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

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.

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.

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.