U.S. researchers collected data from areas around cities with high populations, like New York and Tokyo.
They found temperatures there were all slightly different than computer models predicted they would be.
It has been known for some time that cities, with their greater level of energy use, generate something called “waste heat,” causing higher air temperatures.
Scientists discovered that the warmer air from cities rises and is carried hundreds of miles away by the prevailing winds aloft.
Temperatures downwind can be altered by as much as a whole degree Celsius (1.8 degrees Fahrenheit).
The heat also widens the jet stream and alters global weather patterns.
"The burning of fossil fuel not only emits greenhouse gases but also directly affects temperatures because of heat that escapes from sources like buildings and cars," says National Center for Atmospheric Research scientist Aixue Hu, a co-author of the study.
While some areas are warmed by the urban heat island effect, parts of Europe are cooled by it as much as a degree Celsius, especially in the autumn. But the overall effect of cities on the average global temperature is just 0.02 degrees Fahrenheit, researchers say.