Global warming may seem like an overwhelmingly complex problem to tackle. But one scientist thinks the answer is brilliantly simple: all we need is white paint. A lot of white paint. David Adam reports

Hashem Akbari has a vision of a shiny, happy world. He sees polished roads and cities that gleam in the sunlight. Rooftops are bright and pavements light. Akbari wants to turn our cities into a giant mirror and he needs your help. And paint, lots of it.

Akbari is no architect and his grand plan is no conceptual art project. Based at the prestigious Lawrence Berkeley National Laboratory in California, Akbari is a scientist who has come up with a new way to fight global warming. It could be the easiest solution you've never heard of.

His big idea is based on principles as old as the whitewashed villages that scatter the hills of southern Europe and North Africa. Turn enough of the world's black urban landscape white, he says, and it would reflect enough sunlight to delay global warming, and grant us some precious breathing space in the global struggle to control carbon emissions.

Akbari is poised to launch a campaign to paint the world white. He wants dozens of the world's largest cities to unite in an effort to replace the dark-coloured materials used to cover roads and roofs with something a little more reflective.

It sounds simple, but the effect could be dramatic. Study after study has shown that buildings with white roofs stay cooler during the summer. The change reduces the way heat accumulates in built-up areas - known as the urban heat island effect - and allows people who live and work inside to switch off power-hungry air conditioning units.

Aware of the benefit, California has forced warehouses and other commercial premises with flat roofs to make them white since 2005, and, if such an effort could be extended, the results could make a big difference.

Together, roads and roofs are reckoned to cover more than half the available surfaces in urban areas, which have spread over some 2.4% of the Earth's land area. A mass movement to change their colour, Akbari calculates, would increase the amount of sunlight bounced off our planet by 0.03%. And, he says, that would cool the Earth enough to cancel out the warming caused by 44bn tonnes of CO2 pollution. If you think that sounds like a lot, then you're right. It would wipe out the expected rise in global emissions over the next decade. It won't solve the problem of climate change, Akbari says, but could be a simple and effective weapon to delay its impact - just so long as people start doing it in earnest. "Roofs are going to have to be changed one by one and to make that effort at a very local level, we need to have an organisation in place to make it happen," he says. Groups in several US cities, including Houston, Chicago and Salt Lake City, are on board with his plan, and he is talking to others.

The idea is a form of geo-engineering, a broad term used to cover all schemes that tackle the symptoms of climate change, namely catastrophic temperature rise, without addressing the root cause, our spiralling greenhouse gas emissions. And if altering all of the world's roofs and roads sounds extreme, then take a look at some ideas from the other end of the geo-engineering scale: giant mirrors in space, shiny balloons to float above the clouds and millions of fake plastic trees to suck carbon from the air. An increasing number of climate scientists argue that the world has little choice but to investigate such drastic options. Carbon emissions since 2000 have risen faster than anyone thought possible, mainly driven by the coal-fuelled boom in China, and a global temperature rise of 2-3C seems inevitable. Last year a special edition of a Royal Society journal dedicated to geo-engineering said the geo-engineering schemes "may be risky, but the time may well come when they are accepted as less risky than doing nothing".

Akbari says his plan is more workable than other geo-engineering ideas. The science is simple. Sunlight reflected from a surface does not contribute to the greenhouse effect, which drives global warming. That problem comes when dark surfaces soak up sunlight and send it back up as thermal energy, at just the right wavelength to rebound off CO2 in the sky.

The problem with shiny cities, according to Kevin Anderson of the Tyndall Centre for Climate Change Research at Manchester University, is more simple science. "It won't tackle global warming because carbon emissions are still rising," he says. Like all geo-engineering schemes, it will need to be kept up indefinitely, he says, and does not address the growing acidification of the oceans, caused as extra CO2 dissolves. The cooling effect and energy savings in cities would be welcome though, he adds.

Akbari says his idea is not intended to replace efforts to cut carbon emissions, but to work alongside them. "We can give the atmosphere time to breathe," he says. "I just don't see a downside to this idea. It benefits everybody and you don't have to have hard negotiations to make it happen."

