Hydroelectric dams

Background and context
Dams have existed for thousands of years for various purposes, such as irrigation and flood-control. In the 19th century, they began to be used for hydroelectricity, and by the 20th century, they had become a mainstay of electricity generation. As of 2005, hydroelectric power, mostly from dams, supplied 19% of the world's electricity, and over 63% of renewable energy. Yet, dams have been the subject of significant controversy among environmentalists in recent decades, mainly due to the local environmental costs of dams to river ecosystems.
Largely for this reason, new dam construction has wained, and dams have been de-emphasized as a renewable energy source. With the emergence of the global warming crisis, though, hydroelectric dams, which generate massive quantities of electricity without burning fossil fuels, have received attention once again. The main question and debate is whether governments should include, prioritize, and even subsidize modern hydroelectric dams in 21st-century plans to combat global warming? Multiple questions frame this debate: Can dams help substantially reduce greenhouse gas emissions and combat global warming? Are dams carbon neutral? While hydroelectric generation does not itself release greenhouse gases into the atmosphere, what about the energy used in constructing dams as well as the reservoir created behind a dam (deforestation and decomposition)? Do dams generate enough 0-emission electricity to have a large-scale impact in cutting emissions? Can hydroelectricity grow to help replace coal? Are dams "renewable", or do they have a limited life on rivers? How do dams impact river ecosystems? Are hydroelectric dams economical, or do they require significant subsidization? How do dams compare to other electricity sources on price? Do dams provide needed irrigation waters to farms? Do they supply needed water to surrounding populations? What human rights issues are involved in the construction of dams and the frequent displacement of local peoples? Are dams aesthetic? Do they provide recreational space? Finally, does the political and popular will exist to make hydroelectric dams a major component of plans to combat global warming?
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Yes
•Dams cut emissions and fight main crisis: global warming Many environmentalists complain that hydroelectric dams have environmental costs at the local level to river ecosystems. While this is true, it is important to recognize that hydropower is essentially 100% clean when it comes to global warming. This is because it burns no fossil fuels and so emits no greenhouse gases. If we conclude that global warming is the greatest environmental crisis facing mankind, the fact that hydropower helps fight it should weigh much more heavily than the local ecosystem costs.
•Steps can be taken to limit Methane emissions from dam reservoirs A 2000 World Commission on Dams report concluded: "All large dams and natural lakes in the boreal and tropical regions that have been measured emit greenhouse gases… some values for gross emissions are extremely low, and may be ten times less than the thermal option. Yet in some cases the gross emissions can be considerable, and possibly greater than the thermal alternatives".[1] This indicates that dams can be created that emit "ten times less than the thermal option". The methods used to create these clean dams can and should be followed, can eliminte concerns regarding Methane emissions, and can preserve hydroelectric power as a global warming solution.
•Hydropower is a clean alternative to coal. Coal is the greatest source of electricity in the world. In places like the United States, it constitutes over 50% of the electric energy supply. It is also one of the greatest emitters of greenhouse gases and, therefore, contributors to global warming. Replacing coal, therefore, is one of the greatest priorities in the fight against global warming. Hydrolectric power, as a primary electricity supplier around the world, can act decisively as a substitute to coal electricity generation. Because hydroelectric emits 0 greenhouse gases, it is a highly valuable substitute for coal, and thus a major tool in the fight against global warming.
•Hydropower is capable of scaling to fully replace coal. The World Energy Council Hydropower Data shows that hydropower is presently generating about 3,000 TWh (Terrawatt hours, or millions of megawatt hours annually). If we include both high head (hydropower dams) and low head (run-of-the-river micro plants) systems using current technologies, the world can probably economically generate almost 15,000 TWh annually.[2] This means that hydropower could move from its current position of supply roughly 10% of all electricity demand globally to supplying upwards of 50%. Some argue that "we have enough economically usable river water resources to generate virtually 100% of current global electrical demand."[3] This is a very important conclusion, as it means that hydropower is a viable long-term 0-emissions alternative to coal, the biggest culprit in the global warming crisis.
Hydropower is more proven/reliable than other renewables Unlike the many uncertainties and risks surrounding modern renewable and green alternatives, there are no uncertainties in regards to the technologies used in hydropower, the price-competitiveness of it, and the electricity return for any given dam project. In the face of the global warming crisis, the reliability of hydroelectricity as a 0-emission alterntive source of energy should be embraced.
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No
•Dam reservoirs release large amounts of greenhouse gas Methane "Pros and cons of hydropower dams". Energy consumers edge - "hydropower dams in forested areas can actually produce more greenhouse gasses than fossil fuel plants of the same output. If vegetation is left in place when the lake floods, the dead material will emit methane as it decays. This process can go on for years, indefinitely in areas that are intermittently flooded and dried. The report also states that many hydropower dams also eliminate vast forested areas that had been removing CO2 from the atmosphere."
•Dams eliminate forests that absorb CO2 from the atmosphere Forests and plant life are important carbon sinks, sucking CO2 (a greenhouse gas and contributor to global warming) from the air in the process of photosynthesis. Forests are, therefore, important in the fight against global warming. Killing trees by cutting them down to make room for dams and their infrastructure, by flooding them upstream under a dam's reservoir, or by diminishing downstream nutrient flows all eliminates these important carbon sinks. This only contributes to the greenhouse gas effect and global warming.
•Constructing dams burns massive quantities of fossil fuels Dams are massive steal and concrete objects constructed in remote areas. The amount of energy required in creating and transporting steal and pouring concrete for dams is unparalleled by any other modern engineering endeavor. Fossil fuel is the primary fuel used to power the equipment and vehicles that perform these tasks. Dam construction, therefore, emits massive quantities of greenhouse gases and contributes mightily to global warming.
•The world's rivers are covered with dams
room for expansion is limited. Sixty per cent of the length of the world's large river systems are at least moderately or severely fragmented by dams. In this context, there is not much more room for expanding hydroelectric power, particularly if we value keeping a portion of the world's rivers free of dams. This means that dam hydroelectricity cannot expand very much, and so should not be seen as a major part of new energy plans and solutions to global warming.[4]
•Industrialized countries have tapped most of their hydropower. Industrialized countries have tapped most of their hydroelectric potential. The expansion of it is mainly in un-industrialized countries. This is an important limitation to the future of hydroelectricity.
•Mass of dam reservoirs has shifted Earth's gravitational field. The amount of water captured in dams has shifted the weight of the earth toward the equator which in turn effects the earth's gravitational field. This has the potential to destabilize the earth generally, and possibly in ways that interact negatively with global warming.