HYDROPOWER Is the leading source of renewable energy. It relies on rainfall (and snowmelt) that drains into rivers and flows to dams downstream. There water is channelled to turn a turbine, producing energy in a generator - the amount being determined by the flow or drop of the water. Once a dam is built, hydropower is a very cheap source of energy - with added advantages of no carbon dioxide emissions, or vulnerability to fuel price. But it can attract criticism over its impact on the local environment and communities. In 2003, hydropower produced almost 16% of electricity generated worldwide. One country, Norway, produces almost 99% of its domestic electricity from hydro stations.

WIND Wind power is currently the second most successful renewable source after hydroelectric, although it is more expensive per unit of electricity produced than fossil fuels. The tower-mounted three-blade turbines, typically tens of metres in diameter, are emission-free and quick to install. They can be used onshore or offshore, but production fluctuates according to wind. And critics complain of their obtrusiveness in the landscape. Europe leads the way in wind power.