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We all know the arguments. Greenhouse-gas levels are rising. The planet is warming up. The climate is changing, with strange patterns of extreme weather, melting ice and engulfing deserts. According to some, we may be approaching — or even past — a tipping point where effects feed off each other and climate change accelerates.

What should we do about this? For years now, the answer has been clear: reduce greenhouse-gas emissions. Increasingly drastic targets have been set by many governments for cutting the CO₂ emitted from power stations and transport by increasing efficiency and reducing consumption. However, CO₂ levels continue to rise. The heat is on and what was once seen as crackpot fringe science, strictly for pulp fiction, has now started to seem more plausible: geoengineering.

It is a term that has many meanings, but, for the purposes of combating climate change, it is the use of technology to cool down the planet. There are two main methods of doing this. One is reversing the greenhouse effect by pulling CO₂ out of the atmosphere and locking it away in a planetary version of a carbon capture and storage (CCS) scheme. That may sound ambitious, but it is nothing compared to the other option: create a giant sunshade to prevent some of the sun’s rays from hitting the atmosphere in the first place.

As far fetched as these ideas seem, they are now beginning to gather attention and are the subject of serious research. To some, however, particularly in the environmental movement, they are seen as a distraction from the matter of reducing emissions and developing renewable technologies.

The technologies involved in geoengineering concepts are mostly in their infancy and very few have been trialled at any stage. Those that have are in the field of ‘air capture’ — finding ways to remove CO2 from the atmosphere. ‘The pre-industrial value for CO₂ in the atmosphere is 280ppm and we’re over 380ppm at the moment,’ said Nem Vaughan, co-ordinator of the Geo-Engineering Assessment and Research (GEAR) Initiative at the University of East Anglia, which is just embarking on a study of the issues surrounding geoengineering.

‘Some corners of the science community think that 400ppm is going to be too high to recover from and we’re locked into this increasing trajectory because of our dependance on fossil fuels. So if we have this amount of CO₂ that’s too high, the first step is to stop it growing — but the next is to see whether we can take some of it away,’ she added. However, ocean fertilisation is not proving to be as simple as it seems —something that is likely to be common to all geoengineering techniques. A recent trial called LOHAFEX, carried out by an Indo-German collaboration, saw four tonnes of iron sunk into the South Atlantic. Algae bloomed, but it proved very popular with tiny crustaceans that had an unexpected feast. The amount of CO₂ drawn into the ocean was quite small. ‘But we did learn a lot about the ocean food chain,’ said Wajih Naqvi of the Goa-based National Institute of Oceanography.

This did not surprise Prof John Shepherd of the National Oceanography Centre at Southampton University, deputy director of the Tyndall Centre. ‘I’ve never been a fan of ocean fertilisation,’ he said. ‘It’s always been clear that the vast majority of any carbon fixed by these experiments would be returned to the atmosphere within a year. The numbers bandied around aren’t realistic and never have been,’ he said.

Perhaps more promising is a direct form of air capture championed by Klaus Lackner, a geophysicist at Columbia University. He has developed a device that is best described as an artificial tree: a telephone-box-sized container, holding sheets of an ion exchange fabric that absorbs CO₂ from the air. This sorbent fabric can be regenerated, the CO₂ stripped away into solution, to be purifed and stored in the same way as proposed CCS schemes. At the moment, Lackner’s pilot modules can store less than 100kg of CO₂ per day, but he is planning future models capable of holding one tonne per day.

Another scheme uses a sort of inverse solar generation. Solar troughs concentrate sunlight onto quicklime, heating it to 400°C, at which point it reacts with CO₂, locking it into a new crystalline structure.

‘One common factor that these schemes share is that they’d be slow,’ said Shepherd. ‘You’d have to keep cranking away for several decades to have any noticeable effect. For this reason, we’re considering carbon removal and the other techniques, which we’re calling solar radiation management, together, because you can imagine that they might, to some extent, be complementary to each other.’ Solar radiation management would involve deflecting sunlight before it could heat the atmosphere, he explained, and the effects of this would be almost instantaneous.

However, the technology involved is, to say the least, exotic. The solar radiation management camp includes schemes such as creating highly reflective clouds in the stratosphere, such as by vaporising seawater and pumping it into the air from a fleet of autonomous robot ‘cloud ships’, proposed by wave-energy pioneer Prof Stephen Salter of the University of Edinburgh.

Similarly, it has been proposed that pumping an aerosol of sulphur dioxide (SO₂) into the atmosphere, possibly from high-flying jets, would have a global dimming effect similar to that caused by a large volcanic eruption, as SO₂ reacts with moisture in the atmosphere to form sulphuric acid, a strong absorber of solar radiation. The effects of creating a concentration of sulphuric acid in the stratosphere are less clear.

Another concept, championed by astronomer Roger Angel of the University of Arizona, is positively James Bond-like in scale. He has proposed creating a 100,000-square-mile ‘sunshade’ in space, orbiting the Sun at the inner Lagrange Point (L1), 1.5 million kilometres from the Earth in a direct line between star and planet, where it will keep station and reduce the amount of sunlight hitting the Earth.

