Environment - Power paradox: Clean might not be green forever

 

Global energy consumption has been rising by around 2 per cent per year this century (Image: Andy Potts)

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 diagram: "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.

So although nuclear fusion could in theory provide an effectively unlimited source of energy, if our energy demand keeps growing we will not be able to use it freely without significantly warming the planet.

It seems Chaisson's mentor, Carl Sagan, was right. "Sagan used to preach to me, and I now preach to my students," says Chaisson, "that any intelligent civilisation on any planet will eventually have to use the energy of its parent star, exclusively." More specifically, they will be limited to the solar energy that is normally absorbed by their planet - anything extra, including space-based solar, is out.

Waste-heat warming

In theory an advanced alien civilisation could produce a lot of waste heat and still maintain a stable climate by using geoengineering to counteract waste-heat warming. On Earth, though, there is probably little scope for reducing greenhouse gas levels much below preindustrial levels, because plants need CO₂. Shading the planet or increasing its reflectivity would be problematic, too.

Chaisson accepts that warming from waste heat is not important now. Nevertheless, he argues that we might as well switch to solar-based energies as soon as possible. "Everyone agrees that something must be done to stop the rise of CO₂ in the near term, and then we need to worry about excess heating of our atmosphere by energy usage in the long term," he says. "My point is that if we can do both at the same time, then why not take the steps now to do just that?"

That's music to the ears of Mark Jacobson of Stanford University in California. He has been pushing an ambitious plan for a wholesale switch to renewable energy by 2030. He envisages wind and solar providing 90 per cent of this (Energy Policy, vol 39, p 1154). Yet on these kinds of scales, even renewable power sources could begin to affect the climate.

Take wind power. In 2010, Somnath Baidya Roy at the University of Illinois in Urbana-Champaign reported that wind farms affect their local climate. Long-term data from a wind farm at San Gorgonio, California, confirmed his earlier model predictions: surface temperatures behind the wind turbines were higher than in front during the night, but as much as 4 °C lower by day.

Roy thinks the turbulence created by the turbines sucks air down from above. During the day, when the hottest air is usually near the surface, this has a cooling effect. At night, when the air near the ground may be colder than that above, it can have a warming effect.

These effects could be minimised by placing wind farms in areas where there's already a lot of turbulence. But we might not want to minimise them. "Some of these effects are actually welcome for agricultural reasons," says Cristina Archer at the University of Delaware in Newark, who studies wind power. Strategically placed wind farms might keep crops cool in summer and reduce the risk of frost in other seasons. Farmers in California and Florida already use wind machines to fight frost by pulling down warmer air.

Do offshore wind farms affect sea surface temperatures and evaporation rates? Could these local effects add up to produce significant regional or even global effects? Perhaps. Winds obviously play a major role in climate. Slowing or altering wind patterns will alter the movement of heat and water around the planet, and thus temperature and rainfall.

It might seem inconceivable that humans could have a significant effect on the wind, but we may already be doing so. While wind speeds over the oceans are increasing, surface winds over Europe, Asia and North America have slowed by up to 15 per cent on average since 1979. At least half of the slowdown is thought to be due to changes in land use, with more vegetation and possibly more buildings making the terrain rougher (Nature Geoscience, vol 3, p 756).

A 2004 study by David Keith of the University of Calgary in Alberta, Canada, suggested that the climatic effects of wind power might start to become apparent at a level of 2 TW. According to Axel Kleidon and Lee Miller of the Max Planck Institute for Biogeochemistry in Jena, Germany, the impact of wind power depends on what proportion of the available power we extract. They recently calculated how much wind energy there is from the top down, starting with the incoming solar radiation that drives the winds by creating temperature differences in the atmosphere. They concluded that at most 68 TW could be extracted. Further modelling suggested there could be as little as 18 TW available - far lower than other estimates.

Even more controversially, the team claimed that extracting all the available wind power would produce big changes in temperature and precipitation. While they are not suggesting the world will warm overall, according to their model the local changes are comparable in magnitude to those associated with a doubling of CO₂.

Even if this conclusion is correct, we are nowhere near to extracting this level of wind power. At the end of 2011, worldwide wind power generation capacity was just 0.2 TW. And many others in the field are extremely sceptical about the team's conclusions. "I don't believe their results," says Archer.

"The idea that [the impact] is on par with doubling of CO₂, that's just nonsense," agrees climate scientist Gavin Schmidt of the NASA Goddard Institute for Space Studies in New York. There will be some impact of large-scale wind-power generation, but Miller's team is overstating it, he says.

According to Archer and Jacobson's bottom-up estimates, which unlike Kleidon's are based on actual measurements of wind speeds, there is 1700 TW of wind power at an altitude of 100 metres over land and sea. Of this, between 72 and 170 TW could be extracted in a practical and cost-competitive manner.

