Northern winter not as cold as expected? It could be urban 'waste heat'

Taken together, these locations form an "urban-heat archipelago" that spans the mid-latitudes of the northern hemisphere, says Ming Cai, an atmospheric scientist at Florida State University in Tallahassee and a member of the team that reported the results Sunday in the journal Nature Climate Change.

This pattern of human development along the coasts has unwittingly put these concentrated heat sources "underneath sensitive regions of atmospheric circulation," says Dr. Cai – either right under the jet stream or under regions of relatively persistent high or low pressure.

The team found that when waste heat in these regions exceeds a certain threshold, the vast column of relatively warm air can rise to set up a blocking pattern – altering the strength of the jet stream at different latitude and the location of its north-south meanders. The effect is most pronounced in winter, when the temperature contrast between the waste heat and surrounding air is the strongest.

The team, led by Guang Zhang, a climate researcher at the Scripps Institution of Oceanography in La Jolla, Calif., ran global climate models to reproduce the global climate over the past 100 years. In one set of model runs, they included all of the human influences – from increasing CO2 emissions to various forms of aerosols and soot. In a second set, they included the heat from the urban-heat archipelago.

During December through January, Russia and northern Asia warm by an average of up to 1 degree C above what one would expect from global warming alone. Eastern China far beyond the urban areas warms by about 0.5 degrees C. In North America, the US Northeast and southern Canada warmed by about 0.8 degrees C. The circulation changes also can cool other regions by comparable amounts.

Moreover, the modeled wintertime results bore a striking resemblance to the measured temperature record itself.

When the team hunted for a mechanism driving the relative warmth at such distances from the sources, changes in circulation patterns from the urban waste heat showed up across the range of model results. 

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