These options reduce f (the fraction of primary energy from fossil fuel) in equation (3). Hydropower
At present about 19 percent of the world's electricity is produced from hydropower (McCarthy et al. 2001). Although this source is reaching saturation in developed countries, continued deployment is likely in many developing countries. Hydropower is a renewable energy source with important benefits, such as flood control and regulation of river flows for agricultural, industrial, and urban use. From the power-engineering viewpoint, hydropower is fast to start in peak electricity consumption periods and can store surplus energy from other sources during low consumption periods, thus increasing overall system efficiency. There can, however, also be a range of negative sociocultu-ral, environmental, economic, and ancillary greenhouse-gas consequences. During dam construction and first filling, few or many (up to millions) of people are displaced from their homes, often leading to impoverishment (World Commission on Dams 2000). Dams alter downstream nutrient cycling patterns, ecosystems, and food-web structures, reduce biodiversity in rivers, affect the viability of fishing industries, and accumulate sediment. WBGU (2003) suggests that at least 20 percent of global rivers should remain undammed. From the greenhouse-gas perspective, the global production of greenhouse gases from hydropower systems is relatively low, but lakes in vegetated areas can incur significant methane production in anoxic sediments (McCarthy et al. 2001).
The solar option has low sociocultural and environmental impacts compared with many other energy systems. Available energy densities are moderate, about 15 W per square meter (m-2) for electric power (Hoffert et al. 2002), so that current global electricity consumption could be met from a square less than 400 kilometers on a side (WBGU 2003). The resource is most concentrated in subtropical semiarid to arid areas, implying development opportunities in these areas, which are largely in the developing world. Extensive decentralization is also possible and is very efficient in some sectors (such as solar hot water), leading to substantial social benefits and microeco-nomic opportunities. On the cost side, large-scale solar energy deployment may involve significant land use changes with consequences for ecosystems; water supply may be an issue in arid regions; energy transport over large distances and across national borders is both a technical and an institutional hurdle; and environmental issues can arise in the production and disposal of photovoltaic cells (silica crystal with traces of heavy metals).
Like solar energy, wind power is considered a clean energy. Its technology is rapidly deployable to many suitable regions around the world, making it the world's fastest growing energy source at present. Its energy density is comparable to solar, but areas for deployment are more restricted. Wind power has very limited negative effects on the environment. The largest concern is probably the visual impact of large-scale wind farms with turbines up to 200 meters tall on mountain ridges or in coastal regions with aesthetic and cultural values. Wind turbines, especially older models, can be noisy. A controversial impact is that on bird communities through killings by turbine blades.
An important option is the substitution of biofuels for fossil fuels, both for transport and for electricity generation from biofuels alone or with fossil fuels in co-fired plants. Biofuels have minimal net carbon emissions because atmospheric CO2 is continuously recycled. The technical potential is large: up to 500 megahectares (Mha) of land (around 3 percent of the global land area) could be made available for biofuel crop production by 2100 (Watson et al. 2000b). This amount would produce in the range of 150-300 EJ y-1 and displace 2-5 petagrams (Pg) C y-1 of carbon emissions. Biofuel production offers many ancillary benefits: with good design, soil erosion can be reduced and water quality improved through increased vegetation cover. From a national viewpoint, dependence on imported oil can be reduced, rural investment increased, work opportunities created, and agricultural products diversified. There are few negative impacts when biomass sources are by-products (organic waste and residues) from agriculture and forestry, though transport infrastructure is an issue because of low energy densities (0.3—0.6 W m-2). The large-scale production of dedicated biofuels can, however, have negative impacts, including competition for other land uses, primarily food production; soil sustainability (extraction of residues should not exceed 30 percent of total production to maintain soil fertility); poor resilience of monocultural plantations; and the implications for biodiversity and amenity (Watson et al. 2000b). The full greenhouse gas balance of biofuel production also needs to account for significant emissions from mechanization, transport, use of fertilizers, and conversion to usable energy. Taking into account some of these constraints, especially competition for other land uses, Cannell (2003) estimated the range 0.2-1 PgC y-1 to be a more conservative achievable mitigation capacity for C substitution using biofuels. This amount is about 15 percent of the potential mitigation described earlier. Other estimates are even lower (WBGU 2003).
Geothermal power is a niche option with small mitigation potential but significant environmental benefits where available because of the clean energy yield. There are some environmental concerns over the depletion of geothermal resources.
Nuclear energy is a non-fossil-fuel source with significant mitigation potential. Environmental concerns include waste and plant (old reactor) disposal and the potential for accidents. There is also a major sociocultural or political issue with weapons proliferation, especially where breeder reactors are used to create additional fissile material. This issue is too large to be explored here. It is notable that the UNFCCC Conference of Parties (COP 7 and 8) has agreed not to include nuclear power in the Kyoto Protocol flexible mechanisms.
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