The sunshade would be made up of hundreds of millions of small solar deflectors, about 60cm in diameter and made from a transparent film, formed into a cylindrical ‘cloud’ 60,000 miles long and about half the diameter of the Earth. Around 10 per cent of the sunlight travelling through the cloud would be deflected to a path where it misses the planet, reducing the amount of incident solar radiation by about two per cent. This, Angel calculates, is enough to offset the consequences of the greenhouse effect.

Of course, it sounds completely far fetched. ‘This sort of thing definitely fits into the crackpot bracket,’ Vaughan said. However, take a closer look at Angel’s proposals and it appears much more feasible. The individual fliers would be controlled by MEMS mirrors that would balance them against the pressure of the solar radiation and would be launched using an electromagnetic system already proposed for rocket launches, accelerating spacecraft holding many deflectors through a series of magnetic fields to escape velocity.

This electro-cannon would be so large that it might have to fit inside a hollowed-out mountain, which seems unfeasible until you consider such feats as the massive chasms of CERN. Angel has even costed his proposal: he believes that it would cost several trillion pounds over 20 to 30 years, and would last around 50 years.

Put like this, it is not outside the realms of possibility that such a mission could be launched. ‘The big advantage of these schemes is that they would be fast,’ Shepherd said. The downside is that the effects of interfering with solar radiation in this way are not known. ‘All of these solar radiation management schemes don’t do anything about the composition of the atmosphere, so there’s a different type of risk involved.’ Some of the effects of

global warming are only just becoming apparent and this drastic form of global cooling could easily have effects that are difficult to predict. ‘One important consideration is: if it goes wrong, can you turn it off?’ Vaughan added.

The reason that such exotic ideas are being considered now is that, over time, these concepts can often become more viable and the urgency of climate-change mitigation is such that many believe it is as well to study the implications now. If the time comes when such methods are feasible — or if the climate starts to change suddenly and catastrophe is imminent — such studies will help plans swing into action. ‘You wouldn’t want to roll something out without having a very good idea of what’s going to happen,’ Vaughan said.

She and Lenton have already studied the potential effects of a variety of geoengineering proposals on global temperature and now, as part of GEAR, they are starting to put a team together to look at wider implications. ‘We’re looking at policy and governance, to see what mechanisms are already in place that could govern these sorts of initiatives. For example, the London Convention on marine dumping has implications for ocean fertilisation,’ said Vaughan.

‘We also want to look at the economics, balancing the potential effects against how much they cost. Some of these measures would only be practical at a certain carbon price, such as air capture. We know we can take carbon out of the air and we know that’s just undoing the emissions of the past, so what drives it is cost. If there’s a monetary value to taking carbon out of the air, that changes the landscape,’ she added.

Moreover, Vaughan said, geoengineering has such a broad definition that some measures are not only non-controversial but are already happening. ‘Reforestation has been part of our mitigation toolkit for decades, but it’s actually geoengineering because it’s a form of air capture.

‘And increasing the albedo of urban areas by making roofs more reflective and lightening the shade of paving and roads also has an effect. Globally, it doesn’t reduce temperature much at all, because only 30 per cent of the Earth’s surface is land and only a fraction of that is urbanised. But more than half the population lives in urban areas and it does have an appreciable effect locally. If you can cool down the areas where people live, it’s going to mitigate global-warming effects for them,’ she added.

This is already state-level policy in California. Of course, it’s ingrained into the culture in the Mediterranean and Africa, with their predominantly white-painted towns.

What matters, Vaughan stressed, is preparation. ‘Carbon-emission mitigation is hard, because everyone has to agree to it for it to have any effect. But I can’t see any evidence that geoengineering would be treated any differently. Projects such as this could, potentially, be launched by one country or a small group, but they’d affect every country and every human on the planet and they’d all have to agree. You’d want to go to the negotiations well armed with good science about which methods you’d use, possible side effects and their magnitude, so you can balance the risks and argue persuasively,’ she said.

Despite this, there is still a feeling that geoengineering is deflecting attention and resources, both financial and intellectual, from the priority issue: preventing climate change by reducing emissions. Shepherd, who is leading a Royal Society working group to produce a report on geoengineering, has some sympathy with this opinion.

‘I don’t personally think that geoengineering is immoral, because it can’t be wrong to try to do something deliberately to reverse the harmful effects of something you’ve done by accident. But there is a question of moral hazard. If we invent such schemes as these, it will take the heat off proper mitigation and emissions reduction and could let people off the hook. There’s a debate to be had there, but I don’t see the end point of that to be a disaster. I don’t think that doing a bit more engineering and a bit less emission would be a bad thing,’ he said.

So are there circumstances in which these schemes could become a reality? Shepherd is keeping an open mind. If, for example, nuclear fusion was found to be a practical energy source, but with a long lead time, during which fossil fuel would still be the dominant fuel for power generation, a scheme such as Angel’s Lagrange sunshade might provide a stopgap to prevent warming and allow development to take place.

Equally, Salter’s high-albedo stratospheric clouds from floating cloud ships could provide the breathing space for the effects of Lackner’s artificial trees to become apparent. ‘If things got sufficiently hairy, if we pass a tipping point and Greenland’s ice starts melting rapidly, it’s more likely you could achieve the arguments that these measures will be necessary,’ he said. ‘But there’s no point in saying we think they’ll work or we think they’ll be a disaster — we’ll have to go through a well-structured sequence of research, testing and planning before you can even think of deploying it on a large scale. One of the things we want to do is to elucidate that process.’