Another energy source that has been harnessed by humanity for many centuries is the power of flowing water, which has been used for milling corn, pumping and driving machinery. During the twentieth century, its main use has been in the generation of hydroelectricity, and hydropower has grown to become one of the world's principal electricity sources. It currently provides some 2.3 per cent of world primary energy. However, the relative contribution of hydroelectric power (and of other electricity-producing renewables) is under-stated by a factor of about three in most national and international compilations of energy statistics. This is due to a convention whereby the heat produced by thermal power stations (both fossil and nuclear-fuelled) is included as part of their primary energy contribution, even though that heat is normally wasted. The annual electricity outputs of the world's hydro-and nuclear power plants are actually about the same, but due to this statistical convention the primary energy contribution of nuclear is calculated to be about 7 per cent whereas that of hydro is only about one-third of this. Hydropower in 2000 contributed over 17 per cent of world electricity.
The original source of hydroelectric power is solar energy, which warms the world's oceans, causing water to evaporate from them.
In the atmosphere, this forms clouds of moisture which eventually falls back to earth in the form of rain (or snow). The rain flows down through mountains into streams and rivers, where its flow can be harnessed using water wheels or turbines to generate power.
When harnessed on a small scale, hydropower creates few, if any, adverse environmental impacts.
However, many modern hydro installations have been built on a very large scale, involving the creation of massive dams and the flooding of extremely large areas. This often entails the re-location of many thousands of indigenous residents who are usually, to say the least, reluctant to move from their homes. Other impacts include adverse effects on fish and other wildlife, reductions in water-borne nutrients used in agriculture downstream, increases in water-borne diseases – and not least, the rare but catastrophic effects of dam failures. A further problem with large dams is that in certain circumstances trees and other vegetation trapped below water when a reservoir is flooded can decay 'anaerobically' (i.e. in the absence of oxygen). This produces methane which, as we have seen, is a more powerful greenhouse gas than the CO2 that would have been produced if the tree had decayed normally in the presence of oxygen from the atmosphere.
However, the current consensus is that greenhouse gas emissions from hydropower generation are likely to be at least an order of magnitude lower than those from fossil-fuel-generated electricity (United Nations, 2000).