Urban heat crisis: Understanding, mitigating, and adapting

Cities around the world are getting hotter, and the phenomenon is not just due to heatwaves but also urban heat islands (UHIs), which make cities significantly warmer than their rural counterparts. This alarming trend is highlighted by Jan Carmeliet and Dominique Derome, who emphasize the urgent need to understand and mitigate urban heating.

The frequency of extreme heat events in cities is on the rise, a consequence of urbanization and historical greenhouse gas emissions that contribute to long-term climate change. Such events are among the deadliest natural disasters, with the 2003 European heatwave causing approximately 72,000 deaths and the 2022 heatwave resulting in over 61,000 deaths in Europe alone.

Research over the past three decades shows a 60% increase in heat exposure in rural areas, while some cities have seen increases exceeding 90%. The UHI effect significantly exacerbates heat exposure during heatwaves, suggesting that addressing UHIs could greatly reduce heat exposure in urban areas, independent of global climate change mitigation efforts.

The impact of urban extreme heat extends beyond health risks; it also leads to excessive cooling energy and water consumption, environmental damage, economic loss, social inequity, and loss of biodiversity. Implementing heat mitigation and adaptation measures can yield co-benefits, such as reduced health risks, energy and water consumption, and socio-economic inequalities, while enhancing well-being.

Unveiling urban heat hotspots: Analysis

Urban areas, with their dense buildings and impervious surfaces, absorb more heat and offer less evaporative cooling compared to the green, open spaces of rural regions. Cities also generate additional heat from human activities like traffic and industry, known as anthropogenic heat.

A detailed analysis of temperature variations across different city zones can pinpoint hotspots by correlating satellite surface temperatures during heatwaves with urban morphology. An essential factor in this analysis is the sky view factor, which measures the ratio of radiation received and emitted by urban surfaces to the sky, influencing shadowing and cooling effects. This data is crucial for stakeholders, including local governments and urban planners, to guide future city designs. For example, Geneva requires a minimum of 20% vegetated area to mitigate excessive heat during heatwaves.

Despite advancements in urban climate-sensing techniques, their resolution is often insufficient for detailed urban climate studies. Mesoscale meteorological models, typically used for weather predictions, have been adapted to simulate urban climates with resolutions between 2 km and 250 m. Down-nesting these models allows for reanalysis of heat events at a scale of hundreds of meters, considering the city’s impact through parametric modeling. These models reveal intra-urban heat exposure diversity, pinpointing local hotspots, their causes, and potential large-scale mitigation measures.

The models also analyze the heat surface balance, showing the competing effects of heat storage in buildings and roads versus the cooling provided by vegetation. High heat exposure during heatwaves is attributed to accumulated heat storage in cities, where nighttime cooling is insufficient to offset daytime heating, unlike in rural areas. Additionally, studies of the planetary boundary layer height (PBLH) show that extreme urban heat exposure is often accompanied by a heat dome, trapping hot air, as seen across the USA in the summer of 2023.

Models to test different ways to cool

To tackle the problem of cities getting too hot, scientists use special models to test different ways to cool them down. These methods include planting more greenery like trees and bushes, creating shaded areas, adding water features, using roofs that reflect heat or are covered with plants, and using special pavements that stay cool by letting water evaporate from them.

Here are some examples of what they’ve found:

• Adding Greenery: Planting trees and shrubs can cool down cities.
• Creating Shade: Using buildings and trees to cast shadows helps reduce heat.
• Water Features: Ponds and fountains can help lower temperatures.
• Cool Roofs and Pavements: Reflective or plant-covered roofs and special pavements that allow water to evaporate can cool the environment.

These strategies depend on the city’s layout, types of trees, and local climate. No single solution fits all cities; each city needs a unique approach combining different methods. Experts and residents must work together to create effective plans, using detailed models and real-world data to understand and address urban heat.

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