Some solutions to climate change involve manipulating clouds around the world. (istockphoto)

Is it time to get serious about geoengineering our way out of climate change?

Dozens of schemes have been devised to cool the planet. We could launch a vast fleet of ships to whiten the clouds by spraying salt mist, or squirt sulfuric acid into the stratosphere to reflect the sun. Send a swarm of mirrors into deep space. Engineer paler crops. Fertilize the oceans. Cover the world’s deserts in shiny mylar. Spread cloud-seeding bacteria. Release a global flock of micro-balloons.

These schemes are ingenious, but would any of them work? Or would they just make things worse? Scientists have begun to explore some of these idea with detailed calculations and computer models, and as the results of such studies mount up, we’re starting to get an idea of what geoengineering might — or might not — be able to achieve.

Some ideas can be dismissed with relative ease. Covering deserts in reflective plastic, for example, might reflect a lot of sunlight and cool the planet somewhat, but it would devastate ecosystems, alter regional climate patterns and require an immense army of cleaners to keep it going.

Others are beyond our powers today. To shade Earth with a swarm of space parasols would require an estimated 20 million rocket launches. Without some radical new technology, that would be astronomically expensive and fatally polluting. “This is complete science fiction,” says Tim Lenton of the University of Exeter in England. “We ought to stop talking about it.”

Many other schemes, such as painting roofs white, are certainly feasible, but can they actually fix the climate? The basic problem, of course, is that rising levels of greenhouse gases in the atmosphere are acting like a blanket around Earth, trapping heat. To stop the warming, any geoengineering scheme either has to block an equivalent amount of incoming heat from the sun or increase heat loss from the top of the atmosphere by that much. It also needs to work without drastically altering regional climates, while also preventing sea levels from rising. Ideally it should also stop the oceans from becoming so acidic that coral reefs vanish.

Does it make a difference?

But the first test is potency. In 2008, Lenton and Nem Vaughan of the University of East Anglia in England combined various model results with their own calculations to assess the potential cooling power of a couple of dozen proposals. “It was born of frustration,” Lenton says. “I had been at one too many workshops where people were advocating their pet technologies and arm-waving about ‘was this more effective than that?’ ”

They found that many schemes would make little difference. Take the idea of making roofs and roads whiter to reflect more sunlight. Even with optimistic assumptions, this could make at best only a minor contribution to restoring Earth’s heat balance.

A seemingly more promising plan is to fertilize the seas. Plankton consume CO2 as they grow, and sometimes their dead bodies sink to the sea floor and get buried, locking this carbon away. Adding nutrients that are in short supply, such as iron, could boost plankton growth. By the end of the century, this could improve the radiation balance by more than the white roofs, Lenton and Vaughan calculated, but that’s nothing like a game-changer — and, again that’s the top-end estimate, which could fall considerably as we learn more about this process.

But two schemes stand out as being both highly potent and relatively feasible. Both involve shading the sun.

One idea is to whiten marine clouds — specifically, the low, flat stratus clouds that cover a large swath of sky. Ships scattered across the world’s oceans would send plumes of fine salt spray up into the air. The idea is that the salt particles would encourage droplets of water to form into clouds. These clouds would be whiter than normal, and they would reflect more sunlight. Potentially, this could offset the entire warming from a doubling in CO2 .

Cloud-whitening has its upsides, such as not involving any hazardous chemicals. But it is not well understood, so it might not work as well as its proponents suggest. A study published this year found that seeding clouds over the Pacific might alter rainfall patterns in a similar way to the highly disruptive La Nina weather phenomenon, for instance.

The other leading contender is an old one: Fill the atmosphere with a haze of fine particles to reflect back sunlight. In fact, we are doing this already. Sulfur dioxide pollution forms fine droplets of sulfuric acid that act as a reflector. But SO2 from fires and factories doesn’t remain in the atmosphere for long, so its effects are limited. If sulfate gets as high as the stratosphere, however, it can linger for years, so its cooling effect is much greater. The proof comes from volcanic eruptions large enough to inject SO2 into the stratosphere. The 1991 eruption of Mount Pinatubo in the Philippines cooled the planet by up to half a degree Celsius over the following couple of years.

$10 billion a year

To balance the warming effect of a doubling in CO2, we would need to pump up to 5 million tons a year of SO2 into the stratosphere. According to Justin McClellan of Aurora Flight Sciences in Cambridge, Mass., whose team evaluated several ways to deliver the sulfates, this would cost about $10 billion per year.

And a sulfur spray may barely slow the seas’ advance. Sulfur droplets do not linger in polar regions as long as they do in the tropics, making them less effective as polar coolants. So even if aerosol injection brought the average global temperature back down to that of the 1800s, the poles would not be as cold as they were and the ice caps would keep melting.

It is not clear whether a different kind of reflector, such as solid metallic particles or tiny, shiny balloons, would be any better. Pumping out a gas is so much simpler and cheaper, so most studies have concentrated on sulfates.

In 2010, Myles Allen of the University of Oxford and his colleagues used a detailed climate model to look at the effect of varying amounts of sunscreen in the stratosphere . They found that there is no solution that works for everyone. An amount of aerosol that would take China close to comfortable preindustrial temperature and rainfall, for instance, might cool India far too much.

Climate models agree fairly well on the global effects of sunshade schemes, but they produce different patterns of regional climate change, which makes it hard to be sure that any particular scheme will have the desired result.

So instead of blocking sunlight, what about geoengineering to actively scrub CO2 from the air? The concentrated gas could then be pumped into underground reservoirs such as depleted gas and oil fields. But no one has devised an efficient method for doing this.

Another idea is to soak up CO2 with silicate rocks. A chemical reaction that has been occuring for millions of years, called weathering, soaks up vast amounts of CO2, which is eventually returned to the atmosphere by volcanoes. But to deal with just a single year’s worth of emissions, we’d need to grind up enough rock to cover several percent of Earth’s land surface. So this process cannot save us, either.

What about modifying land use and agriculture to capture more carbon? Simply planting forests is a help, although Earth’s geography limits its potential, and all that carbon could end up back in the atmosphere if forests die or burn as the planet warms.

Locking away carbon

One way to lock away the carbon stored by plants is to turn them into charcoal, or biochar, and bury it. Another is to burn crops in power plants fitted with carbon-capture technology. These ideas need land, so they will compete with food production, which is a problem as the world’s population continues to grow. So this looks optimistic, barring some breakthrough such as genetically altering plants to enable them to capture more of the sun’s energy.

Carbon-capture schemes, then, can at best slow the pace of warming over the coming century. They will be of no use, though, if we get near a tipping point, such as the widespread dying of forests, the massive release of methane from thawing permafrost or the collapse of the West Antarctic ice sheet. So is it possible to keep the potent but risky schemes such as sulfur sprays in reserve for the direst circumstances?

Lenton, who helped to define the notion of tipping points in a paper in 2008, is skeptical. “People say that is why we need solar reflection in our back pocket, but they haven’t proved you could get early warning of a tipping point, or deploy in time, or that these schemes would not cause other tipping points,” he says.

Tim Palmer of the University of Oxford says, “You shouldn’t think of this as a magic button that you can press if things get out of control: It may turn out to be a bit of a nightmare.”

Battersby is a consultant for New Scientist, from which this article was excerpted.