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Africa is the world's fastest urbanising continent. In 1950, sub-Saharan Africa had no cities with populations of more than 1m. Today, it has around 50. By 2030, over half of the continent's population will live in cities, up from around a third now. The fastest growing metropolises, such as Nairobi, Kenya's capital, are expanding at rates of more than 4% per year. That is almost twice as fast as Houston, America's fastest-growing metropolis.

Dwarfed by any of the north African nations, it represents an area slightly smaller than Wales but scientists claimed yesterday it could one day generate enough solar energy to supply all of Europe with clean electricity. Speaking at the Euroscience Open Forum in Barcelona, Arnulf Jaeger-Walden of the European commission's Institute for Energy, said it would require the capture of just 0.3% The scientists are calling for the creation of a series of huge solar farms - producing electricity either through photovoltaic cells, or by concentrating the sun's heat to boil water and drive turbines - as part of a plan to share Europe's renewable energy resources across the continent. A new supergrid, transmitting electricity along high voltage direct current cables would allow countries such as the UK and Denmark ultimately to export wind energy at times of surplus supply, as well as import from other green sources such as geothermal power in Iceland. Energy losses on DC lines are far lower than on the traditional AC ones, which make transmission of energy over long distances uneconomic. The grid proposal, which has won political support from both Nicholas Sarkozy and Gordon Brown, answers the perennial criticism that renewable power will never be economic because the weather is not sufficiently predictable. Its supporters argue that even if the wind is not blowing hard enough in the North Sea, it will be blowing somewhere else in Europe, or the sun will be shining on a solar farm somewhere.