Backup plan: Sunglasses in space
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One solar geo-engineering proposal is to send trillions of glass lenses into space to deflect sunlight, a bit like putting a giant pair of sunglasses on the Earth. The reduction in sunlight would cool the planet down, but there are many uncertainties and the project might have unintended consequences.
Without significant cuts in greenhouse gas emissions, the amount of carbon dioxide (CO2) in the atmosphere could soon be double preindustrial levels. Some solar geo-engineering proposals aim to cancel out the warming effect of this extra CO2 by reducing the sunlight reaching the Earth. To compensate for doubled CO2 levels, we’d need to stop about 2% of sunlight reaching our planet. This doesn’t sound much, but considering the Earth gets more energy from the Sun each hour than humans use in a year, 2% is actually a huge amount. If we increase CO2 to more than double preindustrial levels, we’d need to block out even more sunlight to keep global temperature stable.
A space sunshade blocking out 2% of sunlight would cool the Earth quickly. Within a few years, global temperature could drop significantly, even with higher carbon dioxide (CO2) levels. But rapid cooling might disrupt climate patterns and ecosystems, so we could deploy the sunshade in stages to reduce temperatures gradually. To keep global temperature stable, we’d need to keep the sunshade in space for as long as the extra CO2 remains in the atmosphere – decades or even centuries. If the sunshade degraded, or if we abandoned the project while CO2 levels remained high, temperatures could rise again rapidly – possibly faster than if we hadn’t deployed the sunshade in the first place.
The aim of solar geo-engineering using a space sunshade is to divert some sunlight away from the Earth. Some scientists suggest using metal discs to reflect sunlight back into space – these mirror-like discs could be manufactured in space using metals from asteroids. Another proposal is to use a mesh of very thin aluminium threads, which would also reflect sunlight. Other scientists suggest using glass lenses, which refract light passing through them, causing some of the incoming sunlight to change direction and veer away from the Earth. One proposal is to use 16 trillion extremely thin glass lenses, each 60 cm across and weighing just 1 gram – less than a paperclip.
The Earth orbits the Sun because of the pull of the Sun’s gravity. But the Earth, although much smaller than the Sun, also has gravitational pull. There are certain locations round our orbital path – known as Lagrange points – where the pull of the Earth’s gravity balances out the pull of the Sun’s. One of these points, called L1, lies between the Sun and the Earth. An object placed at this point will orbit the Sun in ‘lock step’ with the Earth. Some scientists think this would be a good location for a space sunshade. It would stay in place at all times, constantly shielding our planet from some of the Sun’s energy.
Any solar geo-engineering proposal that involves sending objects, such as lenses, into space raises a question: how do we get them there? A set of small lenses would need more coordination than one large one, since each lens would need to carry a tiny computer-controlled positioning system to govern its movements and prevent collisions between lenses. But small lenses could be launched in smaller spacecraft, and if a few lenses broke it wouldn’t wreck the whole project. Although it would be expensive, some scientists think it could be feasible to deploy 16 trillion lenses by launching stacks of 800,000 lenses at a time – 20 million stacks in total. The whole space sunshade could be assembled over 25 years.
The Sun heats the tropical regions near the Equator more than the higher latitudes. This means that the cooling effect of reduced sunlight would be greatest in the tropics, changing the difference in temperature between the Equator and the poles. This could affect climate patterns – in particular, reducing sunlight could reduce rainfall. Two-thirds of all rain falls in the tropics, where intense sunlight causes a lot of evaporation. Less sunlight means less evaporation and so less rainfall, but the changes would be smaller than those expected if global warming continues at the current rate. Unfortunately, cooling the planet by reducing sunlight won’t simply return the climate to its preindustrial state.