The Heat Cost Of Driving

The first looks at a source of heat often not included in global climate simulations: road traffic. The overwhelming majority of “waste” heat in urban environments comes from four sources — buildings, traffic, industry and our own bodies. The ratio of these different forms of “anthropogenic heat emissions” varies by region. For example, in London in 2008, buildings contributed 80% of total, with traffic and human metabolism contributing 15% and 5%, respectively. But in some urban areas, the share of heat emissions generated by traffic is much higher, upwards of 50% in cities including São Paulo and Daegu.

Despite this, say researchers at the University of Manchester and the U.S. National Center for Atmospheric Research, “most Earth system models do not include traffic‐related anthropogenic heat in their simulations, so they fail to capture cities' real impact on the climate.”

This group set out to change that, developing a traffic heat flux module that can be incorporated into existing climate models. They reported their findings in the Journal of Advances in Modeling Earth Systems. Their module considers traffic volumes, vehicle type and distribution (EVs generate far less waste heat than petrol and diesel vehicles), speed and road characteristics, and it responds to varying weather conditions and the time of day and year.

They incorporated this module into the widely-used Community Earth System Model and tested its performance at two urban sites with quite different climates, urban landscapes and traffic patterns: Toulouse, France and Manchester, U.K. They found that traffic heat increases Toulouse’s average annual air temperature by about 0.4°C and Manchester’s by around 0.25°C.

While these might sound like small increases, they are meaningful. In both cities, the traffic-related warming effect was stronger in winter than summer, but there were differences in when the effects peaked. Writing in The Conversation, the authors say, “In Toulouse, morning traffic heat built up through the day and persisted into the night. In contrast, Manchester's evening rush hour contributed to stronger overnight warming, with its air temperature from traffic peaking around 3am, on average.”

This study shows that the vehicles on our roads are making already-hot cities even warmer. Crucially, it also provides a tool to more realistically simulate cities and more accurately reflect the realities of living in these hotspots.

It’s Warmer Below The City

The next study took a very different approach to quantifying the impact of heat on urbanites. In their short paper published in Nature Cities, researchers at Northwestern University focused on the metro systems of Boston, London and New York. They wanted to understand the extent of thermal discomfort in these environments, but rather than survey sweaty commuters or model the tunnel networks, they mined the internet.

Underground environments are very often much hotter than the surface above them, thanks to the soil and rock that surrounds them retaining the heat. In London, the highest temperature recorded on the Tube was 47 degrees C, exceeding the record for surface air temperature by 6.8 degrees C.

As an aside: In 1927, this subsurface warmth was even used in an advertising campaign by Transport for London, which manages the Tube network.

It turns out that when commuters overheat, they post about it on social media. The Northwestern team crowdsourced more than 85,000 posts published between 2008 and 2024 on X (formerly Twitter) and Google Maps, using natural language processing to identify thermal complaints. The one in four of these posts that qualified as genuine complaints about underground heat were then analyzed.

As you might expect, summer dominates, with the month of July consistently seeing the largest number of thermal complaints. Interestingly, the researchers found that for every 1 degree C rise in outdoor air temperature above 10 degrees C, thermal complaints underground increased by 10% in Boston, 12% in New York and 27% in London.

The authors say that if unaddressed, the extreme heat in the urban underground “is set to continue affecting the user experience of metro systems and create unwanted and deleterious loops with the effects of other forms of extreme weather, such as heatwaves and climate change.”

The Paradox Of Air Conditioning

While I was working on this article, a new high-profile report on global heating was released by the U.K.’s Climate Change Committee. It lays out the challenge they’re facing, saying that U.K. infrastructure “was built for a climate that no longer exists today,” and calls on the government to take urgent action.

Julia King, one of the lead authors, has said that of the many climate risks laid out in the report, extreme heat is the one that poses the greatest threat. “Extreme heat is certainly the most deadly of the climate impacts on the UK, so we need to see cooling rolled out at scale.” Amongst their recommendations is an increase in the number of air conditioning units installed in buildings, describing them as a reliable means of reducing indoor temperatures.

But a study from Singapore published in Sustainable Cities and Society shows that the widespread adoption of these “active cooling” technologies can have unintended consequences.

This group of academics were interested in human behavior — specifically, they wanted to explore the associations between private cooling, household electricity demand and support for “public heat mitigation measures” that can make cities cooler (e.g. shading and urban greening). As a dense, tropical, highly urbanized city with near-universal access to air-conditioning, Singapore was uniquely suited to this study.

The researchers surveyed 416 households (967 adults) and combined the responses with spatial heat data and actual electricity consumption records. They found that widespread access to private cooling systematically weakens people’s motivation to engage in collective climate action, a phenomenon they call “behavioral insulation.”

