Backup plan: Whitening the Earth
You need to have Flash Player 10 or above installed to view this video
One solar geo-engineering proposal is to make clouds whiter so that they reflect more sunlight and cool the planet down. Other whitening proposals suggest painting large areas of the Earth’s surface or covering it with reflective materials. But there are many uncertainties and such projects might have unexpected effects on the weather.
Without significant cuts in greenhouse gas emissions, the amount of carbon dioxide (CO2) in the atmosphere could soon be double its preindustrial levels. Some solar geo-engineering proposals aim to cancel out the warming effect of this extra CO2 by reducing the sunlight absorbed by the Earth’s surface. Only 70% of sunlight reaching the Earth is absorbed. The rest is reflected back to space – an effect known as albedo. To compensate for doubled CO2 levels we’d need to increase the albedo by about 2% by making large areas of the planet more reflective. If we increase CO2 even more, we’d also need to increase the albedo even more to keep global temperature stable.
Some clouds reflect more sunlight than others - from just 10% to over 90% depending on the cloud’s characteristics. Clouds form when water vapour condenses into droplets, often by clinging to a particle of dust, salt or ice. Clouds formed around many such particles will have many smaller droplets, making them denser and whiter. Clouds with fewer particles will have fewer, larger droplets, making them less dense and less white. Larger droplets are also heavier and fall as rain more quickly, so a cloud with many smaller droplets could have a longer lifespan – and reflect more sunlight. These effects mean spraying particles into clouds could increase their albedo, cooling the planet.
The aim of solar geo-engineering through cloud whitening is to cool the planet by increasing the amount of sunlight reflected by the Earth – its albedo. But whitened clouds floating above other bright surfaces, such as ice and snow, wouldn’t be very effective since these surfaces already reflect most of the sunlight hitting them. So the clouds would need to be carefully chosen for maximum effect. Scientists have identified large areas of suitable clouds called marine stratus. They’re often found off the west coasts of Africa and North and South America. Because they float above the ocean, which has a very low albedo (about 10%), whitening these clouds could cool the Earth significantly.
Spraying particles into clouds can increase the number of water droplets they contain, making them denser and whiter. Scientists have calculated that doubling the number of droplets in all marine stratus clouds should reflect enough extra sunlight to cancel out the warming effect of doubled atmospheric carbon dioxide levels. Particles could be sprayed into clouds from above using low-flying aeroplanes, or from below using ships. One proposal suggests using a fleet of about 1500 specially designed ships to extract salt particles from sea water and spray them upwards into the clouds. It isn’t yet clear how effective this type of solar geo-engineering system would be.
In addition to the idea of cloud whitening, some solar geo-engineering proposals aim to cool the Earth by increasing the amount of sunlight reflected by the surface. This could be achieved by painting cities and large land areas white, or by covering deserts with reflective aluminium. But the huge areas involved, together with regular repainting or maintenance costs, could make this very expensive. If successful, whitening the surface would cool surrounding areas significantly – but the effect could be complicated by cloud cover. Reflecting extra sunlight from the surface would be most effective under a clear sky, since any dense clouds would already reflect much of the sunlight themselves, making the whiter surface redundant.
Increasing the albedo of large areas of the planet would be likely to have other effects on the climate apart from simply reducing global temperature. Whitening either clouds or land surfaces would cool the immediate surroundings more than the rest of the planet. This uneven effect on temperatures could alter winds, ocean currents, rainfall patterns or even large-scale climate circulation such as the monsoons. The climate is a very complex system. Climate models can simulate its general behaviour, but there are still many uncertainties. Increasing the albedo of parts of the planet would change temperature patterns and scientists aren’t yet sure what effect this might have on the weather.