EVEN with the best will in the world, reducing our carbon emissions is not going to prevent global warming. It has become clear that even if we take the most drastic measures to curb emissions, the uncertainties in our climate models still leave open the possibility of extreme warming and rises in sea level. At the same time, resistance by governments and special interest groups makes it quite possible that the actions advocated by climate scientists might not be implemented soon enough.
Fortunately, if the worst comes to the worst, scientists still have a few tricks up their sleeves. For the most part they have strongly resisted discussing these options for fear of inviting a sense of complacency that might thwart efforts to tackle the root of the problem. Until now, that is.
A growing number of researchers are taking a fresh look at large-scale “geoengineering” projects that might be used to counteract global warming. “I use the analogy of methadone,” says Stephen Schneider, a climate researcher at Stanford University in California who was among the first to draw attention to global warming. “If you have a heroin addict, the correct treatment is hospitalisation, therapy and a long rehab. But if they absolutely refuse, methadone is better than heroin.”
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Basically the idea is to apply “sunscreen” to the whole planet. It’s controversial, but recent studies suggest there are ways to deflect just enough of the sunlight reaching the Earth’s surface to counteract the warming produced by the greenhouse effect. Global climate models show that blocking just 1.8 per cent of the incident energy in the sun’s rays would cancel out the warming effects produced by a doubling of greenhouse gases in the atmosphere. That could be crucial, because even the most stringent emissions-control measures being proposed would leave us with a doubling of carbon dioxide by the end of this century, and that would last for at least a century more.
The concept of a technological fix is not new (New Scientist, 27 March 2004, p 26). While most remedies have focused on combating greenhouse gases themselves – finding ways to remove them from the air or scrub them from power-plant emissions – only recently have more radical proposals been taken seriously. Last November, a group of researchers from around the US gathered at Stanford University for a workshop to discuss how to reduce the sun’s energy input to the atmosphere, and last year NASA awarded a research grant for one such approach. These small steps suggest that a concept that once was considered heresy is gaining credence.
There are two classes of proposals: reflecting away sunlight within the Earth’s atmosphere, or blocking it in outer space (see Illustration). Each approach has its supporters and detractors. While tinkering with the atmosphere is likely to be much cheaper and simpler, space-based approaches may be longer-lasting and less likely to cause unwanted side effects – though they are much more technically challenging.
The simplest method proposed has been known for decades. That is to inject sulphur dioxide into the upper atmosphere, mimicking the cooling effects of volcanoes. Sulphur is cheap, and the means of releasing it could be as simple as pumping it through a vertical pipe as much as 10 kilometres long to reach the stratosphere. Sulphur dioxide forms sulphate particles that are big enough – about 0.1 micrometres across – to block part of the incoming sunlight, but small enough to allow infrared wavelengths – the heat radiation from the Earth – to escape back into space.
“What’s really important is the size of the particles,” says Stanford physicist Ken Caldeira of the Carnegie Institution’s Department of Global Ecology, who organised last year’s workshop. “It doesn’t matter what they are.” What’s more, Caldeira says, the method is potentially easy and cheap. The amount of particles needed to balance greenhouse gases “would be small enough that a single fire hose could spray into the atmosphere all you would need. This might be a thousand times less costly than transforming energy systems,” he says. “The cheapness, and lack of an international cooperative model for reducing emissions, make it almost too attractive.”
Since sulphur is naturally present in the upper atmosphere as a result of volcanic eruptions, some researchers think the effects of an increase in high-altitude sulphur might not present as many unforeseen risks as some other suggested remedies for global warming, such as seeding the ocean with iron filings or other nutrients to encourage the growth of carbon-consuming organisms.
The approach is not risk-free, however. Anything we do within the Earth’s atmosphere might have unpredictable side effects that turn out to be worse than the cure, such as dramatic changes in regional rainfall or drought patterns, or chemical reactions that might disrupt ecosystems. The sulphur would also require constant replenishment. “If you inject it near the poles in spring, it would wash out next winter, whereas at the equator it might last five years,” Caldeira says. Either way, to depend on such a strategy means you’re committing yourself for centuries, he says.
These drawbacks have driven others to look seriously at larger-scale, more expensive approaches that might carry fewer risks. One that might do the trick is a space-based sunshade system. It may sound wildly implausible but, with a grant from NASA, a team of astronomers and space scientists has been studying the idea and performing small-scale experiments. The concept is simple, though daunting in scale. Roger Angel at the University of Arizona, Tucson, the astronomer responsible for producing many of the world’s largest telescope mirrors, has worked out some of the details and is convinced that it is feasible ().
Dimming with discs
His idea is to manufacture discs of silicon about 60 centimetres across, just a few micrometres thick and weighing about 1 gram using conventional chip-fabricating technology. Each disc would be studded with holes of precisely calculated sizes, close to the wavelengths of visible light, which would scatter incoming light like a lens. The effect would be to produce a slight but imperceptible dimming of sunlight.
The discs would be packed into metal containers in stacks of a million and launched into space using electromagnetic rail guns – a propulsion method that has been tested in labs but never actually used. The containers would be propelled along nearly vertical tracks by magnetic coils activated in sequence, pulling each container along at increasing speed until it reached escape velocity.
It’s the same principle that propels maglev trains, only applied upwards. The acceleration is far too rapid for launching people or delicate equipment, but the method has long been proposed for shooting bulk material into space, such as water, rocket fuel or building materials. It could be cheaper and more reliable than traditional rockets.
