Two smiling young boys sit in a blue plastic container filled with water

Boys cooling themselves on a hot day in Ahmedabad, India, where a heatwave action plan is helping to save lives.Credit: Amit Dave/Reuters/Alamy

Last month’s heatwave shattered temperature records across the western United States and Canada. On 29 June, the Canadian village of Lytton hit nearly 50 °C — an astonishing increase of almost 5 °C on the previous national high. A day later, fire burnt most of Lytton to the ground, killing two people. Elsewhere, the cities of Vancouver, Portland and Seattle saw hundreds of people die during the same three-day heatwave.

As global temperatures rise, the risks from extreme heat — defined as periods when a region’s temperatures are abnormally high compared with the average — are also rising. Heat has always posed a threat to urban living, with heat-absorbing surfaces such as asphalt sending the mercury soaring. But climate change means that heatwaves now happen more frequently and are more intense than in the past. This is one of the most underappreciated hazards of climate change. Researchers say that the Pacific Northwest heatwave, for instance, would have been “virtually impossible” in the absence of human-induced global warming (see

And although heat can kill anywhere, the risk is greater in cities. One study presented at a conference last December estimates that people’s exposure to extreme heat in more than 13,000 cities more than doubled between 1983 and 2016. Another study published last year estimated that air temperatures in two cities — Jacobabad in southern Pakistan and Ras Al-Khaimah in the United Arab Emirates — have already passed the human body’s limits of survivability on their hottest and most humid days (C. Raymond et al. Sci. Adv. 6, eaaw1838; 2020).

Climate researchers have long warned that global warming makes heatwaves such as that seen in North America this year much more likely. This means that urban planners must work harder to incorporate extreme heat into climate-adaptation strategies.

Some city authorities have been preparing for such a scenario. Take Ahmedabad in western India. After a devastating heatwave in 2010, the city developed an action plan with three elements: raising awareness about how people can protect themselves from extreme heat; creating an early warning system for when meteorologists forecast a heatwave; and training medical staff to better recognize and treat people suffering from extreme heat. One estimate suggests that the programme has saved 1,190 lives a year, and a similar approach has been rolled out for more than a dozen other cities across India.

Another idea known as ‘cool roofs’, which are painted white or covered with energy-reflecting materials that absorb less heat, can reduce temperatures inside buildings by 2–5 °C when compared with conventional roofing.

But such climate solutions need to be implemented effectively and efficiently on a city-wide scale for them to have any significant impact. And for that to happen, governments need to require the construction industry to incorporate heat mitigation into their building projects through green building-certification programmes. The provision of subsidies for green buildings is also an option. In Barcelona, Spain, for example, the authorities are subsidizing 75% of the costs of 10 new green-roof projects in the city.

At the same time, cities must target heat-mitigation efforts at those most affected by the heat. That includes people in lower-income neighbourhoods, which have, historically, often been deprived of parks, tree-lined streets and other green spaces that are a common component of wealthier areas. Scientists have also found shocking correlations between race and heat exposure in cities in the United States. Studies show that historical urban policies have left communities of colour at higher risk of heat-related illness or death than people in predominantly white neighbourhoods.

Cool corridors

One pioneer in this concept of ‘heat equity’ is Paris, where officials are building a city-wide network of ‘cooling islands’ — which include spaces such as parks and pools — linked by cool walkways. Meanwhile, Medellín in Colombia has targeted low-income areas of the city for tree planting; more than 10,000 trees have been planted along 36 ‘green corridors’, resulting in a 2 °C reduction in surface temperatures. Government officials must continue to track the results of such experiments and make use of the best available evidence to green their cities.

This week, the mayors of 31 cities in the C40 global network of cities working to fight climate change have committed to ensuring that, by 2030, 70% of city residents can get to a green or blue public space with no more than a 15-minute walk or bicycle ride. Annual accountability check-ins must ensure that true progress is made on this ambitious goal.

In all cases, city and regional governments must better organize their heat-fighting efforts. It’s not feasible to react to heat after the fact — by the time hospitals are overcrowded with people affected by heat stroke, electrical grids have crashed under the weight of demand for air conditioning, and coroners are counting the bodies, it’s too late. Every death from heat is preventable if a person can access shade, water or other means of cooling.

As we face a future with longer, hotter and more frequent heatwaves, cities must escalate their planning for extreme heat. It needs to be on a par with preparations for other disasters such as earthquakes, floods andhurricanes. That applies not only to tropical cities but also to those in temperate climes. Who, after all, would have flagged heat as a major risk factor for Vancouver, at a latitude of more than 49 degrees north? Yet, after June’s deadly heatwave, the city’s officials are now working to incorporate extreme heat into their emergency plans. It is the only way forwards.