Source: IIP Digital (Managed by the U.S. Department of State)
Washington - The heat that seeps out of leaky buildings and the warmth of cars purring in traffic contribute to the mass of heat that pulses into the atmosphere from urban centers. Scientists are now getting a handle on how that heat behaves in the upper atmosphere and influences temperatures elsewhere.
This composite of satellite photos of Earth at night shows the urban centers emitting heat into the atmosphere. (source: NASA)
Research just published in the journal Nature Climate Change shows that urban heat can change the behavior of major atmospheric systems that sweep across North America and Northern Asia. Produced by a collaboration involving the National Center for Atmospheric Research (NCAR), the work finds that temperatures in places far from the urban heat centers may be pushed up by as much as 1 degree Celsius.
The heat swells can also shift the standard paths for global air currents, the researchers found, such that temperatures in Europe are cooling by as much as 1 degree Celsius with the effects of U.S. urban heat on the North Atlantic jet stream.
"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, a co-author of the study. "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."
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," said lead author Guang Zhang of the University of California, San Diego. "This is accomplished through atmospheric circulation change."
On the global scale, however, urban heat is a tiny fraction - just 0.3 percent - of all the heat that is throbbing off the planet and gliding across higher latitudes by atmospheric circulation. But urban heat in air currents may be having some very localized effects, the study suggests, a theory that explains why some areas are experiencing more winter warming than what has been predicted by computer models.
The urban centers in North America and Asia releasing this concentrated heat also happen to lie directly under major atmospheric activity, said another study co-author, Ming Cai of Florida State University.
"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," Cai says. "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."
The scientific team behind this work - including NCAR; the Scripps Institution of Oceanography at the University of California, San Diego; and Florida State University - suggests that climate models be tweaked to factor in the influence of urban waste heat.
The largest source of heat on the planet is the sun, the researchers point out. It warms the Earth, and atmospheric circulation redistributes that energy across the globe. The release of urban waste heat is different because it comes from fossil fuel sources, releasing energy that had previously lain dormant in the Earth for millions of years.
The National Science Foundation sponsors NCAR, which is managed by the University Corporation for Atmospheric Research, a nonprofit consortium of research universities.
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