City sprawl is now large enough to sway global warming over land

Just how much heat does city sprawl add to large-scale warming? That's one longstanding question researchers sought to answer in a new study recently published in the journal One Earth.

Once thought to cover too little of the Earth's surface to affect climate at larger scales, the new work suggests that urbanization does indeed have a detectable influence on global warming over land, with more to potentially follow as cities continue growing.

The effect is most dramatic in some of the world's most rapidly urbanizing areas. In the bustling Yangtze River Basin, for example, home to more than 480 million people (one third of China's total population), urban sprawl contributed nearly 40% of the area's increased warming between 2003 and 2019.

In Japan, where close to 10% of total land is developed, urbanization contributed a quarter of the added warming observed during the study period. The urban signal was less pronounced in Europe and North America, where urbanization boosted roughly 2–3% of warming. That's likely because much of the development there happened before the study period, and proportionally, there is still a great deal of undeveloped land compared to other smaller regions and countries.

Overall, cities added just over 1% of increased land surface warming across the entire globe; 1.3% during daytime and 1.1% during nighttime.

"Urbanization" is an umbrella term of sorts. It encompasses not only built structures in cities, but also the many climate-influencing factors like air pollution, city parks and swelling populations that are tied to their existence.

The urban climate signal

Traditionally, cities are either left out of global climate models or represented in very simplistic ways, according to the study authors. When climate scientists use these models to understand how extreme weather may change in a warmer world, for example, they rarely factor cities into their simulations. But if they're included at all, cities aren't depicted as growing, changing entities, even though the rest of the world is projected to change decades into the future.

That's a shortcoming, said lead author and Earth scientist TC Chakraborty at the Department of Energy's Pacific Northwest National Laboratory. A large body of research demonstrates that cities influence the climate around us in a myriad of ways.

Buildings absorb and trap heat, which means cities take much longer to warm and cool down than rural areas. In some cases, this could mean city dwellers may spend more time in uncomfortable heat than their countryside neighbors. Cityscapes can change the way air moves around us, or even intensify extreme weather.

While the influence of urban land has been clear at the local scale, researchers have questioned whether cities matter at the regional, continental and planetary scales.

"The answer is yes, they do; to a small extent," said Chakraborty. "Urbanization does have a detectable impact on global land warming. That impact is minor but statistically significant at the global land scale, and particularly evident at continental scales. When you zoom into specific regions of the world, the effects can get quite large."