Energy dissipated as heat in cities can cause regional temperature changes, simulations suggest

By Erin Wayman

Web edition: January 25, 2013
Print edition: February 23, 2013; Vol.183 #4 (p. 16)

The waste heat generated by car engines, power plants, home furnaces and other fossil fuel-burning machinery plays an unappreciated role in influencing regional climates, new computer simulations suggest. By altering atmospheric circulation, human-made heat may raise temperatures by as much as 1 degree Celsius during winter in the northernmost parts of the world.

The finding may help explain why current climate simulations, which account for the heat-trapping effects of greenhouse gases but not the heat directly produced by energy consumption, have failed to replicate some winter warming observed in the northern latitudes, researchers report online January 27 in Nature Climate Change.

“The magnitude of their result is quite surprising,” says Mark McCarthy, a climate scientist at the Met Office Hadley Centre in Exeter, England.

It’s well-known that the heat from human energy consumption makes cities hotter than sparsely populated areas nearby, a phenomenon known as the urban heat island effect. But worldwide, waste heat represents only a tiny fraction of the heat produced naturally by incoming solar energy. Previous studies hadn’t found evidence that waste heat significantly influences global average temperatures. Energy consumption’s global warming effect, those studies have suggested, is no more than around 3 percent of that due to carbon dioxide emissions.

The new study suggests that waste heat coming from urban areas is sufficient to influence climate on a regional scale. Climate scientist Ming Cai of Florida State University in Tallahassee and his colleagues ran global climate simulations that took into account energy use in 2006 from 86 of the world’s largest metropolitan areas. Together, these cities — located along the coasts of North America, Europe and East Asia — cover less than 2 percent of Earth’s surface but are responsible for about 42 percent of world energy consumption. The researchers assumed that all energy used in these areas is converted to waste heat — an overestimate, but not an unrealistic one.

The simulations incorporating waste heat found that temperatures in December, January and February were 1 degree warmer in Russia and northern Asia than in simulations that ignored the heat. Parts of the United States, Canada and China experienced winter temperature increases of as much as 0.5 to 0.8 degrees.

“The largest warming is not in the places where the energy is consumed,” Cai notes. That’s because the heat itself doesn’t cause the temperature spikes, according to the simulations. Instead, the heat disrupts normal atmospheric circulation, widening the jet stream and strengthening other circulation patterns in the mid-latitudes. These changes warm some regions in winter and bring cooler air to others, such as Western Europe, the simulations show.

The results demonstrate that climate researchers shouldn’t ignore waste heat, says Mark Flanner, an atmospheric scientist at the University of Michigan in Ann Arbor.

The next step is to improve estimates of waste heat, says David Sailor, a mechanical engineer at Portland State University in Oregon. Not all energy use dissipates as heat, as the new simulations assume. Sailor also calls for adding to the simulations the daily, seasonal and spatial variations in energy use.