The problems with excessive heat are clear to anyone living in the cities where thermometers posted record highs in summer 2021, and increasingly to lawmakers, who see the health and economic impacts of the raging temperatures
UNTAPPED RESOURCE
District cooling systems could be used more widely as a low-carbon way of cooling buildings
ALTERNATIVE TECHNOLOGIES
Cities are looking to cool their urban areas beyond electricity-based solutions
KEY QUOTE
Cooling is one of the mountains in front of us if you look at the drivers of climate change
Dark, impermeable pavements and buildings dominate the grey landscape of most cities. Trees and other vegetation are often lacking. This combination gives rise to the phenomenon of urban heat islands in which the temperature of a city can be several degrees higher than surrounding areas. In Spring 2021, three cities on three continents—Athens in Greece, Miami in the US and Freetown in Sierra Leone—each announced they would set up a new position in each local government of chief heat officer. United States president Joe Biden in September 2021 announced a plan to combat extreme heat, including in urban heat islands. The plan includes initiatives to increase cooling assistance to vulnerable communities and low-income households, better protect heat exposed workers and promote targeted solutions such as tree cover to address urban heat. By 2100, cities around the world could warm by as much as 4.4°C on average, as a result of the urban heat island effect, according to findings of a group of international researchers published in Nature Climate Change in January 2021. This is nearly triple the Paris Agreement objective to limit the global temperature rise to no more than 1.5°C. Since urban areas are projected to house about 70% of the global population by 2050, up from 55% in 2018, cooling will largely be a problem for cities to resolve. Cooling itself is a contributor to global warming, so scaling up while simultaneously decarbonising its energy use will be essential.
DEMAND GROWTH The International Energy Agency (IEA) notes that energy demand for space cooling has more than tripled since 1990 and was responsible for 8.5% of total final electricity consumption in 2019. In a baseline scenario in its 2018 Future of Cooling report, the IEA estimates energy demand for space cooling will roughly triple from its 2016 level to reach 6200 terawatt-hours (TWh) by 2050. Due largely to the increased use of renewable energy, emissions from space cooling are seen rising at a slower, but still worrisome, rate to reach 2050 million tonnes (Mt) in 2050, nearly double the 1135 Mt in 2016. “The need for cooling is seen constantly increasing in all climate scenarios,” notes Irene Skoula of C40 Cities. “In most cities, heat is one of the biggest risks they are facing.” Emerging markets are expected to account for the bulk of new space cooling demand—with India, China and Indonesia alone seen representing about half of the total—the IEA points out, so what cities do in these places is critical. “Cooling is one of the mountains in front of us if you look at the drivers of climate change,” says Iain Campbell at the Rocky Mountain Institute (RMI), a US-based energy research organisation. “Most people have heating so the big push now is to decarbonise it, but many people who will need access to cooling don’t have it.” 
DISTRICT COOLING One largely untapped, sustainable resource for cooling cities is district cooling, in which chilled water transported through insulated pipes from a central source provides cooling to a district or group of buildings. IEA figures indicate less than 4% of cooling energy used for space cooling comes from district cooling, with the greatest penetration in the US, followed by the United Arab Emirates (UAE) and Japan. District cooling is five to ten times more efficient than traditional air conditioning and can reduce energy consumption by 50%, notes Giulia Forgnone of European district energy association Euroheat & Power. Cooling sources can vary from cold water piped up from lakes, seas and rivers to waste cold and heat, which may be brought to the necessary temperature with the use of a chiller, a type of heat pump. Like district heating, district cooling makes sense in densely populated areas. District cooling is often combined with storage, so an excess capacity of chilled water during the night, for instance, can be stored for use during the day, enabling load shifting. Another benefit is that, unlike air conditioning units, district cooling does not eject waste heat. In the emirate of Dubai, which boasts the world’s highest concentration of district cooling and where temperatures exceed 40°C in the summer, district cooling has helped to limit electricity grid expansions. Dubai aims for district cooling to serve 40% of cooling needs by 2030. District cooling may also be used in more temperate climates. A district cooling network in Gothenburg, Sweden, provides about 100 gigawatt-hours (GWh) of cooling a year, supplying offices, supermarkets and other customers that need refrigeration, explains Lars Holmquist of the utility Göteborg Energi. In winter, the cool source is provided by river water and in summer from waste heat. While Gothenburg’s district cooling system is destined to grow, it is not expected to rival the city’s 3.5 TWh district heating network. Holmquist sees the biggest district cooling potential in hot, densely populated areas.
URBAN PLANNING District cooling systems are easier to put into place in new developments. With the rapid rate of global population growth and urbanisation, the market will expand rapidly in the coming years, particularly in places like India and China that are facing increasing cooling demands. According to the United Nations Environment Program (UNEP) three-quarters of the infrastructure that will need to exist in 2050 has yet to be built, much of it in developing countries. Implementing district cooling in existing cities and neighbourhoods is far from impossible, however. “For district cooling, you need to dig holes into the ground, which is always a challenge if you live in a city but the same is true for electricity, gas and fibreoptics, so you could combine this with other people digging holes in the ground,” notes Holmquist. “The more urban planning you have, the easier and more efficient it will be.” Within Europe, Paris boasts the largest district cooling system in Europe. Since 1991, Climespace, a unit of Engie, a French utility, has operated a district cooling network, with the cooling source provided by water from the River Seine and pipes placed within the sewage system to form an 86-kilometre underground network. The cooling system serves hundreds of buildings, including offices, banks, shops, hotels, the Louvre and other museums. While district cooling only has about a 2-3% market share in space cooling in Europe, Euroheat & Power’s Forgnone expects this will increase. “Cities that already have in place district cooling systems—the ones that already know this technology—plan to expand them,” she says. A lack of knowledge about the technology is one of the obstacles to the spread of district cooling in many cities, along with the higher upfront costs compared to traditional air conditioning solutions. “But in the long term, you have a big return in terms of sustainability,” Forgnone adds. 
