Power paradox: Clean might not be green forever

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As energy demand grows, even alternative energy sources such as wind, solar and nuclear fusion could begin to affect the climate

"A better, richer and happier life for all our citizens." That's the American dream. In practice, it means living in a spacious, air-conditioned house, owning a car or three and maybe a boat or a holiday home, not to mention flying off to exotic destinations.

The trouble with this lifestyle is that it consumes a lot of power. If everyone in the world started living like wealthy Americans, we'd need to generate more than 10 times as much energy each year. And if, in a century or three, we all expect to be looked after by an army of robots and zoom up into space on holidays, we are going to need a vast amount more. Where are we going to get so much power from?

It is clear that continuing to rely on fossil fuels will have catastrophic results, because of the dramatic warming effect of carbon dioxide. But alternative power sources will affect the climate too. For now, the climatic effects of "clean energy" sources are trivial compared with those that spew out greenhouse gases, but if we keep on using ever more power over the coming centuries, they will become ever more significant.

While this kind of work is still at an early stage, some startling conclusions are already beginning to emerge. Nuclear power - including fusion - is not the long-term answer to our energy problems. Even renewable energies such as wind power will have to be used with caution, because large-scale extraction could have both local and global effects. These effects are not necessarily a bad thing, though. We might be able to exploit them to geoengineer the climate and combat global warming.

There is a fundamental problem facing any planet-bound civilisation, as Eric Chaisson of the Harvard Smithsonian Center for Astrophysics in Cambridge, Massachusetts, points out. Whatever you use energy for, it almost all ends up as waste heat.

Much of the electrical energy that powers your mobile phone or computer ends up heating the circuitry, for instance. The rest gets turned into radio waves or light, which turn into heat when they are absorbed by other surfaces. The same is true when you use a mixer in the kitchen, or a drill, or turn on a fan - unless you're trying to beam radio signals to aliens, pretty much all of the energy you use will end up heating the Earth.

We humans use a little over 16 terawatts (TW) of power at any one moment, which is nothing compared with the 120,000 TW of solar power absorbed by the Earth at the same time. What matters, though, is the balance between how much heat arrives and how much leaves (see "Earth's energy budget"). If as much heat leaves the top of the atmosphere as enters, a planet's temperature remains the same. If more heat arrives, or less is lost, the planet will warm. As it does so, it will begin to emit more and more heat until equilibrium is re-established at a higher temperature.

See diagramm: "Earth's energy budget"

Over the past few thousand years, Earth was roughly in equilibrium and the climate changed little. Now levels of greenhouse gases are rising, and roughly 380 TW less heat is escaping. Result: the planet is warming.

The warming due to the 16 TW or so of waste heat produced by humans is tiny in comparison. However, if humanity manages to thrive despite the immense challenges we face, and keeps on using more and more power, waste heat will become a huge problem in the future. If the demand for power grew to 5000 TW, Chaisson has calculated, it would warm the planet by 3 ?C.

This waste-heat warming would be in addition to the warming due to rising CO₂ levels. What's more, since this calculation does not take into account any of the feedbacks likely to amplify the effect, well under 5000 TW may produce this degree of warming.

Such colossal power use might seem implausible. Yet if our consumption continues to grow exponentially - it has been increasing by around 2 per cent per year this century despite rising prices - we could reach this point around 2300.

Chaisson describes his work as a "back of the envelope" calculation done in the hope someone would prove him wrong. So far no one has. On the contrary, preliminary modelling by Mark Flanner of the National Center for Atmospheric Research in Boulder, Colorado, suggests that waste heat would cause large industrialised regions to warm by between 0.4 ?C and 0.9 ?C by 2100, in agreement with Chaisson's estimates (Geophysical Research Letters, vol 36, p L02801). Normal climate models do not include the waste-heat effect.

Does this mean human civilisation has to restrict itself to using no more than a few hundred terawatts of energy? Not necessarily. It depends on where the energy comes from. If you turn the sun's energy into electricity and use it to boil your kettle, it won't make the planet any warmer than if that same energy had instead gone into heating up the tiles on your roof. But if you boil your kettle using energy from fossil fuels or a nuclear power plant, you are adding extra heat. "The only energy that is not going to additionally heat the Earth is solar and its derivatives," says Chaisson, referring to sources driven by the sun's heat - wind, hydro and waves.

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