Fig. 8.4. Polluter-emission of traffic in Hungary (1999 = 100%) (www.kti.hu)

Carbon dioxide, which is released by the combustion of fossil fuels, is not a directly harmful gas, but it does increase the CO₂ concentration in the earth's atmosphere. The solar radiation reflected on the surface of the earth is accordingly absorbed by the air, causing the air to heat up (the "greenhouse effect"). Unlike the other air pollutants, emission of the carbon dioxide responsible for the "greenhouse effect" cannot be reduced by catalytic converters.

The following causes of air pollutant emissions are of particular importance:

- inadequately maintained and elderly vehicles with increased pollutant output (carburettors, ignition, fuel injection systems, exhaust systems)

- deficient technical inspection and supervision of both fuel production and vehicles (emission tests)

- low quality fuels (unfavourable combustion processes)

The intensity of air pollutionin a given area depends on:

- the volume of traffic,

- the ratio of internal combustion to diesel engines,

- driving habits (speed and gear selection as well as acceleration and braking behaviour),

- the condition of the engines,

- the quality of the fuel used,

- climatic and topographical conditions (air renewal).

b, Urban runoff from roads and other impervious surfaces is a major source of water pollution too. Rainwater and snowmelt running off of roads tends to pick up gasoline, motor oil, heavy metals, trash and other pollutants. Road runoff is a major source of nickel, copper, zinc, cadmium, lead and polycyclic aromatic hydrocarbons (PAHs), which are created as combustion byproducts of gasoline and other fossil fuels.

Road salts (primarily chlorides of sodium, calcium or magnesium) can be toxic to sensitive plants and animals. Sand can alter stream bed environments, causing stress for the plants and animals that live there.

c, Finally, roads can act as barriers or filters to animal movement and lead to habitat fragmentation. Many species will not cross the open space created by a road due to the threat of predation and roads also cause increased animal mortality from traffic. This barrier effect can prevent species from migrating and recolonising areas where the species has gone locally extinct as well as restricting access to seasonally available or widely scattered resources.

Habitat fragmentation may also divide large continuous populations into smaller more isolated populations. These smaller populations are more vulnerable to genetic drift, inbreeding depression and an increased risk of population decline and extinction.

8.3. 8.3. Nuclear energy

Nuclear power stations use nuclear fission to generate energy by the reaction of U²³⁵ inside a nuclear reactor. The reactor uses uranium rods, the atoms of which are split in the process of fission, releasing a large amount of energy. The process continues as a chain reaction with other nuclei. The energy heats water to create steam, which spins a turbine generator, producing electricity.

Stated estimates for fission fuel supply at known usage rates vary vastly, from several decades to billions of years; among other differences between the former and the latter estimates, some assume usage only of the currently popular U²³⁵, and others assume the factor of a hundred fuel efficiency increase which would come from utilizing U²³⁸ through breeder reactors. At the present rate of use, there are about 70 years left of presently inventoried U²³⁵ reserves. The nuclear industry argues that the cost of fuel is a minor cost factor for fission power; if needed, more expensive, more difficult to extract sources of uranium could be used in the future, such as lower-grade ores, and, if prices increased enough, from sources such as granite and seawater. Increasing the price of uranium would have little effect on the overall cost of nuclear power; a doubling in the cost of natural uranium would increase the total cost of nuclear power with typical present reactors by 5 percent. On the other hand, if the price of natural gas was doubled, the cost of gas-fired power would increase by about 60 percent.

Opponents on the other hand argue that the correlation between price and production is not linear, but as the ores' concentration becomes smaller, the difficulty (energy and resource consumption are increasing, while the yields are decreasing) of extraction rises very fast, and that the assertion that a higher price will yield more uranium is overly optimistic. As many as eleven countries have depleted their uranium resources, and only Canada has mines left that produce better than 1% concentration ore. Some state uranium from seawater is dubious as a source.

Nuclear meltdowns and other reactor accidents, such as the Fukushima I nuclear accident (2011), Three Mile Island accident (1979) and the Chernobyl disaster (1986), have caused much public concern. Research is being done to lessen the known problems of current reactor technology by developing automated and passively safe reactors.

The long-term radioactive waste storage problems of nuclear power have not been solved. Several countries have considered using underground repositories. Nuclear waste takes up little space compared to wastes from the chemical industry which remain toxic indefinitely. Spent fuel rods are now stored in concrete casks close to the nuclear reactors. The amounts of waste could be reduced in several ways. Both nuclear reprocessing and breeder reactors could reduce the amounts of waste. Subcritical reactors or fusion reactors could greatly reduce the time the waste has to be stored. Subcritical reactors may also be able to do the same to already existing waste. The only long-term way of dealing with waste today is by geological storage.

At present, nuclear energy is in decline, according to a 2007 World Nuclear Industry Status Report presented by the Greens/EFA group in the European Parliament. The report outlines that the proportion of nuclear energy in power production has decreased in 21 out of 31 countries, with five fewer functioning nuclear reactors than five years ago. There are currently 32 nuclear power plants under construction or in the pipeline, 20 fewer than at the end of the 1990s.

8.4. 8.4. Hydropower

Hydropower or water power is power derived from the energy of falling water and running water, which may be harnessed for useful purposes. Since the early 20th century, the term is used almost exclusively in conjunction with the modern development of hydro-electric power, which allowed use of distant energy sources. Another method used to transmit energy used a trompe, which produces compressed air from falling water. Compressed air could then be piped to power other machinery at a distance from the waterfall. Hydro power is a renewable energy source.

Water's power is manifested in hydrology, by the forces of water on the riverbed and banks of a river. When a river is in flood, it is at its most powerful, and moves the greatest amount of sediment. This higher force results in the removal of sediment and other material from the riverbed and banks of the river, locally causing erosion, transport and, with lower flow, sedimentation downstream.

Some hydropower systems such as water wheels can draw power from the flow of a body of water without necessarily changing its height. In this case, the available power is the kinetic energy of the flowing water. Over-shot water wheels can efficiently capture both types of energy.

The water flow in a stream can vary widely from season to season. Development of a hydropower site requires analysis of flow records, sometimes spanning decades, to assess the reliable annual energy supply. Dams and reservoirs provide a more dependable source of power by smoothing seasonal changes in water flow. However reservoirs have significant environmental impact, as does alteration of naturally occurring stream flow. The design of dams must also account for the worst-case, "probable maximum flood" that can be expected at the site; a spillway is often included to bypass flood flows around the dam. A computer model of the hydraulic basin and rainfall and snowfall records are used to predict the maximum flood (Picts. 8.2., 8.3.).