Close Login
Register here
Forgot password
Forgot password

Close Inspiration on environmental sustainability, every month.

Currently 5,988 people are getting new inspiration every month from our global sustainability exchange. Do you want to stay informed? Fill in your e-mail address below:


Want to be kept in the loop? We will provide monthly overview of what is happening in our community along with new exciting ways on how you can contribute.

Close Reset password
your profile is 33% complete:
Update profile Close

Architecture architecture General

Green or white roofs to cool urban area's?

Share this post
by: Peter Sant
green or white roofs to cool urban area s

Are white roofs the solution for warming cities?

The summers in the city can be hot - several degrees warmer than in the countryside. However, recent research indicates that this does not necessarily have to be the case. The systematic replacement of dark surfaces with white can reduce the temperature by 2 degrees Celsius or more. The heat island effect will increase with climate change and the ongoing urbanization. There is therefore sufficient reason to look for multiple ways to keep us cool.

The meteorological phenomenon of the ‘urban heat island effect’ has been known since the rise of large cities in the 19th century. The materials with which most cities and roads are built reflect much less solar radiation - and absorb it more - than the vegetation they have replaced. Part of that energy is again released in the air in the form of heat.

The darker the surface, the stronger the warming. Fresh asphalt reflects only 4 percent of the sunlight, compared with 25 percent for grassland and up to 90 percent for a white surface such as fresh snow.
Black pavement, shadow of a man

Approximately 2 percent of the earth's surface is occupied by cities and is subject to a certain level of district heating. According to the American Environmental Protection Agency, New York City is on average 1 to 3 degrees Celsius warmer than the surrounding countryside, and up to 12 degrees warmer on some evenings. The effect is so overwhelming that some climate skeptics have already claimed that global warming is only an illusion created by thousands of meteorological stations that once stood in rural areas but were gradually surrounded by urbanization by more and more buildings.

Climate scientists take this type of deviation from measurements into account, so the claim does not hold. Nevertheless, the effect is real. So, says a recent study published in the journal Nature Geoscience, if dark, heat-absorbing surfaces heat our cities, why not reverse the effect and install white roofs and other light-colored surfaces to reflect the sun's rays?

White roofs from New York to Melbourne

Many white roofs in cirkles seen from above
Photo: under Architecture, Environment, global warming, Green Roof, News & Inhabitat

During a heat wave, when the sun has free rein in a cloudless sky, the creation of lighter land surfaces can 'help to reduce extreme temperatures by 2 or 3 degrees Celsius' in much of Europe, North America and Asia, says co-author of the new study Sonia Seneviratne, which studies land-climate dynamics at the Swiss Federal Institute of Technology (ETH) in Zurich. It could save lives, she says, and the warmer it gets, the stronger the effect. It could save lives, and the warmer it gets, the stronger the effect.

Seneviratne is not alone in defending the reflection of sunlight. There are many small-scale initiatives in cities to make roof surfaces more reflective. For example, in 2012 New York introduced rules about white roofs in the building codes. Volunteers in the city have painted almost seven million square meters of roof covered with tar white. However, this is only about 1 percent of the potential roof surface.

Chicago is trying something similar, and last year Los Angeles started a program to paint road surfaces in asphalt in light gray paint. Outside the United States there are initiatives on cooling roofs in cities such as Melbourne.
However, these remain small-scale programs, the results rather anecdotal. It is therefore important that researchers now gather evidence around the world that shows that the benefits of converting that 1 percent into 100 percent every year can save many lives.

Custom farming

Keith Oleson of the National Center for Atmospheric Research at Boulder, Colorado, looked at what would happen if every roof in large cities around the world was painted white. As a result, the reflectivity of objects - climate scientists would call the 'albedo' - increase from 32 percent today to 90 percent. He discovered that the heat island effect would be reduced by a third. That is enough to reduce the maximum daily temperatures by an average of 0.6 degrees Celsius, and more in hot regions such as the Arabian peninsula and Brazil.
Other studies indicate even greater benefits in the US. In a 2014 publication, Matei Georgescu of Arizona State University shows that 'cooling roofs' can lower temperatures in California to 1.5 degrees and to 1.8 degrees in cities like Washington.

Not only the cities benefit from a whitewashing, as it turns out. Seneviratne and her team suggested that farmers can also cool their residential areas with other farming methods. Modified methods, applied over large areas, can have a considerable effect according to her.
In Europe, grain fields are almost always plowed shortly after harvesting. This makes the fields large, dark surfaces that absorb the sun's rays during the winter. However, if the land is not plowed immediately, the light-colored stubble that remains on the fields after harvesting could reflect about 30 percent of the sunlight, compared to only 20 percent for a field that is released immediately. This may sound like a relative trivial difference, but it can be calculated for large areas of arable land that the temperature in some rural areas may decrease by as much as 2 degrees on sunny days.
Harvest cornfield brown stubbles
In North America, early shifts occur much less often. But Peter Irvine, a researcher in the field of climate and geo-engineering at Harvard University, has suggested that crops can also be chosen on the basis of their ability to reflect sunlight. In Europe, for example, a cereal such as barley, which reflects 23 percent of the sunlight, can be replaced by sugar beet, an economically comparable crop that reflects 26 percent. In other words, farmers can simply choose more reflective varieties to grow.
Again, the difference sounds marginal. But since arable land covers more than 10 percent of the land surface of the earth, about five times more than cities, the potential can be considerable.

