Monday, January 28, 2013

City heat drives warm weather north

Big cities in North America and Asia may generate enough heat to warm areas far to the north of them, according to a new study co-authored by a researcher at Scripps Institution of Oceanography.

The study, published Sunday in the journal Nature Climate Change, suggests that energy consumption in U.S. and Asian cities can alter air circulation, pushing jet streams farther north and heating up parts of Canada and Siberia in the winter.

Its findings, the authors said, could account for unexplained warming in those areas of up to 1.8 degrees Fahrenheit each winter.

“Human activity — light, heat, cooling and transportation — consumes a lot of energy,” said Guang Zhang, a research meteorologist with Scripps Institution of Oceanography. “This energy eventually goes into the atmosphere as waste heat. We were trying to figure out how this excess heat affects climate. To our surprise, we saw that changes for surface temperature are substantial in regions far away from urban areas.”

Meteorologists long have known that cities are warmer than rural areas, as cars and buildings burn energy, and asphalt and roofs absorb heat. That’s called the urban heat island effect, and researchers have long thought that the heat stayed close to the cities.

But the study, based on a computer model, now suggests the heat does something else, albeit indirectly. It travels about half a mile up into the air and then its energy changes the high-altitude currents in the atmosphere that dictate prevailing weather.

The study adds to the evolving body of data examining how human activity affects climate. While scientists have said that carbon dioxide and other greenhouse gases are responsible for warming trends, recent studies, including one published earlier this month, suggest that black soot — from coal or wood fires — plays a greater role than previously thought.

Research on the effect of urban heat on global weather systems might fill in another part in that puzzle, but researchers acknowledge that there are more questions to answer.

Several outside scientists said they were surprised by the study results, calling the work “intriguing” and “clever.” But they said it would have to be shown in more than one computer model and in repeated experiments before they could accept this theory.

“It’s an interesting and rationally carried out study,” said David Parker, climate monitoring chief of the United Kingdom meteorology office. “We must be cautious until other models are used to test their hypothesis.”

Warming trends aren’t uniform around the globe, but instead are more pronounced over high-latitude land areas than elsewhere, said study co-author Ming Cai, a professor with the Department of Earth, Ocean, and Atmospheric Science at Florida State University. Until now, scientists struggled to explain that difference, and climate models couldn’t account for it, he said.

That extra heat could indicate a flaw in the model, natural temperature variability, or some other factor that forces northern areas to warm up in winter, Cai said. When he and his research partners factored urban heat in the model, the equation added up, with 95 percent confidence.

“By adding that heat into the model, this accounts for a substantial part of the missing warming that the models were unable to account for,” Xhang said.

The heat generated in cities doesn’t just warm the immediate area, however, the authors said, but instead may supercharge weather systems that move across the globe.

“The heat perturbs the jet stream,” Xhang said. “It modifies the air circulation pattern and forces heat northward. Basically it pushes the jet stream further up north.”

Urban heat involved represents a tiny fraction of the energy that the sun adds to the climate, Xhang said. So its role is not so much fueling — but rather rearranging — climate patterns. Consequently some parts of the Pacific Northwest and Europe may be a little chillier in winter, he said. And portions of North America and Asia may be cooler in fall.

Global energy use is almost 16 trillion watts, the study stated. Eighty-six cities, including New York, Los Angeles and Tokyo and other metropolitan giants, consume about 42 percent of that amount. The study only looked at that subset of cities, and only studied those through 2006, Xhang said, so future studies should include a more comprehensive estimate of energy use.

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