Editing IPCC:AR6/WGI/Chapter-10 (section)

=== FAQ 10.2 | Why Are Cities Hotspots of Global Warming? ===

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Urban areas experience air temperatures that can be several degrees Celsius warmer than surrounding areas, especially during the night. This ‘urban heat island’ effect results from several factors, including reduced ventilation and heat trapping due to the close proximity of tall buildings, heat generated directly from human activities, the heat-absorbing properties of concrete and other urban building materials, and the limited amount of vegetation. Continuing urbanization and increasingly severe heatwaves under climate change will further amplify this effect in the future.

Today, cities are home to 55% of the world’s population. This number is increasing, and every year cities welcome 67 million new residents, 90% of whom are moving to cities in developing countries. By 2030, almost 60% of the world’s population is expected to live in urban areas. Cities and their inhabitants are highly vulnerable to weather and climate extremes, particularly heatwaves, because urban areas already are local hotspots. Cities are generally warmer – up to several degrees Celsius at night – than their surroundings. This warming effect, called the urban heat island, occurs because cities both receive and retain more heat than the surrounding countryside areas and because natural cooling processes are weakened in cities compared to rural areas.

Three main factors contribute to amplify the warming of urban areas (orange bars in FAQ 10.2, Figure 1). The strongest contribution comes from urban geometry, which depends on the number of buildings, their size and their proximity. Tall buildings close to each other absorb and store heat and also reduce natural ventilation. Human activities, which are very concentrated in cities, also directly warm the atmosphere locally, due to heat released from domestic and industrial heating or cooling systems, running engines, and other sources. Finally, urban warming also results directly from the heat-retaining properties of the materials that make up cities, including concrete buildings, asphalt roadways, and dark rooftops. These materials are very good at absorbing and retaining heat, and then re-emitting that heat at night.

The urban heat island effect is further amplified in cities that lack vegetation and water bodies, both of which can strongly contribute to local cooling (green bars in FAQ 10.2, Figure 1). This means that when enough vegetation and water are included in the urban fabric, they can counterbalance the urban heat island effect, to the point of even cancelling out the urban heat island effect in some neighbourhoods.

The urban heat island phenomenon is well-known and understood. For instance, temperature measurements from thermometers located in cities are corrected for this effect when global warming trends are calculated. Nevertheless, observations, including long-term measurements of the urban heat island effect are currently too limited to allow a full understanding of how the urban heat island varies across the world and across different types of cities and climatic zones, or how this effect will evolve in the future.

As a result, it is hard to assess how climate change will affect the urban heat island effect, and various studies disagree. Two things are, however, very clear. First, future urbanization will expand the urban heat island areas, thereby amplifying future warming in many places all over the world. In some places, the nighttime warming from the urban heat island effect could even be on the same order of magnitude as the warming expected from human-induced climate change. Second, more intense, longer and more frequent heatwaves caused by climate change will more strongly impact cities and their inhabitants, because the extra warming from the urban heat island effect will exacerbate the impacts of climate change.

In summary, cities are currently local hotspots because their structure, material and activities trap and release heat and reduce natural cooling processes. In the future, climate change will, on average, have a limited effect on the magnitude of the urban heat island itself, but ongoing urbanization together with more frequent, longer and warmer heatwaves will make cities more exposed to global warming.

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'''FAQ 10.2, Fig''' '''ure 1 |''' '''Efficiency of the various factors at warming up or cooling down neighbourhoods of urban areas.''' Overall, cities tend to be warmer than their surroundings. This is called the ‘urban heat island’ effect. The hatched areas on the bars show how the strength of the warming or cooling effects of each factor varies depending on the local climate. For example, vegetation has a stronger cooling effect in temperate and warm climates. Further details on data sources are available in the chapter data table (Table 10.SM.11).

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