Modelling by Jacobson's team suggests that extracting 11.5 TW of this wind power would reduce the kinetic energy of wind at 100 metres by less than 1 per cent. The effects on temperature and precipitation are so small they cannot be distinguished from natural variability, he says.

Solar cooling

The science is far from settled. Yet even if wind farms do turn out to have significant climatic effects, we might be able to turn this to our advantage. Perhaps carefully placed wind farms could boost rainfall in arid regions, for instance. It might even be possible to use wind power as a form of geoengineering (see "Generate energy, cool the planet"). "Could some of the climatic impacts of near-surface wind power be desirable? Absolutely," says Miller. But this type of research is only beginning, he points out. What is clear, of course, is that every wind farm that goes up means less CO₂ pumped into atmosphere.

Compared with solar power, though, wind resources are relatively small. "I think that there is simply not enough wind energy capturable on Earth to do much good in the long term," says Chaisson. "Nor with water and waves. The only way to endure is to learn how to utilise the sun's energy." Thousands of terawatts of solar power could be generated just using existing technology.

Even solar power can affect climate, though, because solar panels can alter the reflectivity, or albedo, of the surface. One recent study modelled the effects of building a 1-TW solar power plant in the Mojave desert in California. It concluded that placing so many dark solar panels over light-coloured sand will warm the air above by 0.4 °C, affecting temperature and wind patterns within a 300-kilometre radius.

If we develop much more efficient solar panels in the future, though, a similar solar plant would cool the local area. The heat would end up wherever the energy is eventually used. Indeed, even existing solar panels can have a local cooling effect if they are placed over dark surfaces, such as black roofs. "Solar panels will basically take 20 per cent of sunlight and convert it to electricity," says Jacobson. "That cools down your house."

What's more, many other human activities, from building cities to planting crops, alter albedo, and these activities have a much greater impact because they affect a far greater proportion of Earth's surface. Air temperatures in south-eastern Spain have fallen more than 0.6 °C since 1983 because there are so many reflective greenhouses in the area, for instance.

So while the large-scale use of solar power could potentially affect the climate, the effects will be relatively minor so long as we don't capture hundreds of terawatts that would otherwise have been reflected straight back into space. Careful design and placement of solar plants should minimise any negative consequences.

Some regard any discussion of the climatic effects of renewable energy, and waste heat, as a distraction from the far more urgent task of cutting greenhouse gas emissions. But if we do not start thinking about it now, we may one day discover that in trying to solve one climate problem, we have created another.

Generate energy, cool the planet

When we talk of extracting wind energy, it's mainly from wind at an altitude of about 100 metres. But wind speeds increase the higher you go. In the four jet streams that circle Earth more than 10 kilometres up, wind speeds of well over 100 kilometres per hour are typical.
Exploiting this energy will not be easy, not least because of the way the jet streams meander and change location, but several groups are developing ways to do it. Most involve tethered turbines or kites that turn generators on the ground.
According to some estimates, the available energy in the jet streams is about 100 times the current global energy demand. Simulations by Cristina Archer at the University of Delaware in Newark and Ken Caldeira of Stanford University in California suggest that extracting enough energy from high-level winds to meet all our current energy demands would have no significant impact on global climate. But their model suggests that extracting larger amounts would have a big impact. In the extreme case of extracting 1000 TW, mean surface temperatures fell nearly 10 °C, total rainfall decreased by about 35 per cent and sea ice cover doubled (Energies, vol 2, p 307).
The reason, says Caldeira, is that slowing down the high-altitude winds would slow the heat transfer between the equator and the poles. This would cause the equator to warm and the poles to cool, increasing sea ice cover. More sea ice means more heat is reflected from the poles. The end result is that the equator warms slightly, but the poles cool significantly.
This effect might actually be desirable to counteract global warming, given that the Arctic is warming faster than any other area on Earth and losing sea ice fast. So could we deliberately induce it? "This is one of the things we plan to look at in the future," says Caldeira.
However, Axel Kleidon and Lee Miller of the Max Planck Institute for Biogeochemistry in Jena, Germany, claim Archer and Caldeira have massively overestimated the amount of energy that could be extracted. They think the high wind speeds in the jet streams are a result of a near lack of friction, rather than a constant input of energy. As a result, they estimate that only about 7.5 TW of power could be extracted from the jet stream, and that even this would have a major effect on climate (Earth System Dynamics, vol 2, p 201).
From an energy perspective this would be bad news, but it makes cooling the planet this way seem more feasible. According to their model, though, the planet would cool just 0.5 °C, with the Arctic getting 2 °C cooler but the Antarctic warming by 2 °C, among other effects. We will obviously need to have a far better understanding of the changes before we even begin to entertain the notion of geoengineering, Miller says.

Anil Ananthaswamy is a consultant for New Scientist based in Berkeley, California.

http://www.newscientist.com/