Dams

Introduction

Dams are structures built across streams or rivers, usually to block water flow and cause the formation of a lake upstream of the dam. The water accumulated or impounded behind a dam can be used for drinking, irrigation of crops, or generating electricity. Electricity generated by dams, termed hydroelectricity, is made by allowing water to run downhill through fan-like turbines, a modern application of the ancient waterwheel principle for extracting work from falling water. The turbines turn generators that produce electricity. About 19% of the world's electricity is hydroelectricity.

Dams have long been criticized for flooding landscapes, destroying ecosystems, and in some cases, forcing the evacuation of millions of people. However, for many years it was assumed that dams are a climate-neutral way to produce electricity since they burn no chemical fuel and produce no carbon dioxide (CO₂) emissions in daily operation. In the 1990s, researchers realized that some dams, especially in the tropics, are significant sources of methane, a potent greenhouse gas. Research published in 2006 reported that dams may account for more than 4% of anthropogenic warming.

Historical Background and Scientific Foundations

Overview

The first large dam was built by the Egyptians around 2800 BC. It was a massive barrier 37 ft (11 m) tall and capable of impounding 20 million cubic ft (566,000 cubic m) of water. The dam failed after a few years when water overflowed its crest, eroding the downstream side of the structure and weakening it until the dam collapsed.

Since that time, most civilizations have built dams of various sizes. However, the building of truly large dams did not occur until the twentieth century. During the 1930s, engineers began building dams large enough to block whole river basins. The Hoover Dam was completed across the Colorado River in the United States in 1936, becoming the world's largest at that time. When the lake behind the dam had filled, it backed 110 mi (177 km) upstream and covered 247 square mi (639 square km). Hoover is still the second-tallest dam in the United States.

Since Hoover, thousands of dams have been built worldwide. Over half (172 out of 292) of the world's large rivers are now dammed at one point or more. All of the world's largest 21 rivers are dammed. Globally, there are more than 45,000 large dams, that is, dams over 49 ft (15 m) high, which altogether store more than 1,557 cubic mi (6,500 cubic km) of water, an amount equal to 15% of the world's annual freshwater runoff. There are over 300 very large dams, namely those over 492 ft (150m) high or impounding over 6 cubic mi (25 cubic km) of water. The latest of these giant dams, and the most controversial and environmentally destructive to date, is the Three Gorges dam across the Yangtze River in China. It will impound over 9 cubic mi (39 cubic km of water) after filling and is forcing the evacuation of over a million people.

As of 2004, 2.2% of the world's primary energy was obtained from hydroelectric dams, or about 19% of the world's electricity. Irrigation water from dams is used to grow 12–16% of the world's food.

Dams and Climate Change

There are several connections between dams and climate change. First, hydroelectricity was long assumed to have less global-warming impact than electricity generated using fossil fuels. Although CO₂ is emitted during the manufacture of the cement, concrete, and steel that go into making a modern dam, these emissions, for a typical dam, amount to less than 10% of the emissions from generating the same amount of electricity by burning fossil fuels. However, starting in 1993, researchers began

noting that some large dams might be significant sources of the greenhouse gas methane (CH₄), which has about 23 times the global-warming effect of CO₂, ton for ton. As of 2008, this issue continued to be hotly debated, with some scientists challenging the objectivity of scientists working for (or partly funded by) the hydropower industry, who have done most of the research on dam methane to date.

Methane is produced after the dam is constructed. When a typical tropical dam is built, large areas of upstream forest are drowned. Wood and other types of organic (carbon-containing) matter that decay under water are mostly broken down by anaerobic bacteria—bacteria that thrive in the absence of oxygen—using digestive chemistry that produces CH₄ as a waste product; in contrast, microorganisms that digest organic matter where oxygen is present produce CO₂. The anaerobic rotting of wood and other organic matter under water produces methane.

An initial pulse of methane is released as the trees, soil carbon, and forest litter covered by the filling of a new dam lake rot. However, this is not the major source of dam methane, which continues to be produced as long as the dam stands. In most tropical areas, rainy seasons alternate with dry seasons. During the dry sea- son, water continues to flow through the dam and the lake water level lowers. This creates large mud flats and shallow zones, because tropical river valleys are generally wide and gently sloping rather than narrow and steep. Water weeds and other soft, quick-growing vegetation flourish in these flats and shallows.

When the rainy season comes, the water level rises again, drowning these plants, which quickly decay underwater, releasing methane. In effect, the rising and falling lake is a vast, solar-powered machine for harvesting carbon from CO₂ in the air and re-releasing it as CH₄ with greatly increased greenhouse warming effect.

In 2005, Philip Fearnside, one of the most-cited researchers on methane production by dams, found that in 1990, 13 years after its lake filled, the Curuá-Una Dam in Brazil emitted 3.6 times more greenhouse gas than oil-burning plants generating the same amount of electricity would have emitted. That is, the dam released enough methane to cause 3.5 times as much global warming as the CO₂ released by the oil plants would have.

Ivan Lima and colleagues, among other scientists, have suggested that at least some of this methane could be collected and burned to produce electricity. This would both extract useful energy and convert the methane (CH₄)to CO₂, greatly reducing its greenhouse potential. Most dams allow deep, high-pressure, methane-rich water to run through their turbines. This causes the release of much of the methane in the water and may account for up to 70% of methane emissions from dam lakes (the rest is emitted from the lake surface). Some or most of this methane might be collected for burning. As of 2007, however, such technologies had not yet been demonstrated on a practical scale.

