Can White Roofs Help Put a Lid on Urban Heat Waves?

Last summer shattered temperature records, and this year shows no signs of cooling down. With experts warning of even hotter summers ahead, adapting to rising temperatures is critical. Cities, especially in India, face the worst of it, turning into heat islands that retain heat long after the sun sets.

Adapting to warming cities requires various solutions. The good news is simple, tried-and-tested methods can bring relief from the heat. Planting shade trees and cooling our roofs can make a big difference.

Coating city roofs with white reflective coating can bounce back the sun's rays instead of absorbing them, reducing the urban heat island effect, potentially lowering temperatures and making heat waves more bearable.

The Autonomous University of Barcelona proposes cooling overheated urban areas by replacing dark roofs with reflective white ones, potentially lowering citywide temperature by nearly 5°F (2.5°C) or more.

"Cities with urban heat islands could paint their rooftops white," says Sergi Ventura, a UAB meteorologist involved in the Barcelona study. "In our models, we've detected an average temperature reduction of 1.4°F (0.8°C) during heat waves. And peaks of up to 7.6°F (4°C) lower at specific locations and times."

Urban heat buildings rapidly

The urgency to defuse urban heat islands is increasing as climate change raises global temperatures.

Extreme heat isn't just uncomfortable; it can be deadly, especially for vulnerable groups like the elderly. The 2003 European heat wave led to over 70,000 deaths, many at night when people couldn't escape the heat.

Ventura and other researchers suggest that changing roof albedo could be a simpler, lower-cost alternative to planting greenery and boosting parks for urban cooling.

Countering urban heat Island

The urban heat island effect has been observed for over a century, since the growth of the world's first mega-cities in the industrial era. Cities are significantly warmer than surrounding rural areas due to buildings and paved surfaces absorbing and radiating the sun's energy.

"Fresh asphalt reflects only 4% of sunlight, compared to 25% for natural grassland and up to 90% for fresh snowfall,"

                      says Keith Oleson, A scientist at the National Center for Atmospheric Research (NCAR) studying urban heating.

In a 2010 study, Oleson modelled the effects of making all urban roofs and pavement brilliant white worldwide. His simulations predicted a one-third reduction in the urban heat island effect, taking the edge off summertime peaks by around 1.3°F (0.7°C) on average.

"Our research shows white roofs could reduce heat," says Oleson. "It's unclear if making roofs white is feasible, but the idea warrants further investigation."

Other studies found greater localized cooling benefits. In the U.S., Matei Georgescu of Arizona State University estimated white roofs could reduce temperatures by up to 2.7°F (1.5°C) in California and 3.2°F (1.8°C) in cities like Washington D.C.

Reflecting heat or creating it?

Not everyone is convinced that white roofs are worth the risks. Some climate scientists worry that increasing urban surface reflectivity could worsen global warming by reflecting more sunlight into the atmosphere.

"Using white roofs to cool your house at the expense of warming the planet isn't a good trade-off," cautions Mark Jacobson, a Stanford University researcher. "There are more effective ways to reduce global warming."

In a 2009 study, Jacobson and colleagues modelled the effects of replacing all roofs and paved surfaces worldwide with white coverings, accounting for climate feedback loops. Their simulation projected a slight global temperature increase.

The study found that white surfaces reflect sunlight into space, but some of that energy gets absorbed by atmospheric particulates like soot, generating heat and offsetting the cooling effect.

"Our simulations suggest the urban heat island effect may contribute 2-4% to global warming, but more verification is needed," says Jacobson. His work estimates greenhouse gases drive about 79% of warming while heat-absorbing particulates like black carbon account for 18%.

Switching to white roofs worldwide could slightly warm the Earth due to a complex domino effect. The study did not account for potential emissions reductions from decreased air conditioning in white-roofed buildings.

Sonia Seneviratne, a climate scientist at ETH Zurich, advocates for increasing urban albedo, acknowledging concerns but suggesting they may be overstated for local temperature management.

"Local radiative management differs from global geoengineering because it doesn't affect global temperatures, so global effects would be negligible," Seneviratne says. "It's a measure of adaptation, not an attempt to geoengineer the entire planet's climate system."

Room for compromise

Some researchers see potential for a combined urban heat island mitigation strategy using high-albedo roofs and solar panels, which capture sunlight to generate energy and have a net cooling influence.

A 2022 study from the University of New South Wales found that solar panels cool daytime temperatures like white roofs. The study estimated a citywide photovoltaic array could lower summertime temperature peaks by around 2°F (1°C) in Sydney, Australia.

Barcelona resident Saintot integrates rooftop solar with white reflective surfaces, a sensible compromise. "Having both would be ideal," he says. "Anything that helps with renewable electricity and keeps buildings cooler is a win-win against climate change."

2021 study suggests large solar farms in deserts could cause a "heat island" effect by insulating the ground and preventing nighttime cooling. Complex modeling needed to assess deployment strategies.

Lack of quick action troubling

To Adrian Fernandez of Greenpeace Spain, nuances are secondary. The environmental activist fears inadequate measures are being taken to adapt Barcelona and other cities for heat waves and climate change impacts.

"We need to make sweeping changes in the city's layout," says Fernandez. "Roof color changes might help, but with more frequent and severe heat waves, we need broader solutions."

He cites a lack of urgency from authorities despite Barcelona's climate emergency declaration in 2020: "Things are not moving fast enough. More progressive actions on buildings, transportation, energy systems, and green spaces need to happen simultaneously. The time to start was years ago."

For Keith Oleson of NCAR, reflecting roofs are just one tool to evaluate as part of a holistic approach to make cities more resilient to rising temperatures.

Understanding climate change's impact on vulnerable urban areas, home to most of the world's population, is crucial," says Oleson. "White roofs could help, but we must explore other options."

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