Dark roofs reflect about 10-20% of sunlight, while white surfaces tend to send back at least half. In technical terms, the percentage of light reflected by a surface is called its albedo - so a perfectly reflective surface has an albedo of 1. Coloured paints have an albedo of 0.1-0.3, and white paints an albedo of 0.5-0.9. Asphalt road surfaces have albedos as low as 0.05, so they absorb up to 95% of the sun's energy. Concrete has an albedo of up to 0.3, tar and gravel just 0.1. Akbari's mission is to get individuals, local authorities, builders and communities to think about albedo alongside cost, colour and design when it comes to repairs, maintenance and new construction.

"This is not just a question of painting things white. Roofs and roads are routinely repaired and replaced and, when it comes to a householder changing their roof, we want them to look at reflective options. That's the time to target people." He says an "aggressive" programme could convert all cities within 10-20 years.

It is fairly easy to persuade, or require, the owners of buildings to select white materials for flat roofs, because the colour is only noticed by passing air travellers. But sloping roofs, found on most houses, are a different issue because they are visible from the ground. As pretty as snow-coated Alpine villages may look, skiers wear powerful sunglasses for a reason. Streets of white-roofed houses would dazzle in the sun. The same is true of road surfaces - too light a colour and too much light reflects as glare into the eyes of motorists.

No problem, Akbari says: reflective materials need not be white. Lighter colours such as grey are good too. And there are other ways to increase the albedo of materials. Pigments that bounce back infrared light can raise the reflectivity of dark surfaces by 40% without any obvious change in colour. They are not as effective as white, which bounces back visible wavelengths of light too, but they are much better than conventional materials.

The Public Works Research Institute in Japan has experimented with paints with such pigments applied to conventional asphalt surfaces. They made a road that reflects 86% of infrared light, which helps keep the surface cool, yet reflects just 23% of visible light, to keep down glare. The researchers were nervous that the extra infrared bouncing off the shiny road could cook pedestrians, but volunteers recruited in summertime to "stand on the paint-coated pavement and conventional pavement" said they actually preferred the painted version. This could be because the coated road kept their feet cool, the researchers said.

There are other benefits too. Computer simulations of Los Angeles show that resurfacing about two-thirds of roads and rooftops with reflective surfaces, as well as planting more trees, could cool the city by 2-3C. That would reduce LA smog as much as a total ban on cars and lorries, and cooler roofs could also save a fortune in electricity bills. On hot days in North America, up to 40% of all electricity can be consumed by air-conditioners, and each degree a city such as LA warms is reckoned to see the air-con turned up enough to need another 500MW - the output of a decent sized nuclear power station. Akbari estimates that widespread use of cooler rooftops could slash $1bn from electricity bills in the US alone.

That may be very well for places such as California, with its 300 days of sunshine a year, but what about gloomy northern Europe and the UK, with a measly 100 sunny days? The effect would not be as great, admits Surabi Menon, who works with Akbari at Lawrence Berkeley, but she says anywhere that needs air-conditioning, or has cities warmer than the outer rural areas would benefit. Akbari says his estimates of the global cooling potential of reflective cities are based on a global average, so the cloudier places will be made up for by the sunnier spots. "It's absolutely worth doing in the UK," he says. And, he adds, he might just have found a way to pay for it.

Each 10 sq m of urban surface changed from dark to white, he says, has the same cooling effect as preventing the release of a tonne of carbon dioxide. So why not include such resurfacing in carbon offset schemes? Just as money from green consumers and firms anxious about their carbon footprint is used to fund projects that plant trees, fit green lightbulbs and develop renewable energy, in exchange for carbon credits, so it could pay people across the world to paint, coat and resurface. At today's carbon prices, changing the colour of an average roof could net the householder £150, and Akbari's global scheme could together generate more than £500m. "We want to target 30 to 40 cities initially, but within a few years we hope it will mushroom around the world," he says. Go on, paint your town white.