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Man-made climate change is happening. Its impact will be massive, serious and unevenly spread; its cost enormous. Until recently the focus of governments around the world has been on climate change mitigation supported by policies to reduce CO2 emissions. The approach has failed; global efforts to reduce emissions have fallen far short of the reductions needed to avoid significant climate change.

In acknowledging their collective failure, the same governments are now adopting climate change adaptation policies. The focus is now on measures to ensure critical assets, such as power stations, water supplies, transport systems and the urban environment, are able to function in a more extreme climate.

But time is running out. According to NASA, 鈥渢he evidence for climate change is compelling鈥�. The organisation cites a sea level rise of 170mm in the last century, 10 of the warmest years since 1880 have been in the last 12 years, with temperatures regularly setting new records, and increasing numbers of intense rainfall events. Clearly we need to buy more time to find solutions, which is why scientists are starting to give geoengineering serious consideration.

Geoengineering must be explored even if only as an insurance policy in the event of some catastrophic climatic tipping point

Mark Lynas, journalist

Geoengineering is the deliberate large-scale manipulation of the planetary environment to counteract climate change resulting from human activity. There are two main geoengineering categories: carbon dioxide removal from the atmosphere and solar radiation management. Carbon dioxide removal methods address the root cause of global warming, but they are generally slow to take effect and are costly.

Solar radiation management is more of a sticking plaster solution; it comprises technologies to reflect sunlight back into space such as mirrors between the sun and the earth, to help offset the effects of global warming but does nothing to tackle the causes of climate change.

However, Mark Lynas, journalist, author of a number of books on climate change, and speaker at the Ecobuild conference on geoengineering, says on his web post: 鈥渢he addition of mirrors at the Lagrange point [the point where objects can be placed and unaffected by the gravitational pull of the Earth or moon] between the sun and the Earth would be prohibitively expensive鈥�. According to him, solar radiation management through the addition of sulphates in the upper atmosphere is a more realistic option: 鈥淲hat makes this one scary is that it would probably actually work, and be achievable at reasonable cost of 拢0.3bn to 拢1.6bn a year鈥�.

Lynas says geoengineering is an option that must be explored, 鈥渆ven if only as an insurance policy in the event of some catastrophic climatic tipping point鈥�. Some of the more promising solutions are outlined below:

Adding nutrients to the ocean

Carbon dioxide is fixed from surface waters by algae. Some of the carbon they take up sinks into the deep ocean as organic matter and faecal material where it is consumed by bacteria and other organisms re-releasing the CO2 into the deeper water. The ability to draw carbon down into the oceans depths is limited by the supply of nutrients to allow algal growth on the ocean鈥檚 surface. Geoengineering sets out to speed up the process by increasing the supply of these nutrients through the addition of nitrogen, phosphate or iron depending on the limiting nutrient in a particular area of ocean.

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Speeding up natural weathering processes

Carbon dioxide is naturally removed from the atmosphere by processes involving the weathering of carbonate and silicate rocks, which are the most common types of rocks on the planet. The rock鈥檚 silicate minerals react with CO2 in the atmosphere and oceans to form carbonates. Over time this weathering will decrease CO2 concentrations, however, the rate at which these reactions take place is around one hundredth of the rate at which CO2 is currently being emitted. As a result, there are a number of geoengineering proposals to artificially increase the rate of CO2 absorption including:

• Adding silicate minerals to agricultural soil - this would take about 7 km³ of silicate minerals to remove as much CO₂ as is currently being emitted.
• Reacting crushed and ground carbonate rock with concentrated CO₂ captured from power stations and releasing the resulting bicarbonate solution into the sea.
• Liming the ocean by adding limestone carbonate rocks, which have been heated to drive off the CO2 to the oceans to increase their alkalinity and hence the uptake of CO₂. This solution has the added complication that the CO₂ released by heating the rocks must then be captured and sequestered.

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Capturing CO₂ from air

This is not as easy as it might at first sound because CO₂ is present in the air at concentrations as low as 0.04%. Small scale commercial systems already exist for industrial processes but, as yet, there are no cost-effective large-scale solutions. Air capture systems will require energy inputs and the CO₂ removed would need to be stored.
The three technological possibilities under consideration are:

• Large scale absorption of CO₂ on solids, similar to the systems being developed for power stations
• Absorption into highly alkaline solutions
• Absorption into moderately alkaline solutions with a catalyst to accelerate the reaction.

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Enhancing ocean currents

This solution involves speeding up the rate at which atmospheric carbon is transferred to the deep sea by using vertical pipes to pump water from deep in the ocean to the surface and to promote downwelling of dense water in sub-polar oceans. The effectiveness of this is limited and there is a danger that disturbing existing circulation patterns may even release more carbon into the atmosphere.

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Increasing ground reflectivity

Increasing surface reflectivity by painting roofs white, planting highly reflective crops and covering deserts with reflectors.

Painting roofs, roads and pavements white would increase the albedo (the ability of a surface to reflect energy back into space) of less than 1% of the land surface, too small an area to have a global impact. It would also be expensive to implement.

Crops and grasses make up a much larger area so there is a suggestion that by careful choice of crop and grass species it may be possible to increase the reflectance of vegetated surfaces. Modelling suggests that summertime cooling of up to 1掳C may be possible in Northern Europe and North America but the consequences of large-scale changes in land management would be huge.

As a short-term solution seeding clouds from aircraft is a possibility

Hot deserts make up only 2% of the earth鈥檚 surface. However they experience high levels of solar radiation so one proposal is to cover these areas with a reflective surface. Locally the effect would be significant, but the ecological consequences of covering deserts with man-made materials are unknown and this approach has the potential to change atmospheric circulation and the occurrence of monsoons.

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Increasing cloud reflectivity

Whitening clouds over the oceans by spraying sea water high into the air would produce localised temperature reductions. A spray generator has yet to be developed capable of the task and a fleet of up to 1,500 vessels would be needed to carry the units. As a short-term solution seeding clouds from aircraft is a possibility. However there are concerns that the cooling effect could affect ocean circulation patterns.

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Stratospheric aerosols

Evidence from volcanic eruptions has proved the potential of sulphate aerosols to reflect sunlight, although other forms of aerosol are being considered. Stratospheric aerosol methods have the most potential of all solar management techniques because they could produce large and rapid global temperature reductions by reflecting sunlight. In addition, their effects would be more uniformly distributed than for most other methods and they could be readily implemented.

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Mirrors in space

Space-based geoengineering solutions involve positioning sun shields in space to reduce the amount of solar energy reaching the earth. Proposals include placing mirrors in orbit around the earth, creating a Saturn-like ring of 2 billion tonnes of dust particles in the equatorial plane to shade the tropics.

Another solution is to place about 3 million km2 of reflectors at the point between the earth and sun where the gravitational attraction between the two bodies is equal. Proposals for the sunshade include a mesh of aluminium threads about a millionth of a millimetre thick, trillions of reflecting discs fabricated in space from asteroids, and a reflector of lunar glass, which could be made on the moon. Another possibility is a cloud of 10 trillion refracting 600 mm diameter discs fabricated on earth and launched into space, a million at a time every minute for about 30 years.

If long-term geoengineering is adopted then the Royal Society suggests in its report Geoengineering the Climate that these methods might eventually become the most cost-effective option, although development of the necessary technology is likely to take many decades.

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