Cities affect temperature a thousand miles away

Posted by Kate Taylor

Large cities can affect the weather as far as 1,000 miles away, say scientists, warming some areas and cooling others.

The effect is quite separate from the urban heat island effect. Waste heat from buildings, cars, and other sources in major Northern Hemisphere urban areas affects the jet stream and other atmospheric systems, in turn altering temperatures across thousands of miles.

In northern North America and northern Asia, this manifests itself in higher temperatures, with some remote areas increase as much as 1.8 degrees Fahrenheit warmer than they would be otherwise. Meanwhile, areas of Europe are cooler by as much as 1.8 degrees F on average, with much of the temperature decrease occurring in the fall.

The net effect on global mean temperatures is nearly negligible - about 0.02 degrees F - because the total human-produced waste heat is only about 0.3 percent of the heat transported across higher latitudes by atmospheric and oceanic circulations.

However, the noticeable impact on regional temperatures may explain why some regions are experiencing more winter warming than projected by climate computer models, the researchers conclude. They suggest that models be adjusted to take the influence of waste heat into account.

"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 NCAR scientist Aixue Hu. "Although much of this waste heat is concentrated in large cities, it can change atmospheric patterns in a way that raises or lowers temperatures across considerable distances."

The team analyzed the energy consumption - from heating buildings to powering vehicles - that generates waste heat release. The world's total energy consumption in 2006 was equivalent to a constant-use rate of 16 terawatts, of which an average rate of 6.7 TW was consumed in 86 metropolitan areas in the Northern Hemisphere.

Using a computer model of the atmosphere, the authors found that the influence of this waste heat can widen the jet stream.

"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," says Zhang. "This is accomplished through atmospheric circulation change."

Although the amount of human-generated energy is pretty small compared with that transported by nature, it's highly concentrated in urban areas. In the Northern Hemisphere, many of those urban areas lie directly under major atmospheric troughs and jet streams.

"The world's most populated and energy-intensive metropolitan areas are along the east and west coasts of the North American and Eurasian continents, underneath the most prominent atmospheric circulation troughs and ridges," says Ming Cai of Florida State University.

"The release of this concentrated waste energy causes the noticeable interruption to the normal atmospheric circulation systems above, leading to remote surface temperature changes far away from the regions where waste heat is generated."