City sprawl contributes to global warming, study finds

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Researchers at the University of Houston have found that city sprawl significantly impacts global warming over land.

This discovery, published in the journal One Earth, challenges previous beliefs that urban areas were too small to affect the climate on a large scale.

Urbanization, or the spread of cities, has now been shown to have a noticeable influence on global warming over land, with the most dramatic effects seen in rapidly growing areas.

For instance, in China’s Yangtze River Basin, home to over 480 million people, urban sprawl contributed nearly 40% of the region’s warming between 2003 and 2019.

In Japan, where 10% of the land is developed, urbanization accounted for a quarter of the observed warming during the same period.

However, in Europe and North America, the impact was smaller, contributing only 2-3% to the warming because much of their urban development occurred before the study period.

Overall, cities added just over 1% to the increased land surface warming globally: 1.3% during the day and 1.1% at night.

Urbanization affects the climate in various ways, including air pollution, city parks, and growing populations. Traditionally, cities have been either left out of global climate models or represented too simply. Climate scientists rarely factor in the dynamic nature of cities when simulating future weather changes.

This oversight can lead to incomplete predictions.

Professor TC Chakraborty from the Department of Energy’s Pacific Northwest National Laboratory, the study’s lead author, emphasizes the importance of including cities in climate models. Buildings in cities absorb and trap heat, causing urban areas to warm up and cool down more slowly than rural areas.

This means city residents may experience more prolonged periods of uncomfortable heat. Additionally, city landscapes can change how air moves, potentially intensifying extreme weather.

While the local impact of cities on climate is well-known, this study confirms that urban areas also influence climate at regional, continental, and even global scales.

For example, in Greenland, where urban land expansion was minimal, city sprawl had little impact on large-scale warming. Conversely, in rapidly urbanizing areas of Asia, the study observed significant warming due to urbanization.

Despite the substantial influence of cities, their impact remains minor compared to greenhouse gas emissions and other human activities. This finding counters the common belief that the urban heat island effect is a major contributor to global climate warming.

Interestingly, the study found that some arid desert cities experienced partial cooling as they developed, thanks to the addition of parks and farmland. This was observed in parts of India and Africa, where urban cooling signals were detected as cities grew.

In India, this cooling is due to rural irrigation and additional atmospheric aerosols from cities.

Between 1992 and 2019, global urban land cover increased by 226%, adding about 448,113 square kilometers of urbanized land—an area almost equivalent to California. Most of this growth occurred in Asia, with urban areas in the United States expanding by 181%, India by 366%, and China by 413%.

As more people move to cities, improving the representation of urban areas in climate models becomes increasingly important. Accurate models are crucial for predicting regional climate changes and informing strategies for mitigation and adaptation.

“People are using climate models more and more for regional assessments, with plans to run global models at finer resolutions as our computational capabilities improve,” said Chakraborty. “When you do this, you need to account for urban areas and their impacts on regional temperature, cloud cover, precipitation, and air pollution.”

Satellites play a key role in these assessments by providing comprehensive temperature data across large areas, overcoming the limitations of ground-based measurements.

Researchers at PNNL are working to better incorporate urbanization into climate models. Co-author and Lab Fellow Yun Qian highlights that the Department of Energy is investing in improving climate models to inform the development of technologies that protect diverse communities.

By understanding the full impact of urbanization on climate, scientists can improve predictions and develop more effective strategies to address climate change.