Â鶹´«Ã½

Last Word is Â鶹´«Ã½â€™s long-running series in which readers give scientific answers to each other’s questions, ranging from the minutiae of everyday life to absurd astronomical hypotheticals. To answer a question or ask a new one, email lastword@newscientist.com

How much difference would it make to global temperatures if the roofs of all the buildings on Earth were painted white?

Mike Follows
Sutton Coldfield, West Midlands, UK

When I answered a similar Last Word question 20 years ago, I expressed the hope that someone would eventually use a climate model to predict the effects of painting roofs white. Since then, there has been a good deal of research into the impact of cool roofs – painted white or with light colours to reflect sunlight – particularly in relation to the urban heat island (UHI) effect, where cities tend to be warmer than the surrounding countryside.

At first glance, cool roofs should increase Earth’s albedo – the fraction of sunlight reflected from the surface. This ought to cool the planet, or at least counter some of the warming that has occurred since the industrial revolution. They should also reduce the need for air conditioning and the associated greenhouse gas emissions. However, some of those benefits are offset by increased heating demands these roofs cause in winter or in places above roughly 40° latitude.

Cool roofs could have saved up to 249 lives during London’s hot summer of 2018, according to a . Widespread adoption might have reduced average city temperatures by 0.8°C, preventing nearly a third of the 786 heat-related deaths.

That said, a 2011 global climate model by researchers at Stanford University in California found that, while the albedo effect of white roofs may lower global urban temperatures by 0.02°C, this could be at the expense of warming Earth overall by 0.07°C. The cool roofs enhanced convection, which increased rainfall downwind of cities. However, reduced cloud cover overall allowed more solar radiation to reach the surface – a positive feedback. It also led to greater solar absorption by airborne soot particles, which absorb both incoming and reflected sunlight.

Meanwhile, green roofs, which are planted with vegetation, are also gaining attention. These require sturdier structures and more materials to support the weight of soil and plants. Although they help cool urban areas during the day by absorbing solar energy, they release that heat at night, raising nighttime temperatures. Their overall effect on the UHI is thought to be neutral – but they do bring other advantages, such as improved biodiversity.

Nick Baker
Rowhedge, Essex, UK

Painting all roofs white wouldn’t make much difference. This is mainly because the area of roof compared with the area of unbuilt land is very small. A second reason is that solar radiation delivers about half its energy as infrared and ultraviolet light, which is absorbed by most white paints.

But a white-painted roof would bring a large benefit to those living or working under it, particularly if the roof were badly insulated, such as a rusting sheet of corrugated iron. It is difficult to imagine a worse-performing solution in respect of overheating, and such roofs would benefit hugely from a coat of white paint.

Furthermore, in urban areas, where rooftops form a much higher proportion of the surface intercepted by solar radiation, there would also be a benefit, reducing the so-called heat island effect by several degrees.

Hillary Shaw
Newport, Shropshire, UK

Firstly, what counts as a “building”? A shed, a bus shelter, a pier? Would you also whiten car parks, umbrellas, boats, cars, clothing? What about the sides of buildings? In many areas the sun spends more time below 45 degrees elevation than above it. That could be harsh visually.

Secondly, the process would need a lot of paint to be produced, generating carbon dioxide. Then you have to transport the paint and the scaffolding needed to apply it. Thirdly, you would need to repeat the process every few years, as the paint would become dirty and less reflective. Fourthly, the economic resources for this process might crowd out other green investments.

Maybe just emit less, and consume less too.

To answer this question – or ask a new one – email lastword@newscientist.com.

Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.

Â鶹´«Ã½ retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.

Terms and conditions apply.