Cool Cities
To understand how the world is projected to warm by 2050 under our current trajectory of greenhouse gas emissions, imagine the place you live with the climate of a location 600 miles closer to the equator. For example, New York City of 2050 would resemble present day Arkansas, while London’s climate would be akin to Barcelona, and San Francisco that of Los Angeles.
These shifts in climate will have dramatic effects on water availability, agriculture and the daily life of billions of people, yet they will be felt most acutely during future heat waves.
As reported by Mark Lynas in Our Final Warning: Six Degrees of Climate Emergency, what are extreme heat events in the present day will become commonplace in a world that has warmed 2˚C above pre-industrial levels. For reference, 2˚C is the outcome from achieving the Paris Agreement’s goals for emissions cuts.
In a 2˚C world:
Europe’s 2003 heat wave, which saw 35,000 - 70,000 heat fatalities, and Russia’s 2010 event, when 55,000 people perished, would become “normal” most years.
Australia’s exceptionally hot summer of 2012-2013, when temperatures averaged 104.5˚F, would be 50% more likely.
China’s worst heat wave in 140 years, which occurred in 2013, “would be considered on the cool side” according to Lynas.
In India, Lynas finds that “severe heat waves are projected to rise by 30 times compared to the current climate”.
These are just a few examples. Overall, a 2˚C hotter climate would see a 25% increase in hot days, exposing 37% of the global population to severe heat waves every 5 years and 70% of people to severe heat every 20 years, according to Carbon Brief.
Extreme heat poses a serious threat to human life, as U.S. data shows that extreme heat already causes many more deaths than floods, hurricanes, fires, extreme cold or other hazardous weather events. Prolonged heat waves exert stress on the human body, particularly when nights stay hot or in combination with high humidity levels, which can prevent the body from cooling itself off and prove fatal. Heat waves are also linked to increases in violence and murder.
The impacts of extreme heat will be exacerbated in cities due to the urban heat island effect and concentrations of vulnerable populations, who may lack access to weather forecasts, air conditioning or reliable water supplies for hydration.
The urban heat island effect is largely caused by the cumulative effect of the paved surfaces and buildings in a city, which absorb heat from the sun and then radiate that heat back into the air. Other contributing factors include reduced airflow in urban canyons, as well as waste heat from cars, industry and air conditioning units.
Credit: The Architect’s Newspaper
According to the EPA, the urban heat island effect adds up to 7˚F daytime and 5˚F nighttime temperatures to a city. As the world population increases to 9 billion, and the share of humanity residing in cities grows to 68%, the urban heat island effect will increase as paved surfaces expand outward.
A study convened by the C40 Cities Climate Leadership Group found that by 2050, 970 cities with a cumulative population exceeding 1.6 billion “will be living with extreme high summer temperatures”, compared to 200 million in the present day. Of these 970 cities, many do not currently experience extreme heat, so adaptation will be necessary to protect their residents. In particular, their 215 million residents living in poverty, who “may lack access to adequate drinking water, shelter, air conditioning, and medical assistance”, will need these resources and support, or significant heat fatalities will occur.
For those who can pay, air conditioning (AC) is the obvious response to endure heatwaves, and in a world warmed to 2˚ or 3˚C , demand for AC will skyrocket. According to the International Energy Agency, energy demand via air conditioning is projected to triple by mid-century, equivalent to the energy consumed by the United States, European Union and Japan - combined.
While using AC is completely justified for people’s survival, such massive increase in demand for mechanical cooling will incur significant costs on households - particularly on economically developing nations in the global south - and create an added burden on the already-challenging task of decarbonizing the electricity grid.
Finally, heat will negatively impact economic productivity: the Imperial College in London found that some parts of the world could see 30-40% of daylight hours too hot for work each year, affecting the ability of four billion people to earn income. Asian nations alone could lose $4.7 trillion in gross domestic product annually by 2050, due to outdoor workers and associated economic activity being sidelined by extreme heat and humidity, according to the McKinsey Global Institute.
Given the threat to human life, the high costs of AC and the degradation of economic productivity from extreme heat in urban areas, cities must be proactive to meet this challenge. There are a number of steps they can take to do so:
Planting trees and living walls or roofs covered with vegetation, as plants cool the air by boosting moisture transpiration.
Improving air flow in urban canyons to facilitate the mixing of warmer and cooler air currents.
Communicate heat wave warnings to vulnerable populations, provide public cooling shelters and access to water.
Cities can also take inspiration from ancient builders in places like the Cyclade Islands of Greece, whose traditional homes are painted white to stay cooler in their hot Mediterranean summers.
Island of Santorini, Greece.
Painting buildings in more reflective colors can be applied to modern cities as well - a practice I’ll call urban albedo enhancement - to reduce the urban heat island effect, protect residents against extreme heat, cut demand for AC and support continued economic productivity, as well as quality of life.
By painting roofs, streets, sidewalks and parking lots with lighter, more reflective pigments across an entire urban region, the amount of solar radiation, and therefore heat, that gets absorbed by surfaces is reduced. This provides cooler conditions on the ground (or on the roof) where albedo enhancement occurs, and more broadly across the urban region if applied at a sufficient scale.
