The majority of the world’s energy is currently produced by the burning of fossil fuels. It is likely that fossil fuels will continue to be a major contributor to the production of global energy for the foreseeable future. Unfortunately, the burning of fossil fuels produces carbon, which pollutes the atmosphere and adversely affects human health and the Earth’s climate.

Throughout the world, there is growing recognition that fossil fuels should be replaced by other sources of energy that generate little or no carbon; and that this should be done wherever it is physically, economically, and politically feasible. There also is recognition that, where fossil fuels are used, appropriate actions should be taken to reduce the carbon emissions to the lowest practical level. This is significant in cost comparisons of alternative sources of energy, since reducing carbon emissions normally increases the costs of using fossil fuels.

The sources of energy that do not generate carbon pollution are solar, wind, geothermal, hydro-electric, and nuclear. The first three are also classified as renewable energy sources. For all practical purposes, they are inexhaustible and do not need renewal. Hydro-electric requires a dependable supply of flowing water, which is vulnerable to drought and, in the water-energy-food nexus, may compete with agriculture for the available water. Nuclear power does not generate any atmospheric pollution, but the cooling water may become mildly (non-hazardous) radioactive, and disposal of used fuel can be difficult.

Energy from biomass is often classified as renewable. It may be, but its sources may compete for water with food production and may also compete for land and energy.

Solar, wind, and geothermal energy each require specific climate or geophysical conditions. This places limits on their use.

Solar has potential to be the major source of power only in regions where the climate is dry, with few clouds, and no significant periods of overcast and rain. Most of these regions already are significant users of water purification technologies. Using solar power for desalination instead of fossil fuels has great potential to be economically and environmentally beneficial. Saudi Arabia, the largest user of desalination, and the United Arab Emirates, the largest per capita user, have initiated development and demonstration programs to replace fossil fuels with solar power for desalination.

Solar power can also be used as a zero carbon, low operating cost power source to augment more constant power sources in a grid. Solar power also scales down relatively well. It can be used as the major power source for smaller power requirements where a standby diesel generator can be used during temporary periods when weather conditions are not suitable for solar.

Wind energy does not require any water in its production, and can be very useful in reducing overall carbon emissions. However, very few places have nearly constant strong winds, so wind energy is likely to be used to augment more constant forms of energy in a grid, or for specific tasks, such as pumping water.