Although only covering 3% of Earth’s land surface, cities are home to over half of the world’s population. Temperatures around the world are rising as a result of anthropogenic climate change and urban areas are particularly sensitive to changes in temperature, experiencing a phenomenon known as the urban heat island (UHI) effect. Cities in the US are, on average, 5.5°C warmer than their surroundings. What does this mean for future temperatures of cities? A recent study has found that cities could experience temperatures 4°C warmer than the surrounding rural area.
The urban heat island (UHI) effect is a phenomenon where urban areas experience significantly warmer weather than the rural areas surrounding it. Many buildings and surfaces in cities are constructed from dark materials. These absorb and retain the sun’s radiation, later releasing this heat at night.
Lei Zhao, a professor at the University of Illinois University and lead author of a paper published earlier this year, describes how “There’s a disproportionate relationship between the size of an urban area and its impact on the human experience of climate change.”
Until recently, there has been a lack of data surrounding how temperatures are likely to change, specifically in urban environments, taking into account the UHI effect. This is primarily due to an absence of urban areas in global-scale models.
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“Almost all the models do not have urban representation,” Zhao describes. “Although cities occupy such a small area, that’s where a lot of the human impact [of global warming] takes place. So we closed this gap by providing multi-model climate projections which are specific to urban areas.”
The study determined that by the end of this century, cities could be up 4.4°C warmer (for the highest greenhouse gas emissions scenario) or 1.9°C warmer for the mid-level emissions scenario.
Whilst temperatures are projected to increase in urban areas, this is also expected to lead to a drop in humidity because warmer air is able to hold more moisture. This influences temperatures further since the release of moisture from plants and vegetation leads to a cooling effect.
The study also found that cities in Southern Europe and the US Midwest will experience the greatest amount of warming. This is due to a combination of factors, including the densely populated nature of many cities in these locations as well as the greater levels of warming experienced in higher latitudes.
Whilst in theory this information could be useful for urban planning in order to mitigate the impacts of climate change, in practice the granularity of the results aren’t enough to be of significant use.
Vivek Shandas, an urban planning professor at Portland State University states how we need “approaches that provide intra-urban variation and take into account differences in the built environment and the complex atmospheric dynamics that amplify neighbourhood heat.”
The authors of the study do highlight that one of the most effective approaches to help mitigate the UHI effect would be to invest in green infrastructure. This may involve initiatives such as planting vegetation to help release more moisture in order to cause a local cooling effect. Planting trees can also help to provide more shade in urban areas, reducing the amount of direct sunlight reaching urban surfaces.
Even though the results from this study are limited by their level of detail for urban planners, they still act as a stark reminder of how the future of cities is fragile and still requires significant attention, especially when considering that around 70% of the world’s population is expected to live in urban areas by 2050. The findings made by Zhao are therefore a step in the right direction, and hopefully will fuel a need to accurately model how different cities will respond to climate change.