Rising temperatures are already impacting millions of city dwellers. What is happening and what might be done about it?
Every year, around half a million people die from heat-related causes and the health of millions more suffers from heatwaves exacerbated by global warming.
Cities are particularly at risk, as temperatures in urban areas are regularly higher than in the surrounding countryside. This heat does not hit all city dwellers equally though, with some especially vulnerable due to age, poverty and pre-existing health conditions.
Research looking at 38 cities, published last year, suggests that in half of them it could take less than a decade for the cumulative number of heat deaths to exceed annual deaths from Covid-19 during the pandemic.
“We know what is driving it: fossil-fuel-charged, human-induced climate change. And we know it’s going to get worse,” said UN Secretary-General António Guterres in 2024. “Extreme heat is the new abnormal.”
This is what we know about urban heat, how bad it might get, and what can be done.
Cities are getting hotter
One way to measure dangerous heat is the number of days per year when the temperature exceeds 35C. Above this threshold, health impacts manifest with worrying frequency.
In the last three decades, the average number of days above 35C in 43 major global cities has risen 26%, with 1,612 such days occurring in 2024.
This number is likely to increase. Data from the Intergovernmental Panel on Climate Change (IPCC) shows that, at 1.5C of global warming above pre-industrial levels, South Asian cities will experience on average 95 of these very hot days every year. If warming reaches 3C this number will likely increase to 134.
Recorded outdoor air temperatures only tell part of the story.
“With the way we build now, indoor temperatures are much higher,” explains Kurt Shickman, a senior fellow at the Ross Center for Sustainable Cities, part of the World Resources Institute. “People may be experiencing a 32-degree day outside, but they’re living and working and playing and learning in spaces that are far hotter than that.”
Billions will be exposed to extreme heat
Complicating matters is the fact that many cities are growing. The UN predicts that two-thirds of humanity will live in urban areas by 2050. That amounts to 2.5 billion more city dwellers globally, 90% of them in Africa and Asia.
This growth will increase the “urban heat island” effect which makes cities hotter than the countryside that surrounds them due to waste heat from energy use, lack of vegetation and more heat-absorbing surfaces like concrete. A 2019 study found that by 2050, urban expansion could result in average summer daytime and nighttime warming of 0.5C to 0.7C, and up to 3C in some cities. Depending on the location, this extra warming is about half, and sometimes two times, as strong as that predicted to be caused by greenhouse gas emissions.
Even in optimistic future climate scenarios, between 2070 and 2100 more than 3.5 billion people living in cities will be subject to at least one two-week-long heatwave with a daily average temperature of over 42C on the “heat index”. That is, how hot it feels to the human body when both air temperature and humidity are considered.
In the worst-case scenario, this could rise to 5 billion by that same period, with Bangladesh, China, Nigeria, India, Indonesia, the Philippines and Pakistan most affected. In comparison, over the 1950-2009 period, around 1.2 billion urban dwellers are likely to have experienced this level of heat.
Heatwaves will last longer
As well as more heat, city dwellers are also likely to experience heat for longer.
Recent modelling work by the World Resources Institute looking at 996 of the world’s largest cities found their longest heatwave each year could last for an average of 16 days in a 1.5C-warmer world. That jumps to 24 days with 3C of warming.
The researchers behind this study defined a heatwave as three or more consecutive days where temperatures reach or exceed the top 10% of daily high temperatures, determined by data collected over the 40-year period from 1980.
The lengthening effect varies massively by region, with cities in the Middle East and North Africa potentially facing 36-day longest heatwaves in a 3C world, nearly two weeks longer than they are likely to suffer at 1.5C.
More people will die
At a certain point, the human body starts to buckle under extreme heat. The heart and kidneys have to work extra hard to keep your body cool, and they have limits.
Even if temperatures don’t reach life-threatening highs, going for extended periods without cooling down can put cumulative stress on the body.
Most heatwave deaths are indirect. People typically fall to existing illnesses like heart, lung or kidney disease, made worse by the hot weather.
Human-caused climate change is increasing the risks. A recent analysis of 854 European cities found climate change was responsible for two-thirds of heat deaths last summer, totalling nearly 16,500 people. Put another way, three times more people lost their lives than would have done without climate change.
Modelling looking at the same cities by scientists at London School of Hygiene and Tropical Medicine found the expected future rise of heat deaths substantially outnumbers any potential drop in cold deaths from a warmer climate.
Cooling demand will soar
In a world of long, blistering hot summers, demand for ways to keep buildings cool will rocket.
In its recent work looking at the world’s largest cities, the World Resources Institute (WRI) estimates that at 3C of warming, 194 million people could need twice their historical cooling demand. This is quantified using “cooling degree days”, which measures the difference, in degrees celsius, between the daily average and a comfortable temperature. For example, if comfortable is set at 21C, as it was in the WRI study, then a 24C day gives you 3 cooling degree days (1 day x 3 degrees).
The overall additional demand would be greatest in India, whose 189 largest cities combined would have 58,873 more cooling degree days per year.
Improving access to air conditioning will help, but it may not be a feasible or equitable solution in the short term.
As Shickman points out, air conditioners need electricity, and extreme heat often arrives at the same time as other disasters. “After a hurricane or tornado, you may not have the power to run your AC.” The cost of that electricity can also be a barrier to access, leading to what some researchers call “cooling poverty”.
Plus, to build the air conditioning infrastructure at the scale needed will take years, whereas urban heat is an immediate health threat.
“Passive cooling has to be our primary line of approach for every building,” he asserts. “There are some [solutions] that can be applied just about everywhere: albedo modification. That is, cool roofs, changing the colour of roofs, walls, pavements, and shade. Those are really applicable in any context, irrespective of climate and water availability.”
“It’s like a buffet or a smorgasbord. The stuff in the trays is the same for every city, it’s what you put on your plate that’s going to be a little different.”
More city trees are needed
Reintroducing trees and vegetation is one way to cool cities down. Trees naturally lower the air temperature nearby by providing shade and through evapotranspiration, their version of sweating. They can reduce air temperatures around them by up to 8C.
The effect is especially pronounced in tropical, arid and continental climates. Research published in 2024 looked at the cooling effects of urban trees in 110 cities around the globe. In 83% of those with comparable data, the air cooling achieved by planting trees was enough to lower the average temperature during the hottest month to below 26C.
But according to the 2025 Lancet Countdown report on health and climate change, at a global level the density of vegetation in cities has remained largely unchanged in the last decade, growing by just 0.2% on average since 2015.
All these numbers paint a sobering picture of a sweltering future.
Yet, Shickman offers a note of guarded optimism. Life-saving measures such as cool roofs and tree planting “are city transformations that we can make, all with available materials and technologies today”.
“We are not talking about something we need to innovate out of. We know what to do. We have the tools. They are available in large parts of the world,” he reflects. “It’s a matter of doing it.”
Unless otherwise indicated, all the graphs and associated data included in this article have been reproduced with permission from the owners, allowing republishing under Creative Commons.
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