run, technologies such as wind and solar seem much better bets to provide genuinely clean and affordable power," Pomeranz says.
There are few such problems with run-of-the-river dams, which don't involve building up an enormous reservoir to maintain a constant flow of highly-pressurized water. Instead, the turbines of the dams spin with the river's current.
The problem with such dams is that they involve a trade-off: ecological health is received, but a constant assured energy supply, particularly for rivers that have highly variable seasonal flows, is given up. Smaller dams don't present such problems, and while they necessarily have some ecological impact, it's far less than that entailed by what are basically man-made mountains of concrete.
Smaller dams or run-of-the-river systems are both feasible and sustainable. But the massive engineering projects that come to mind when we hear the word "dam," especially in tropical or sub-tropical under-developed countries, probably aren't nearly so sustainable as they're imagined to be.
(Photos: Three Gorges Dam (top)/tmadeiramd/Flickr Commons; Montana dam/gaby53mt/Flickr Commons)