There are other relationships between dams and climate change. Dams in high latitudes (far from the equator) do not produce large amounts of methane, and so can indeed produce electricity with less greenhouse impact than comparable fuel-burning plants would. This does not reduce their other environmental impacts, however. Counting all dams worldwide, the greenhouse impact of dams is less than half that of the least-polluting fuel-burning alternatives.

Dams are also linked to climate change because they may serve as storage reservoirs in places where precipitation patterns and flood risk may be altered by climate change. Thus, some dams may help with mitigation of and adaptation to the effects of climate change. Finally, some dams may have to contend with decreased rainfall and water flow from changing climate, which might reduce water and energy benefits from those dams.

Impacts and Issues

Reports in the 1990s of significant methane emissions from dams led to intense debate on the climate-change mitigation merits of large dams. In 1997, a workshop was convened by the International Union for the Conservation of Nature and Natural Resources (an 83-state organization headquartered in Geneva, Switzerland) and the World Bank to discuss the question. The two groups established the World Commission on Dams to investigate the effectiveness and costs of dams, including their relationship to climate change.

WORDS TO KNOW

ANAEROBIC BACTERIA: Single-celled creatures that thrive in anaerobic environments, that is, environments lacking free molecular oxygen (O₂). They digest organic matter and release methane, a greenhouse gas.

ANTHROPOGENIC: Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities.

CLEAN DEVELOPMENT MECHANISM: One of the three mechanisms set up by the Kyoto Protocol to, in theory, allow reductions in greenhouse-gas emissions to be implemented where they are most economical. Under the Clean Development Mechanism, polluters in wealthy countries can obtain carbon credits (greenhouse pollution rights) by funding reductions in greenhouse emissions in developing countries.

FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

HYDROELECTRICITY: Electricity generated by causing water to flow downhill through turbines, fan-like devices that turn when fluid flows through them. The rotary mechanical motion of each turbine is used to turn an electrical generator.

METHANE: A compound of one hydrogen atom combined with four hydrogen atoms, formula CH₄. It is the simplest hydro-carbon compound. Methane is a burnable gas that is found as a fossil fuel (in natural gas) and is given off by rotting excrement.

WORLD BANK: International bank formed in 1944 to aid in reconstruction of Europe after World War II, now officially devoted to the eradication of world poverty through funding of development projects. Although ostensibly independent, the bank is always headed by a person appointed by the President of the United States.

In November 2000, the commission released its report. Although stating that “dams have made an important and significant contribution to human development, and benefits derived from them have been considerable,” the report also said that “in too many cases an unacceptable and often unnecessary price has been paid to secure those benefits, especially in social and environmental terms, by people displaced, by communities downstream, by taxpayers and by the natural environment.” The commission estimated that 40–80 million people had been displaced by dams and many species driven to extinction; it also stated that most of the benefits from 1% to 28% of anthropogenic (human-caused) greenhouse emissions might come from dams. A study by Ivan Lima and colleagues stated that 4% of global emissions are from dams.

The World Commission on Dams made a number of recommendations for how proposed dam projects should be judged, including whether people who would be displaced consent to the project. Its findings have been controversial because one of the commission's founders, the World Bank, is the world's largest funder of dam projects, having provided a total of $75 billion (1998 dollars) for 538 large dams in 92 countries since its inception in 1945. The World Bank has not accepted the commission's recommended international standards for dam-building. Europe's two largest public banks, the European Investment Bank and the European Bank for Reconstruction and Development, announced in 2005 that they would abide by the commission's standards. A directive of the European Union mandates that carbon-trading hydropower projects under the Clean Development Mechanism must abide by the commission's standards.

See Also Clean Development Mechanism; Energy Contributions; Methane.

BIBLIOGRAPHY

Periodicals

Fearnside, Philip M. “Do Hydroelectric Dams Mitigate Global Warming? The Case of Brazil's Curuá-Una Dam.” Mitigation and Adaptation Strategies for Global Change 10 (2005): 675–691.

Lima, Ivan B. T., et al. “Methane Emissions from Large Dams as Renewable Energy Resources: A Developing Nation Perspective.” Mitigation and Adaptation Strategies for Global Change (February 2007).

Yardley, William. “Climate Change Adds Twist to Debate Over Dams.” New York Times (April 23, 2007).

Web Sites

“Citizen's Guide to the World Commission on Dams.” International Rivers Network, 2002. < http://www.irn.org/wcd/wcdguide.pdf> (accessed November 13, 2007).

“Climate Change and Dams: An Analysis of the Linkages Between the UNFCCC Legal Regime and Dams.” United Nations Environment Programme, November 2000. < http://www.dams.org/docs/kbase/contrib/env253.pdf> (accessed November 13, 2007).

“Dams and Development: A New Framework for Decision-Making.” World Commission on Dams, June 22, 2007. < http://www.dams.org//docs/report/wcdreport.pdf> (accessed November 13, 2007).

Grahame-Rowe, Duncan. “Hydroelectric Power's Dirty Secret Revealed.” New Scientist, February 24, 2005. < http://www.newscientist.com/article/dn7046.html> (accessed November 13, 2007).

Larry Gilman