Photograph by Raymond Gehman, National Geographic

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Cities such as San Francisco, California (pictured) can affect the weather in places as far away as Siberia.

Photograph by Raymond Gehman, National Geographic

Urban Heat May Warm Faraway Places

New research shows that a hot city can heat up distant locales by almost 2ºF.

The massive amounts of heat produced by cities may be heating up rural areas 1,000 miles (1,600 kilometers) away, atmospheric researchers have found in a new modeling study.

Scientists have long invoked the "urban island heat effect" to explain why cities are generally hotter than suburban and rural areas. More people, as well as more cars, houses, and paved surfaces, turn energy into heat, which is radiated into the atmosphere.

But new modeling research from the Scripps Institution of Oceanography at the University of California, San Diego, suggests that cities in the Northern Hemisphere can also increase the heat of faraway rural places up to 1.8ºF (1°C)—a substantial aggregate increase.

The reason comes down to global air flow.

Heat produced by cars and people travels about 2,500 feet (762 meters) into the atmosphere, where it disturbs part of the jet stream that continually circulates a belt of cool air around the top of the planet. When hot air intercepts the jet stream, it pushes the belt upward, allowing warmer air from the Equator to come farther north, warming parts of northern Europe and North America that normally would have been cooler.

Over the Northern Hemisphere, 86 major metropolitan areas cover only 1.27 percent of the Earth's surface. But those areas consume 6.7 terawatts of energy annually—representing 42 percent of annual global consumption.

Those cities' influence on global climate is therefore magnified, according to the study, published Sunday in the journal Nature Climate Change.

Until now, climate researchers have mostly pointed to greenhouse gasses as the sole cause of climate change. But the planet has been warming in some areas faster than models have predicted.

The impacts of urban heat produced by energy consumption may account for some of that extra warming, said Guang Zhang, a meteorologist at Scripps who conducted the study: "Essentially, we are now able to account for a missing part of the warming."

Zhang found that the areas most significantly impacted by this urban heat effect were Siberia and northern Canada, which can see temperatures rise 1.4º to 1.8°F (0.8° to 1°C) due to urban heat in faraway cities like New York or San Francisco.

Further south, areas like Minnesota and Michigan might see a 0.5°F (0.3°C) increase. The modeling was conducted with data from the National Center for Atmospheric Research, a leading source of global climate data.

While it's true that the Earth's aggregate climate affects every spot on the planet, the impacts of city heat were most pronounced in the Northern Hemisphere, where nearly 90 percent of the global population lives. And the most dramatic cool usually comes in the fall, the researchers said, for reasons that can't yet be fully explained.