Once in space, the containers would travel to the Earth’s “L1” point – the place between the Earth and sun where their gravitational fields cancel out, allowing objects to remain stationary relative to the two bodies. This is where the discs would be released. Each one would have movable fins that could be powered by electricity generated by built-in solar cells and controlled via radio receivers. Like a rudder in a current, the mirrored fins would allow the disc to adjust its position and orientation. Angel thinks these discs could be kept in place for 50 years or more, unlike dust clouds – another proposed method – which would get blown around and need replenishment.
Together with Pete Worden of NASA’s Ames Research Center, who developed the experimental DC-X rocket, Angel has calculated that 20 rail guns, each 3 kilometres high, working round the clock and launching one bundle of discs every 5 minutes for 10 years, could put enough “pico-satellite” discs into space to provide the required 1.8 per cent reduction in the sunlight reaching Earth. The cost would be in the region of a few trillion dollars. According to Angel, the discs would cancel the warming effects of the carbon emitted to power their launch a thousand times over.
It sounds far-fetched, but nobody is disputing the technical analysis. “The pico-spacecraft have enough intelligence on board, like swarms of insects, to maintain their position,” says Worden. “If you decide you want to move them out of the way, just turn them off.” While the principles behind the control system are based on existing technology, he says, the challenge lies in fine-tuning the design and fabrication to minimise the weight and cost. The launch technology would be new; so far the researchers have only done small-scale tests, for example, using a magnetic device to send up a 4.5-kilogram weight to a height of about 2 metres.
Independent computer simulations run by Caldeira show that the space sunshade could almost cancel out the temperature changes expected from global warming, except for a small area around each pole. That’s because while greenhouse warming is uniform, the poles receive less sunlight than the tropics, so the effect of changes in sunlight is weakest at the poles. This regional difference in cooling might cause unpredictable changes in weather patterns. And since the poles would see less of an effect from the dimming, they might still experience a significant loss of ice cover. Caldeira points out that the sulphur dioxide option has the possible advantage of compensating for this by injecting more particles at the poles.
So which approach has the edge? It comes down to costs and feasibility. If we were suddenly faced with a climate catastrophe, Caldeira says, the sulphur-particle approach is cheap enough to be essentially free. “The engineering is simple enough that we could put it up in a couple of years. It makes it much more likely to be deployed,” he says. The space sunshade, though attractive, seems unlikely to be implemented. “If cost were no object, you’d want to use something like Angel’s scheme,” Caldeira says, “because it’s very clean and controllable, and would likely minimise any secondary effects. But it’s very expensive. If you want to go to that much effort, it would be simpler just to change our energy systems.”
What’s more, geoengineering in general has major drawbacks. It does nothing about the CO2 in the atmosphere, which would still produce effects such as ocean acidification (New Scientist, 5 August 2006, p 28). “Everybody knows that CO2 is a weak acid,” says Schneider. “Carbonic acid is the reason that rocks erode. When it runs into the ocean, the ocean gets more acidic. This could be a big deal. You might have problems with coral reefs and with phytoplankton that use carbon-based shells. Nobody disputes that will happen, the only question is how much it matters to basic ecosystems.”
There is also the risk that our global climate could become dangerously dependent on geoengineering. Caldeira and Damon Matthews, also of the Carnegie Institution, recently published a study suggesting that if the biosphere adapts to less sunlight and current levels of carbon emissions, then any failure of the sunscreen system could have swift and severe consequences, including warming rates up to 20 times greater than those at present ().
Despite these risks, is it still a good idea to research such measures? Or does talking about them provide too much cover for those who would prefer to do nothing about emissions? Although some scientists still feel that airing such possibilities in public is dangerous, most now think the basic studies are worth doing. “We ought to do the research and say whether this is an option,” says Worden. “I can’t predict as a scientist what the ups and downs might be. With every scientific discovery, there’s no such thing as a solution that doesn’t have huge dangers. It’s folly to say certain technologies should be off the table.”
“Nobody wants to have to do this,” adds Angel, “but if you get to the point where the alternative is 6 metres of sea-level rise, we might want to have this as an option.”
Schneider, who is a forceful proponent of making the drastic cuts in emissions needed to counteract the build-up of greenhouse gases, agrees that it is important to do the research so that the techniques could be drawn upon as a last resort. “We’re not going to implement it,” he says, “but you certainly have to know what’s possible. It’s like emergency back-up surgery: you never want to do it, but you still have to practise it.”
Geoengineering may be risky…
Stephen Schneider, physicist, engineer and professor of biology, Stanford University, California
“Never mind these exotic space programmes that are super-expensive, let’s just talk about putting a stratospheric haze up there to balance the heat trapped by the greenhouse effect. In principle you can do it – probably cheaply too, compared to putting the coal industry in the tank. But dust in the stratosphere will only last two to four years. The CO2 anomaly will last 200 to 400 years. So we would need centuries of trusted global climate-controllers. They would have to operate continuously, reliably and by consensus. My objection to geoengineering is not that we couldn’t figure out a way to do it – though there are side effects we can’t anticipate – I’m more worried about centuries of trusted controllers. Also, if you’re going to do climate modification you’d better have no-fault disaster insurance, because any casualties will be laid at your doorstep.”
…but we need to explore it
Wally Broecker, geologist and palaeoclimatologist, Columbia University, New York
“Geoengineering should be an insurance policy in case of unacceptable global warming. The space-based approach is enormously expensive, but it’s the ultimate kind of solution if we want to control the climate. I would vote strongly for any research to do that. When you put things up in the stratosphere, on the other hand, you’ll get different rainfall, different this, different that. If we’re thinking about that, we’d better put effort into experiments. One thing you can do is test the effects at a lower strength. To keep CO2 from doubling, we have to go to zero net emissions by 2070. That’s a huge enterprise. So we should explore all the options.”
