Thursday, February 2

Mirror roofs, white asphalt, artificial rain: the desperate battle of cities against the heat

In the early 1990s, during Bill Clinton’s presidential campaign, political consultant James Carville made his fortune with the maxim “It’s the economy, stupid.” Today, three decades later, the slogan could be added a tagline at the local level: it’s finance… and is also the weather. Aware of the impact of what are known as “urban heat islands”, temperature accumulations Generated in large metropolises by factors such as the abundance of materials that absorb solar radiation, the scarcity of green areas and bodies of water, industrial activity or pollution, local authorities have been seeking different strategies for years.

The objective: to lower the “thermostat” of the cities.

It is estimated that the average annual temperature of the air of a city in which a million or more inhabitants reside can be between 1 and 3 ºC higher than that of its periphery. And that during daytime. At night the difference shoots up in some cases to reach ten degrees.

It’s not just that your neighbors live more comfortably, but also. There are different factors at play. Even including the one Caville pontificated about in the 1990s. As detailed in the US Environmental Protection Agency, the rise in temperatures in urban centers translates into an increase in energy costs, pollution levels —the higher the temperature, the greater spending on cooling systems— or heat-related illnesses and mortality. A study published in 2014 in the United Kingdom, for example, estimated that deaths associated with heat will increase in the country by 257% for mid-century.

From vaporizers to vertical gardens

What strategies are cities following to reverse the effect of “urban heat islands”?

On the table they have various tools, some so old that they can be found in buildings from centuries ago, such as the construction of fountains and pools to take advantage of the refreshing effect of water. It is estimated that rivers, lakes, ponds or wetlands, for example, can help reduce the ambient temperature of a city between 1 and 2 degrees centigrade, value that increases to an even larger range, about 3 to 8 ºC, when other artificial systems are used to enhance their effect, such as sprinklers or evaporative cooling.

Another of the great allies of cities are trees and, in general, landscaped areas. how to pick up a study published in The Conversation, it is estimated that a 10% increase in tree canopy cover leads to significant drops in ambient temperatures. In the afternoons the descent can be around a degree or a degree and a half. In addition to plantations in parks and other urban public spaces, vegetation is a valuable ally in the buildings themselves thanks to balconies or vertical gardens deployed along the walls. In some cases, at least, have contributed to lowering the temperature of the interiors between 2 and 3 ºC.

One of the factors that explains the difference in temperature between large metropolises and their outskirts is the use of dark and hard materials, for example asphalt, which absorb solar radiation and contribute to rising mercury. The use of alternatives of low thermal conductivity, with a high solar reflectance, helps to mitigate the heat. The use of most suitable materials and coatings It is already common, in fact, on the facades and roofs of buildings.

One recent study carried out in India, in a particularly hot region, suggests that using reflective covers, the temperature of the ceilings can be reduced by up to 30 ºC and the interior values ​​can be softened by 3 to 7 ºC. In addition to white paints, different coatings can be used in buildings, such as elastromeric or special membranes.

The strategy is also valid for urban public spaces. A study published in 2021 by MIT researchers shows that the use of special asphalts reaches reduce the temperature by more than two degrees. To achieve this, films made with mixtures that better reflect sunlight, light colors or concrete capable of achieving high reflectivity are used.

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Thanks to the combination of paraffins and waxes, among other components, a spanish consortium has developed a pavement capable of increasing solar reflectance by 173%. Result: a reduction of two degrees in ambient temperature and up to 15 degrees in radiated heat.

From the theory to the practice

Beyond the theory How are the measures applied?

Over the last few years, different cities have shown how to alleviate the effect of urban heat islands. Medellin, for example, launched a project in 2017 to transform the margins of 18 roads and 12 waterways into natural routes where their neighbors could find shade. Thanks to that deployment of 30 “green corridors”, totaling around 20 kilometers, and by planting tens of thousands of trees, the City Council of the Colombian city has managed to calm the heat. According to your data, the temperature has dropped by 2 ºC.

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To reach its goal of carbon neutrality in 2050, New York has paid special attention to the roofs. Your NYC CoolRoofs Program —promoted, among others, by its Consistory— offers facilities for its inhabitants to install reflective covers. “Cool roofs can reduce internal building temperatures by up to 30%. Each 2,500 square feet of roof (232 m2) covered can reduce the city’s carbon footprint by one ton of CO2 and help combat climate change”, details the web program, which recalls that the temperature of the urban core of New York it can register up to five degrees higher than its surroundings.

Something similar has been done in the city of Ahmedabad, west of India, where the thermometer reaches up to 50 ºC in summer. As part of a pilot initiative, a couple of years ago, more than 3,000 roofs in the city were painted with white lime and a special reflective coating. The objective was the same, to reduce the absorption of solar radiation and achieve reductions in interior temperature of up to 7 degrees. In Los Angeles they have directly opted for paint some city streets white and in Phoenix they have tested, as a pilot experience, with a mixture based on water, soap, asphalt, polymers and recycled materials that manages to reflect more sunlight.

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In some cities, like Chongqing, in China, have experimented with devices that spray some bus stops with water at 5 or 7 ºC, a solution that in Europe is seen more and more frequently on the terraces of bars in summer. The initiative falls short in any case when compared to the artificial rain systems proposed in Dubai to alleviate high temperatures. He has plenty of experience too. Singapore, which has spent decades embarked on the project of becoming a “garden city”, with green roofs that extend over the ground, but also over the buildings themselves, up to the terraces, homes and offices.

Images Ken Banks Y Geoff Henson