"One of the key findings is that experiencing heat does not automatically translate into lower-energy behavior or stronger collective climate action," said lead author, Natalia Borzino. "People may become more climate-aware and more vocal about heat, while still relying heavily on energy-intensive cooling to manage daily life."

This research shows that climate adaptation is about more than just technology or physical infrastructure. It has social and behavioral aspects, too, which must be considered in our warming future.

The Green Space Gap In Cities

Urban greening is one of the most frequently cited tools in the heat mitigation toolkit. Trees shade surfaces. Vegetation transpires, cooling the surrounding air. Greenery improves air quality, offers somewhere for urbanites to relax outdoors and supports biodiversity. The cooling benefits of green spaces have been demonstrated time and time again. But much of the existing evidence has focused on specific cities, regions or countries. Assessments at a global scale — which connect the population exposure to the actual cooling effects of urban green spaces — are much rarer.

A team of Chinese researchers set out to fill this knowledge gap, using Landsat thermal infrared imagery (taken in summer), alongside WorldPop population data. Their findings appear in the latest issue of Cities. They selected 522 cities to study — 197 from China, 140 from Europe and 185 from the U.S.

The effects of Chinese cities rapidly investing in urban green spaces in recent decades is clear in the data — their maximum cooling intensity rose from 2.27 degrees C to 3.39 degrees C between 2000 and 2020. In contrast, the U.S. experienced a decline in urban green space cooling effects, with the maximum cooling intensity of its cities decreasing by 0.16 degrees C. The cooling capacity of European cities remained fairly stable over the same period. The researchers also showed that if the goal is temperature reduction, tall, dense, structurally complex vegetation is more impactful than lawns or other small-scale planting.

When it comes to the question of how many people actually benefit from the cooling provided by urban green spaces, this study found considerable spatial disparities. In the U.S., close to half (44.86%) of all of the cities studied “provide effective cooling services to a substantial portion of the population.” In Europe that figure was 31.43%, with most of those based in the “economically developed regions of Central Europe.” Despite China having a higher cooling intensity of urban green spaces, the proportion of the population who actually benefit from it is significantly lower than in Europe or the U.S. It seems that while China’s green spaces are growing, they’re not where the people are.

When Cooling Is Out Of Reach

Access to cooling is also at the heart of the final (and arguably, most consequential) paper on my list. Published this month in Nature Sustainability, it reports on an analysis of a vast dataset — demographic and health survey data from 1,155,106 households (~3 billion people) across 28 countries, combined with historical climate records, satellite-derived urban form data including green and blue spaces, and policy inventories.

This group of researchers from across Europe and the U.S. had a goal to quantify the extent and nature of people’s ability to remain thermally safe as climate change worsens. To do this, they scored each household across five dimensions: climate exposure, infrastructure and assets, social and thermal inequalities, health, and education and working standards. From this, they produced a composite “systematic cooling poverty” index for hundreds of sub-national regions.

They found that SCP is widespread and unevenly distributed, and that almost 600 million people experience high levels of cooling poverty. South Asia and Sub-Saharan Africa are the regions with the strongest deprivation. Malawi had the highest overall SCPI (66 / 100), followed by the Democratic Republic of Congo (65), Nepal (63) and Senegal (62).

Yet countries facing similar extreme conditions can have different outcomes. The overwhelming majority of Indonesia and Bangladesh’s populations (96% and 95%, respectively) are exposed to the same hazardous humid heat. But Indonesia’s better physical infrastructure — e.g. buildings, streets, pipes and green spaces — and its healthcare system means that its systemic cooling poverty index is much lower that of Bangladesh (40 vs. 60).

As the authors wrote in The Conversation last week, “Systemic cooling poverty is not about whether a person can afford air conditioning, but rather how surrounding infrastructure, institutions and design expose someone to harmful heat and then fail to protect them from heat. It extends beyond the home to workplaces, schools and health care systems, where heat can have serious consequences for health, productivity and well-being. It reaches further into the systemic causes that determine who suffers most: inequality, discrimination, patriarchy, ableism and racism.”

What This Means For Cities Around The World

These five studies occupy different scales — household surveys, social media posts, satellite imagery, population studies and climate models. But taken together, they describe a problem that is simultaneously physical, social, infrastructural and political.

Heat in cities is not just a matter of air temperature, nor is it solely a climate change issue (though, of course, that is a significant factor). Our ability to cope with ever-hotter summers is just as much about the infrastructure we design, the urban planning decisions we make, the populations we prioritize, the models we rely on, our access to healthcare and other public services and our responses at both the individual and collective level.