Hot in the city. But it doesn't have to be - we have answers

Pablo Lazo is the Director of Urban Development for WRI Ross Center for Sustainable Cities. Eric Mackres is the Senior Manager of Data and Tools at WRI Ross Center for Sustainable Cities.

A new study has found that 4 billion people - half of the world’s population - endured at least one extra month of climate-fuelled extreme heat last year. 

In the last two months, cities from Delhi in India to Jalisco in Mexico and Phoenix in the United States have endured temperatures of over 100 degrees Fahrenheit (around 38 degrees Celsius).

Around the world, cities like these are coping with a scorching present and facing an even hotter future. 

But cities have much more power than they realise to help their citizens survive in a hotter world. 

While many environmental disasters require large regional infrastructure solutions, city leaders can make quick progress toward cooling their cities down by focusing on a few critical urban elements - like greenery, streets, buildings, and design - that are well within their power to transform.  

The impact that these changes could have on cities’ very fabric is enormous - allowing people to work, move and socialise without risking their health. Because today, heatwaves threaten everything from people’s health and wellbeing to local and national economies.  

Already, extreme heat is causing an average of 500,000 excess deaths a year, disproportionately affecting the most vulnerable.

And according to the World Health Organization, deaths from heat are expected to more than double by 2050.  

In that same time, cities are anticipating major impacts to their productivity. Without concerted action to mitigate heat, economic losses stemming from dangerous conditions are also expected to double by 2050.  

Cities face unique risks: heat-absorbing infrastructure, dense populations, and lack of nature all exacerbate heat’s impacts. 

Right now, cities are stuck in reactive mode as heatwaves have exploded in intensity in the past few years. Their citizens are paying the price. But there are clear steps cities can take today to get ahead and heat-proof themselves for the future.  

How to cool down

First, they should focus on easily scalable infrastructure. While solutions like early warning systems and pop-up cooling centers can be helpful, these solutions do nothing to improve a city’s baseline heat resilience. 

To stop this pattern of constantly reacting, cities need to embed heat-resilient infrastructure into the city itself. 

Cities can redesign and reconfigure their streets, buildings, and parks to create a stronger foundation of heat resilience. This approach can work at the neighbourhood scale and also across an entire city. 

Adding new trees to one street can cool a block; a citywide afforestation plan, like Medellín’s Green Corridors, can cool a city.   

Second, cities need to measure what matters. Understanding which resilience projects to pursue requires cities to know how to measure what matters. 

Each city, and each person within it, experiences extreme heat differently. Identifying which heat-related challenges are most acute - and which metrics can shed light on those challenges - can help cities pursue the most effective solutions.  

In places like Singapore and Belém, Brazil, where heat and extreme humidity collide, “wet bulb globe temperature” shows how those factors merge with sun exposure and wind speed to amplify heat stress. 

Singapore requires employers to use this metric to determine if outdoor work can be carried out safely, and what measures they need to implement, like mandated breaks, to keep workers safe.  

Thermal comfort indices, such as the Universal Thermal Climate Index (UTCI), go even further. They offer a more comprehensive measurement of heat by factoring in a wide range of environmental influences - not just temperature, humidity, wind and sunlight, but also reflected radiation and radiant heat from surfaces - to give a fuller picture of how people physically experience heat.

Cities like Los Angeles have started using this index to assess how shaded walking routes can reduce heat exposure for pedestrians, helping them decide where to expand tree cover and shade infrastructure.

Choosing the right metrics to analyse and connect data to residents’  lived experiences can help city leaders and planners decide which communities and interventions to prioritise, set actionable targets, and measure and report progress against those goals. 

Using WRI’s Cities Indicators Dashboard, Freetown, Sierra Leone realised that 99% of the city’s built-up areas have low surface reflectivity - meaning they trap warmth and exacerbate the urban heat island effect.

With this data in hand, the Freetown City Council installed reflective roofs and market shade covers to protect people - like vendors and residents of informal settlements - who are vulnerable to heat exposure and tracked the effectiveness of the interventions over time. 

Initial assessments show that in houses where cool roofs have been installed, temperatures dropped by as much as 6 degrees Celsius.

Put the community at the centre

Finally, for any heat data and resilient urban infrastructure to work, community members must be at the centre. City officials must start and end their efforts to combat extreme urban heat by working with people to understand their experience of heat.  

In Nairobi, the grassroots group SDI Kenya has been leading a community vulnerability mapping effort to work hand-in-hand with residents of informal settlements to understand their heat challenges. 

Recognising the need for more nuanced data on heat stress in informal settlements, residents were invited to design and implement a heat-sensor programme to generate more localised data. 

By working closely with communities, cities and residents can together create better policies - from health clinics to shade structures in exposed public spaces - that centre the needs of the most vulnerable.   

Extreme heat will only get worse. For cities to adapt, it’s essential they clearly understand both the risks of heat and which solutions work best for whom. By measuring heat risks, deploying resilient infrastructure, and working with communities, cities can not only mitigate heat risks now, but plant the seeds for a safe, thriving, and resilient future.