White roofs offer cities a simple way to fight extreme heat

White paint could help cities cope with rising temperatures by reflecting sunlight, cooling buildings and their surroundings by a couple of degrees, Yle reports.

Several severe heatwaves have hit southern and central Europe this summer, with temperatures rising above 40 degrees Celsius in many places. The highest reading, 48.4C, was recorded in mid-July in Sardinia, Italy.

Life has been particularly difficult, and in some cases fatal, in large cities. During the heatwaves, mortality in Paris and its surrounding suburbs nearly doubled.

The problem is linked to the urban heat island effect. People and traffic raise temperatures, but sunlight has the greatest impact. Heat from the sun becomes trapped in concrete and asphalt during the day, making cities warmer than the surrounding countryside.

“Temperatures eight to 10 degrees higher have been measured in large cities, but typically the difference is a few degrees,” says Leena Järvi, a professor of urban meteorology at the University of Helsinki.

The effect continues at night as heat remains trapped between streets and buildings, making it difficult for residents to sleep. Older people and those who are ill are particularly vulnerable, Järvi says.

White roofs cool buildings and streets

Heatwaves are becoming more frequent and more intense because of climate change. One surprisingly simple way to ease the heat in cities is to use white paint.

The idea is not new. Buildings in Mediterranean countries have been painted white for centuries. White surfaces reflect sunlight, lowering the indoor temperature while also keeping the surrounding area cooler. They can reduce the need for energy-intensive air conditioning.

New York has been painting the roofs of apartment buildings for around 20 years. About one million square metres of roof has been covered so far, with more than half of the potential work still remaining.

“When the city cools down, people stay healthier, energy is saved and it is more pleasant to live here,” Mehdi Pourpeikari Heris, a professor of urban planning, said recently.

Reflective paint can also be applied to roads, as has been done in California. Research into the cooling effect of reflective roofs has increased in recent years, and the approach is commonly known in English as a “cool roof”.

A reflective roof can lower temperatures both inside and outside a building by a couple of degrees. Somewhat surprisingly, white paint works better for cooling than green roofs, Järvi says.

Järvi says more research would be needed before painting roofs white in northern cities such as Helsinki. “I would like someone to paint one roof and study what it does before we go ahead and paint every roof or several roofs,” she says.

Ultra-white paint raises wider risks

Ordinary white paint reflects 80 to 85 percent of light back into space. Researchers at Purdue University in the United States have developed an ultra-white paint that reflects up to 98 percent.

Jeremy Munday, a professor who researches green technology, has calculated that covering 1 to 2 percent of the Earth’s surface with such paint could stop the rise in the planet’s temperature. The area would be vast; the total area of the United States, for example, is about 2 percent of the Earth’s surface.

Hannele Laine, director-general of the Integrated Carbon Observation System, rejects the idea as too risky. If the painting were not carried out evenly, she says, it could significantly affect atmospheric circulation and alter when and how much it rains.

That could have major consequences for agriculture and other aspects of people’s lives, Laine says.