Energy and the environment

New materials and more trees, cities challenge heat islands

Urban centres struggling with rising temperatures. Green around a building can reduce air conditioning by 50%

by Elena Comelli

5 July 2024

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4' min read

Key points

In the field of chief heat officers
Heat as silent killer
Melbourne plants 3,000 trees a year

4' min read

2023 was the hottest year since pre-industrial times and 2024 is well placed to surpass it. This May was the hottest May on record since measurements began in the mid-19th century, according to Copernicus, the EU monitoring programme. The average global temperature over the past 12 months was the highest ever, exceeding the pre-industrial average by 1.6°C and for the first time the 1.5°C limit set by the Paris Agreement. A limit that Europe has already crossed some time ago, reaching an annual average temperature of 2.3°C above pre-industrial levels in 2023, with over 70,000 deaths attributed to the heat. We are in what UN Secretary General António Guterres has called 'the era of global boiling'.

In the field the chief heat officers

The problem is particularly acute in cities, which are burdened by classic 'heat islands' that push temperatures 2 to 5°C higher than the regional average and are home to more than half of the global population. Hence the emergence of a new figure, called chief heat officer, who works with officials in the most forward-looking metropolises. Seven chief heat officers, all women, have already started working in Miami, Los Angeles, Santiago de Chile, Melbourne, Dhaka, Freetown and Athens to plant trees, create 'pocket parks', clear 'tombed' waterways, open 'wind corridors' between buildings, install fountains and teach people how to counter the effects of extreme heat on the human body.

The role of chief heat officer was launched three years ago by the Atlantic Council, an American think tank that also funded the establishment of these figures through the Adrienne Arsht-Rockefeller Foundation Resilience Center. The proposal has taken root in several municipalities, where heat waves are becoming increasingly deadly as the greenhouse effect generated by fossil fuel consumption pushes temperatures into uncharted territory. In Delhi, the thermometer this year touched 50°C for three weeks in a row between May and June, causing over 70 deaths. And over 1,300 people died from the heat in Mecca during the Hajj pilgrimage in mid-June.

The heat as silent killer

"Heat is the deadliest climate danger, it is a silent killer," explains Elissavet Bargianni, appointed chief heat officer in Athens last year. "There are still people who do not understand the lethal impact of extreme heat, or the difference between what used to be normal heat and the dangers of abnormal heat," Bargianni comments. Children, the elderly, pregnant women, farmers and construction workers are the most vulnerable. A recent Ilo report estimates that almost 19,000 people die each year from work-related injuries due to excessive heat. Athens was the first city in Europe to classify heat waves from level 1 to 3, helping residents to decide whether to stay indoors, cancel sporting events or temporarily close tourist sites, such as the Acropolis.

3,000 trees are planted annually in Melbourne

Heat islands depend on the materials from which cities are built: dense agglomerations of concrete, steel and asphalt, filled with cars and air conditioners, which in turn emit heat. Under the sun, asphalt can reach 75ºC and concrete 65ºC, while the temperature of grass does not exceed 42ºC. The most effective measure to cool cities, therefore, is to increase vegetation. Planting trees around a building can reduce the use of air conditioning by up to 50%. It is proven that the taller the trees, the more they mitigate the heat island, providing shade and cooling through evapotranspiration.

In Melbourne, where temperatures soared to 39°C in March, chief heat officer Krista Milne has championed the City Council's measure of planting 3,000 trees a year, with the aim of reducing the 4°C spikes. Milne's task is to ensure that all urban planning decisions are passed through a 'heat lens'. Medellin, Colombia, has created a network of 30 shaded pathways through the city known as 'green corridors' to combat heatwaves, planting thousands of trees along the pavements to allow people to walk around without directly exposing themselves to the sun. Another option is green roofs: plants on the roof increase albedo and retain moisture, which has a cooling effect. In addition, they help absorb rain and prevent flooding. Similar benefits result from green surfaces instead of asphalt in courtyards. In Los Angeles and other cities, attempts are being made to increase albedo by painting roofs and other surfaces white. The use of light-coloured concrete, which can reflect up to 50 per cent more light than asphalt, is favoured for new pavements.

Shade Covers

In Freetown, Sierra Leone, chief heat officer Eugenia Kargbo has promoted the construction of solar-panelled shade covers for the largest street markets, which provide shelter for more than 2,300 women traders. In Bangladesh's capital Dhaka, a crowded city with few green spaces and little shade, chief heat officer Bushra Afreen is struggling to improve the situation. In April, temperatures in Bangladesh - ranked the seventh most vulnerable country to the climate crisis - soared above 40°C, forcing schools to close. 'And to think that in the future these record-breaking temperatures will probably be remembered as the coolest summers in years,' Afreen reflects.