Where the electricity comes from to power the next generation of ground vehicles is also vitally important. Electric cars charging up using coal power will deliver little or no benefit to the climate. The best solution will be a dramatically upscaled combination of renewables and nuclear. Of course, the land-use implications of different energy options remain an issue – and here nuclear wins hands down. The reason why is basic: renewables work by harvesting diffuse energy like sunlight and wind over large areas, whilst nuclear fission delivers prodigious amounts of energy from tiny amounts of source material. Compared weight-by-weight, uranium 235 delivers a million times more energy than coal, which itself already represents chemical energy in a highly concentrated form. Just how much energy nuclear fission releases is described by Einstein’s famous equation E=MC2, where E is energy, M is mass and C the speed of light, about 300 million metres per second. Clearly even with a very small amount of fissionable material, multiplying it by the square of 300 million yields a very big number.
My argument is not in favour of nuclear and against renewables, however – both zero-carbon energy options, and various others, are essential to tackling climate change. Renewables can often be deployed in ways that minimise land-use and biodiversity impacts. Where building-mounted solar photovoltaics are cost-effective, no additional land – and therefore wildlife habitat – is used at all. Even the energy-hungry US could in theory supply all of its electricity use, according to one study, using solar PV on an area equivalent to 0.6 per cent of the country – just a fraction of the existing developed and urban space.
Sex and the city
Rural depopulation and urbanisation in developing countries are often decried by those who are concerned about the relentless expansion of megacities, which seem terribly unsustainable because of their noise, sprawling slums, congestion and pollution. But from the perspective of sustainable land use and habitat protection, the more that growing numbers of people can be persuaded to herd themselves into relatively small areas of urban land, the better for the environment.
In many parts of the world, if you want to marry the person you choose, be gay, be female and economically successful, or avoid daily backbreaking labour carrying water or fetching firewood, then you probably need to move to the city. In 1975 there were just three megacities of over 10 million people. Today there are 21. It sounds scary, but this unstoppable shift towards urbanisation actually ranks as one of the most environmentally beneficial trends of the last few decades. As the UN Population Fund wrote in a recent report: ‘Density is potentially useful. With world population at 6.7 billion people in 2007 and growing at over 75 million a year, demographic concentration gives sustainability a better chance. The protection of rural ecosystems ultimately requires that population be concentrated (pdf) in non-primary sector activities and densely populated areas.
City living is seldom lauded by environmentalists, but it may be our most environmentally friendly trait as a species, because urban dwelling is vastly more efficient than living in the countryside. Shops and other services are more concentrated in town and city neighbourhoods, and urban residents are much more likely to use public transport, share heating and housing, and have lower carbon footprints than their rural brethren. Given the scale of global population growth, the challenge still seems daunting: the world will need to accommodate 2 billion more urban dwellers (pdf) by 2030, a rate of expansion equivalent to building about 13 great cities (each with over 5 million inhabitants) per year, almost all in developing countries.