ENERGY EFFICIENCY Although cities have varying degrees of responsibility for building codes, they can enforce existing energy efficiency requirements and sometimes strengthen them, while also working with other government bodies for tighter standards. “The way buildings are designed and built, including the choice of materials used in their construction, can have a huge impact on the need for ACs [air conditioners] and the subsequent energy needed to provide cooling services,” the IEA notes. It believes that policies for more efficient air conditioners combined with policies for more efficient buildings, “Could actually keep the energy demand for cooling flat while allowing strong growth in access to cooling for populations around the world.” In Los Angeles in the US, a 2019 sustainable city plan required all new buildings to be net-zero carbon by 2030 and all new municipal buildings and major renovations to be completely electrified. Meanwhile, the Toronto Green Standard lays out the Canadian city’s sustainable design requirements for new private and city-owned developments as it moves towards zero emissions for all new buildings by 2030, building on the Ontario region’s building code.
BEYOND ELECTRICITY Not all of the solutions for cooling need to be electricity-based. RMI’s Campbell says one way to help cool cities is to, “See what you can do through urban form and design, for example, by using green and blue spaces in the city and optimising them, using reflective surfaces, removing industrial processing from centres so you reduce anthropogenic heat,” and take action to limit the impact of transport, which gives off waste heat as well as adding to greenhouse gas emissions. To effectively reduce “urban heat stress on large scales,” researchers involved with the Nature Climate Change study advise “green infrastructure intervention” is needed. Trees and vegetation lower temperatures through shading and evapotranspiration, which draws heat from the air to evaporate water. Bringing more trees and vegetation to cities has become a priority of more and more local governments. An Urban Nature Declaration signed by 31 cities that are part of the C40 Cities network in July 2021 commits them to secure 30-40% of the city’s surface areas for green or blue infrastructure, including trees, urban forests and parks, along with sustainable urban drainage systems and permeable pavements. In New York City, former mayor Michael Bloomberg in 2007 launched plans to plant one million trees in a decade, an objective that was reached in late 2015 and increased the city’s urban tree canopy by 20%. In Milan, Italy, the first of three million trees the city has pledged to plant by 2030 were put into the ground in 2019. Athens, the European city ratings agency Moody’s deems to be at the highest risk of extreme heat, is “Doing a lot on green and blue spaces, not only planting trees but looking where to place them to maximise their benefit,” notes C40’s Skoula. To combat urban heat islands, Medellin, in Colombia, planted trees and other vegetation alongside roads and waterways. The city says the average temperature in its new “green corridors,” which were concentrated in areas lacking green spaces, has declined by over 2°C. The new greenery is also a matter of social equity given that in Medellin and many other cities around the world, lower-income people tend to live in neighbourhoods with less vegetation and higher temperatures. 
The Big Apple New York will be one million trees richer within a decade
ROOFS AND PAVEMENTS Cities are not only expanding areas dedicated to greenery and planting trees in the ground but also putting vegetation on building tops. According to the US Environmental Protection Agency (EPA), green roofs can be about 17°C-20°C cooler than conventional ones during the daytime, helping to reduce building temperature, cutting overall energy use and reducing peak energy demand. Green roofs also have non-energy benefits like improving aesthetics, reducing noise and helping with stormwater management. They also tend to last longer than conventional roofs. Many cities have approved regulations or provided incentives to encourage their use. Known as the “green roof capital of Europe,” Stuttgart, Germany, has subsidised green roofs since 1986, while Basel, Switzerland has made green roofing mandatory for new buildings and retrofits with flat roofs for over a decade. Toronto, Canada, made green roofs mandatory on larger new developments in 2009. Since 2017, San Francisco has required most constructions to cover 15-30% of rooftops with either vegetation or solar panels. Indeed, in addition to providing clean energy, solar panels are considered to be an effective rooftop cooling technology, as they absorb solar radiation and prevent rooftop heat gains. Green roofs and cool roofs can also increase the yields of solar panels, researchers have found. Cool roofs are designed to absorb less heat by using highly reflective coatings, shingles or tiles and also lead to benefits in terms of lower building temperatures and energy requirements. They are frequently white or a lighter colour. According to the urban heat island group at the Lawrence Berkeley National Laboratory in the US, on a typical summer afternoon, a clean white roof reflecting about 80% of sunlight will be about 31°C cooler than a traditional grey roof that only reflects about 20% of sunlight. While heating costs may go up in colder climates in the winter, it notes that the winter heating penalty is typically small compared to the summer cooling benefit. In 2017, authorities in Ahmedabad, India, launched a cool roofs initiative to paint hundreds of roofs in heat-vulnerable slums with white lime reflective paint as part of its heat action plan. In New York City, the NYC CoolRoofs campaign aims to cover one million square feet of rooftops with reflective white coating. The city claims every 2500 square feet of rooftop that is coated can reduce its carbon footprint by one tonne of CO2. •
TEXT Heather O’Brian PHOTOS Erica Carr & Nazarizal Mohammad