Unpleasant consequences for other regions

At first glance, such initiatives seem appropriate if countries have difficulty with the consequences of climate change. But there is also concern that if large parts of the world take such policy measures to reduce local heat waves, this may lead to noticeable and possibly unpleasant consequences for temperature and rain in neighboring regions.
'Local management of solar radiation differs from global geoengineering because it is not aimed at influencing the global temperature and global effects are therefore negligible'

Proponents of local projects, such as suppressing the heat island effect, say that they are only trying to reverse the consequences of unintentional geo-engineering through urbanization and the growth of arable land. Moreover, they state that local adjustments will only have local effects. 'If all French farmers stop plowing in the summer, their impact on the temperature in Germany will be negligible', says Seneviratne.

'Local solar radiation management differs from global geo-engineering because it is not aimed at influencing the global temperature and global effects are therefore negligible', she says. It is only an 'adjustment measure'.

But sometimes things are not that simple. For example, lowering local temperatures would limit evaporation and thus potentially affect rainfall. A model study by Irvine concluded that rumbling with reflection of sunlight in larger areas such as deserts could cause a "large decrease in the intensity of Indian and African monsoons in particular." But the same study also concluded that changing albedo in cities or likely to have no significant effect on agricultural land.

Cool cities, save lives

What is clear is that tackling the heat island effect by increasing solar reflection is not enough to ward off climate change. According to Oleson's calculations, the whitening of every urban roof and square in the world would only slow down global warming by eleven years. But the potential value of alleviating the most serious effects of overheating in cities can save lives.

The heat island effect can be a killer. Counter-intuitively, the biggest effect is often at night. Vulnerable people, such as the elderly who suffer from the heat during the day, need the night to cool down again. Without this possibility they can succumb to heat stroke and dehydration. Research from this month shows that temperature peaks also cause a peak in heart attacks. This happened during the big European heat wave of 2003, where about 70,000 people died, mostly in houses without air conditioning. Doctors said that the killer was not so much the daytime temperature of 40 degrees Celsius or higher, but the fact that the nights were warmer than 30 degrees.
Woman under fountain water
Heatwave 2003. Photo:

Such nightmares are likely to occur more and more in the future, because the urban area is increasing, and because of climate change.

Taking into account the predicted urban expansion in the US this century 'the temperature near the earth's surface can be expected to increase by 1 to 2 degrees Celsius across large regional parts of the country', says Georgescu's paper from 2014. Similar scenarios threaten other parts of the country. world that is rapidly urbanizing, including China, India and Africa. These areas are expected to multiply their urban land area by 2030 compared to 1970. 'Vulnerable populations are thus exposed to climate change driven by land use.'

Several studies suggest that climate change itself can fuel the heat island effect. Richard Betts of the British Met Office Hadley predicts that in some places this will increase the difference between urban and rural temperatures by as much as 30 percent, especially in the Middle East and South Asia, where deaths during heatwaves are already prevalent.

The combination of rising temperatures due to climate change and high humidity will make it likely that large parts of the Persian Gulf area will be the first in the world to become uninhabitable. And a study published in February predicted temperatures could rise to 10 degrees in most European cities by the end of the century. No wonder the call for cooling of cities sounds louder.

Not white but green

Buiding with green roofs and balconies
Photo by:

Another option is not to spray roofs white, but to make green roofs. This is already being applied in different cities. In 2016, San Francisco became the first American city to make the installation of green roofs mandatory on some new buildings. New York announced last year a program of 100 million dollars for cooling neighborhoods by planting trees. So, what's better, a white roof or a "green" roof?

According to Georgescu, the direct cooling effect of white roofs is greater. Vincenzo Costanzo from the University of Reading, has a similar conclusion regarding Italian cities. But green roofs have other advantages. An investigation in Adelaide, Australia, has shown that in addition to cooling in the summer they also serve as an insulating layer to keep buildings warmer in the winter.

Whitewashed walls, photovoltaic cells and fields full of stubble can all provide local relief during the sweltering decades that are coming.

white roofs and walls houses
Photo by:

There is also a third option: covering roofs with photovoltaic cells. They are dark and therefore do not reflect much solar radiation in space. But that is because it is their job to capture the energy and convert it into sustainable electricity.

Houses with solar panels on the roof

Solar panels 'cool day temperatures in a way that is comparable to increasing albedo via white roofs', say scientists at the University of New South Wales. Their research, published last year in the journal Scientific Reports, showed that solar panels in a city like Sydney in Australia could lower the temperature to 1 degree.

That is the theory. The question is whether it will also work in practice. After all, research into the influence on local temperatures of large solar parks in deserts has yielded contradictory findings. Because while they prevent the sun's rays from reaching the desert surface, they also work at night as an insulating blanket, so that the desert sand cannot release the absorbed heat.

The conclusion is then that light, reflective surfaces can have a major impact on cooling the ambient air - in cities, but also in rural areas. Whitewashed walls, photovoltaic cells and fields full of stubble can all provide local relief during the sweltering decades that are coming.

But policy makers beware. It does not always work that way. There may be unintended consequences, both for temperature and for some other aspects of the climate such as rainfall. Even local geo-engineering must therefore be handled with care.

By: Mondiaal Nieuws (MO). Cover photo:

Share this post
Being involved in sustainability activities has changed my view on this subject a lot. Climate change and pollution are borderless and thus solutions and information has to be shared globally. Rich, 'developed' countries have to start supporting countries that don't have the means and knowledge to improve their situation. Sustainability movement is as strong as its weakest link - is a helpful platform to speed up the X-Change of Global Sustainability.  
Being involved in sustainability activities has changed my view on this subject a lot. Climate change and pollution are borderless and thus solutions and information has to be shared globally. Rich, 'developed' countries have to start supporting countries that don't have the means and knowledge to improve their situation. Sustainability movement is as strong as its weakest link - is a helpful platform to speed up the X-Change of Global Sustainability.  
Get updates on environmental sustainability in your mailbox every month.


Whatsorb info

whatsorb whatsorb whatsorb