Tajikistan is a mountainous, land-locked country in central Asia. It borders Afghanistan, Uzbekistan, and Kyrgyzstan, and lies close to Pakistan. The largest river in Tajikistan is the Vakhsh. The Vakhsh originates in the southwestern, glacier-covered mountains of Kyrgyzstan, and it crosses the entire length of Tajikistan before joining with the Panj River to form the Amu Darya at the border of Tajikistan and Afghanistan.
The Vakhsh is Tajikistan's major source of electricity. Five dams are currently in place on the Vakhsh, including the Nurek Dam, which, as of 2009, is the tallest dam on earth. The Nurek includes nine hydroelectric turbines, which generate 3 gigawatts of power. Altogether, the five hydroelectric dams on the Vakhsh generate 90 percent of Tajikistan's electricity. This output has not been enough to keep up with Tajikistan's electrical needs, however, and the nation has been forced to ration electricity at times. To increase capacity, four more dams are planned along the river. One of them, the Ro-gun Dam, will be taller than the Nurek and is expected to generate 3.6 gigawatts of power. In addition to the electricity it generates, the Vakhsh is vital as a source of irrigation for agriculture.
Because it is so centrally important to Tajikistan, anything that threatens the Vakhsh is a threat to the entire country. One danger is from earthquakes; landslides have occasionally blocked portions of the river and damaged dam equipment. Another potential long-term threat is global warming. The Vakhsh originates in mountain glaciers. If temperatures warm enough to reduce the size of those glaciers significantly, the flow of Tajikistan's major electricity source might be changed irreversibly.
bubbles rushing to the surface . . . except that "The difference in pressure between a closed and open bottle of Coca-Cola is minor compared to the pressure at depth in a hydroelectric reservoir," according to researcher Philip M. Fearnside.19 The methane that bubbles out of the water enters the atmosphere—where it contributes to climate change.
In fact, methane "is estimated to be more than 20 times as powerful," as carbon dioxide in contributing to climate change.20 Thus, hydroelectric dams essentially function as gigantic engines for taking the relatively low-impact carbon dioxide stored in plant matter, changing it into high-impact methane, and releasing it quickly into the atmosphere. The problem is exacerbated in "tropical regions where temperatures are high and decomposition is more rapid," according to Gautier.21 Some environmentalists have even argued that the greenhouse emissions from a hydroelectric dam do more damage than ones from a comparable coal plant.
Whether this is true or not is difficult to determine. Gautier notes that, right after they begin to function, dam emissions are especially high, "as a result of the decomposition of flooded vegetation and soils, as well as construction-related deforestation." After that, the amount of methane emitted varies from dam to dam. Dams with deep, narrow reservoirs produce fewer emissions relative to their power output than coal plants do. Reservoirs that are broader and shallower, however, may produce more emissions per watt than coal plants do.22
Some researchers are working on an innovative means of increasing the efficiency of the dams. Methane does not have to be a waste product; it could itself be a source of energy. If the methane released by the dams could be captured and burned, it would release a huge amount of energy. In fact, "Some hydroelectric plants in the Amazon hold an added energy capacity of 27 to 53 percent, taking into account the methane bubbles released from the water as it passes through the turbines and spillways."23 Brazil's second largest hydroelectric dam at Tucurui could yield one million metric tonnes (907,000 tons) of methane. When burned, that amount of methane would release enough energy to make the construction of several other hydroelectric dams unnecessary. Scientists are still working on perfecting a technique to cap ture the methane. If they succeed and implement the technology worldwide, it "could prevent emissions equivalent to more than the total annual burning of fossil fuels in the UK," according to reporter Tim Hirsch.24 Thus, in the future, hydroelectric dams, tidal power, and wave power may all make important contributions to the moderation of climate change.
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