Smithsonian.com

Ambient heat produced by a city’s buildings and cars often gets lifted into the jet stream and affects temperatures in places thousands of miles away. Image via Wikimedia Commons

The urban heat island effect—in which heat trapped by large-scale construction and paving cause a city to be several degrees warmer than the surrounding countryside—is a well-documented phenomenon that’s been studied for decades.

Now, though, a group of atmospheric researchers have discovered that through a different mechanism, cities can also alter the weather over a much wider area—causing temperatures to rise or fall by nearly 2 degrees Fahrenheit thousands of miles away. As described in a paper published today in Nature Climate Change, they discovered that ambient heat generated by a city’s buildings and cars often gets lifted up into the jet stream, leading to weather changes over a massive area.

“What we found is that energy use from multiple urban areas collectively can warm the atmosphere remotely, thousands of miles away from the energy consumption regions,” said lead author Guang Zhang of the Scripps Institute of Oceanography. “This is accomplished through atmospheric circulation change.”

In studying the excess heat generated by daily activities in cities around the Northern Hemisphere, Zhang and colleagues from the National Center for Atmospheric Research and elsewhere found that a significant amount of the heat is lifted into the jet stream, causing the fast-moving current of air to widen. Overall, this causes an average of 1.8 degrees Fahrenheit warming during the winter for most of North America and Asia, and 1.8 degrees Fahrenheit cooling during the fall for Europe.

The explanation for this phenomenon is fairly simple: A disproportionate amount of the excess heat produced by human activity is concentrated in a few key areas, and many of these areas (the East and West coasts of the U.S., as well as Western Europe and East Asia) lie underneath the jet stream and other prominent air circulation belts. When the heat is taken up into the system, it disrupts the normal flow of energy and can cause surface temperatures to change in distant locales affected by the same air circulation patterns.

The overall effect of this trend on the climate, the researchers say, is negligible—it’s easily dwarfed by the effect of greenhouse gases in trapping heat and causing long-term climate change. It does, however, account for various anomalies in the difference between warming predicted by computer models and what’s actually been observed. Future models will need to take into account this phenomenon as they attempt to simulate the impact of climate change in various areas.

For residents of rural locales, the surprising finding means something more tangible: on an unexpectedly warm (or cold) day, they might have city-dwellers thousands of miles away to thank for the “waves” of warmth emanating from an urban heat island.