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Cities alter weather for thousands of miles downwind, study finds

Jan. 27, 2013

'Waste heat' from big metropolitan areas changes atmospheric systems, altering weather patterns for regions thousands of miles away. The finding helps explain why some regions have seen more warming than computer models predict. Brett Israel The Daily Climate

Had a slushy ski trip to the mountains this winter? The nearest big city might be to blame.

A region's weather can be swayed by the nearest big city, even if it's 1,000 miles away, according to new research. Scientists found that the heat streaming from major metropolitan areas can widen the jet stream and tweak other workings of the atmosphere, according to the study, published Sunday in the journal Nature Climate Change.

Many of the northern hemisphere's cities lie directly under major atmospheric troughs and jet streams. Those changes can affect the weather for thousands of miles, heating up or cooling down regions along the way.

This hot air, or waste heat, comes chiefly from buildings, cars, and other energy-burning sources in big cities across the Northern Hemisphere, where energy use is highly concentrated. Many of the hemisphere's urban areas lie directly under major atmospheric troughs and jet streams. Waste heat from these cities can cause winter warming across large areas of northern North America and northern Asia. The study is the first to demonstrate this effect.

'Far-reaching impact' "Nobody has seen ... such a far-reaching impact," said Aixue Hu, an atmospheric scientist at the National Center for Atmospheric Research and a co-author of the study.

Waste heat's influence on regional temperatures may explain why some areas have more winter warming than projected by climate computer models, according to the researchers, who suggest that models be adjusted to take the influence of waste heat into account.

The average winter temperatures in some remote spots increased by as much as 1.8º Fahrenheit (1º Celsius), according the study, a collaboration between researchers at the Scripps Institution of Oceanography at the University of California, San Diego; Florida State University; and NCAR.

Waste heat can also cool some areas. For instance, waste heat chilled parts of Europe by as much as 1.8ºF, mostly during the fall.

"Waste heat does not really contribute to global mean temperature change," Hu said. "But it does affect regional climate."

Waste heat caused an average temperature increase worldwide of just 0.02ºF (about 0.01ºC), according to the study.

Waste heat is not the same as the so-called urban heat island effect. Urban heat islands are formed as heat is collected and re-radiated by pavement, buildings, and other urban features. Such heat remains concentrated fairly locally, raising annual mean temperatures as much as 5.4ºF in a city of 1 million people, according to the U.S. Environmental Protection Agency.