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

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Indeed, the effect is so small that, while research on the climate effect of waste heat began some 50 years ago, it didn't get far because modeling at the time put the global average effect on temperatures well within the bounds of natural variability. Moreover the experiments at the time had a few sources spread randomly over the globe.

But highly urbanized regions, such as the US Northeast, the San Diego to San Francisco corridor, western Europe, and eastern China, represent concentrated sources of 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.

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