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

Waste heat has a smaller impact on global climate than does CO2, but heat from highly urbanized northern regions appears to explain observed deviations from climate forecasts, a study says.

By Staff writer / January 28, 2013

A frozen fountain at Bryant Park attracts a visitor on Friday in New York.

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Waste heat from burning oil, coal, and gas to fuel everything from cars and homes to power plants in large urban areas provides enough warmth to alter the northern hemisphere's climate at significant distances from the sources of the heat, according to a new study.

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Indeed, the researchers say, waste heat's impact may close a gap between the winter and autumn temperatures that climate models project for some regions of the northern hemisphere and the warmer-than-projected temperatures that have been measured for those areas.

Waste heat is distinct from the so-called urban heat-island effect, in which cities are warmer than surrounding rural areas. The urban heat-island effect stems from a city's rough texture, dark asphalt surface streets, and dark rooftops, which absorb the sun's heat and release it back into the air. Essentially cities are recycling locally energy that is already part of the climate system.

Waste heat represents energy added to the system as long-buried fossil fuels are unearthed and burned. Some of the heat is converted to other forms of energy, such as electricity. But no conversion process is 100 percent efficient. So heat also heads out the smokestack or tailpipe.

Averaged over the entire globe, waste heat's effect on warming is tiny compared with the effect from carbon dioxide and other greenhouse gases humans are pumping into the atmosphere as they burn fossil fuel and change land-use patterns, the researchers say.

Where CO2's effect on climate has increased global average temperatures by around 1 degree Celsius (1.8 degrees Fahrenheit) over the past century, urban waste heat contributes about 0.01 degrees C to the climate's global average temperature in a given year as a kind of low-level background, the researchers estimate.

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.

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