White Roofs in Queens, New York City. Credit: NASA
In experiments, Lawrence Berkeley Lab (LBL) has found that a clean, white roof reflects away 80% of sunlight and is 55˚F cooler than a typical gray roof, which only reflects 20% of sunlight. Likewise, LBL demonstrated that lighter colored pavements stay 30˚F cooler than darker pavements.
Deployed on a meaningful scale across an urban area, increasing roof albedo is estimated to reduce maximum temperatures by 1˚F, and expanding brighter pavements to 35% coverage could lower air temperature another 1˚F. Overall, increasing the albedo of city surfaces could offset 33% of the urban heat island effect.
Advanced paint formulas hold promise for further increasing this cooling effect: in 2021, researchers from Purdue unveiled a paint that reflects 98.1% of light. This new formula also had the unique effect of staying 8˚ - 18˚ F colder than the ambient air, meaning it would cool the surrounding environment, instead of merely reducing the amount of warming.
Conventional vs Cool Pavement in Los Angeles. Credit: Cool Streets LA
However, highly reflective white paint may not be the best idea to apply everywhere. Researchers from Arizona State have raised concerns that cool pavements may increase air temperature felt by pedestrians, since more sunlight is being reflected up from the ground, and high albedo roads may cause glare problems for drivers. Reflective roofs may bounce light onto nearby buildings, causing their temperatures to rise, according to a study from University of California San Diego.
We ought to use the brightest paints where impacts are minimal, such as multistory apartment buildings at the same height, since white roofs can’t reflect light onto one another. Where issues could arise from super reflective surfaces, we can instead utilize new nonwhite pigments, which offer improved albedo levels, are less likely to cause side effects and can accommodate any aesthetics concerns.
LBL has developed multiple hues of higher reflective surface coatings and roof tiles, which appear virtually the same to the human eye as conventional products. One ruby red product created by the Lab even has similar reflectivity to white roofs.
SR = Solar Reflectivity value. Credit: Cool Colors LBL
These alternative pigments could be used where white paint is not appropriate, such as roads, streets and building rooftops that may reflect light onto adjacent buildings. Though the gain in albedo would be lower than white pigments, they could be applied more broadly across the urban landscape.
For cities, enhancing surface albedo can save lives during heat waves by reducing day and night temperatures, with one study finding that cool roofs alone could eliminate 25% of mortalities in cities.
By installing cool roofs, individual buildings can cut energy consumption and costs by 15%. Even in areas with cold winters, where the warming effect of dark surfaces reduces energy demand for heating, LBL found that the financial value of cool roofs in reducing summer AC costs still offered a greater benefit.
Of course, broad scale albedo enhancement will incur upfront costs, likely in the millions each year for a given city. For example, as of 2019 New York City had painted 10 million square feet of rooftops white at a cost of about $5 million, based on a ballpark estimate of $.50/square foot based on a city report. Thus, to cover 30% of NYCs roof space with reflective coatings would cost $8.7 million per year over 30 years.
Painting a Cool Roof in New York City. Credit: OneNYC2050
When expanded to consider cities across the globe, the install costs certainly appear expensive. However, the regional cooling effect of albedo enhancement could yield savings from reduced AC consumption that could offset installation costs.
For Phoenix, Arizona researchers estimate that each 1˚F reduction regionally results in $15 million per year in avoided electricity costs from AC. With cool roofs and roads each offering a cooling effect of 1˚F, Phoenix residents could save $30 million per year. Besides AC costs, the cooling benefits for economic activity, quality of life and reduced risk of power system failure should also be considered.
We have an example of how albedo enhancement can cause regional cooling, in the southern Spanish province of Almeria. Located on a coastal plain of the Mediterranean, Almeria has actually gotten 2.9˚ F cooler over recent decades, even as Spain a whole has warmed faster than the global average. The secret?
Almeria grows more than half of the fresh fruits and vegetables that Europe consumes, in thousands of white greenhouses. Collectively, the result is a significant increase in albedo and sunlight reflected away, causing a region-wide cooling effect.
Greenhouses in Campo de Dalias, Almeria viewed from space. Credit: NASA
In terms of the rollout, pavements need to be resurfaced every 10 years, and roofs replaced every two to three decades. Cities could mandate that high albedo options are utilized when replacing or resurfacing, to ensure widespread adoption over time. Simultaneously, cool roof and cool street programs could be applied in neighborhoods that get the hottest or have higher concentrations of low income residents, which often overlap.
The benefits to cities of cool surfaces are sufficient to warrant investing in, yet there may be a final boon: if a critical mass of cities globally were to install cool roofs and pavements, the cumulative effect could be a modest, yet valuable cooling effect for the planet as a whole. Increasing the albedo of urban surfaces by 10% across temperate and tropical regions could have a cooling effect globally, equivalent to offsetting 7 months to 5 years of global emissions over time, based on findings from two studies from 2010 and 2012.
Cool cities, therefore, could slow the rate of global warming, while providing the slew of local/regional benefits described previously. Combined with other projects to enhance Earth’s surface albedo in arctic sea ice, marine clouds and deserts, the cumulative effect could be substantial, providing protection from climactic extremes and stretching out the time horizon to zero out emissions and restore our climate through CO2 drawdown.
Special thanks to Michael Smith, an intern at San Francisco State, for providing research and input on this article.