News•January 27, 2013

Waste Heat From Cities May Be Altering Weather Patterns

The carbon dioxide that belches from tailpipes and smokestacks is a proven greenhouse gas that has been driving global temperatures more or less steadily upward over the past century. But the heat that leaks directly into the environment from hot exhaust pipes, boilers and chimneys has also contributed to temperature increases in some places, according to a study published Sunday in Nature Climate Change, especially in winter— not directly, but rather because the waste heat may be altering the flow of the jet stream, and thus the way warm and cold air move around the globe.

Waste heat coming from hot exhaust pipes, boilers and chimneys—and mostly from cities like New York—may be helping alter weather patterns.

The regions in question, particularly northern Eurasia and North America, have warmed about a degree Celsius’ (1.8°F) more in winter over the past several decades than many climate models say they should have. The authors say, though, that once you factor in waste heat, the discrepancy disappears. Its direct effect on global temperatures is insignificant compared with the heat trapped by greenhouse gases, said co-author Aixue Hu, of the National Center for Atmospheric Research (NCAR), in an interview, “but it does affect regional weather patterns.”

Most of this waste extra heat is produced in cities — naturally enough, since cars, power plants and heating systems are concentrated there — but it’s distinct from the well-known urban heat island effect, in which sunlight absorbed by roads and buildings is re-emitted to warm the surrounding air.

Hu and his colleagues say this extra heat has an outsize effect because major cities tend to be located in coastal areas, where ridges of high atmospheric pressure and troughs of low pressure also tend to form. These ridges and troughs help fix the path of the jet stream, which drives weather systems around the northern hemisphere.

If the ridges and troughs are disrupted by heat rising from coastal cities, that could plausibly limit the amount of Arctic air that moves southward and tropical air that moves northward, affecting temperatures across the northern part of the hemisphere. It’s more of a rearrangement of heat than an overall increase, they say. While parts of the northern hemisphere are warmer in winter than they would otherwise be, parts — notably Northern Europe — end up cooler in summer and autumn.

The word “plausibly” is key here, however. “My colleagues at NCAR agree that the heating should have some kind of impact, but since this line of research is so new, we can’t really say whether it’s right or wrong,” Hu said. “The basic physics behind it is true, but the magnitude of the impact is something that needs more research.”

On that last point, Jennifer Francis, an atmospheric scientist at Rutgers University, who wasn’t involved in the new study, agrees. “It’s interesting, but it’s surprising to me that the energy consumption imposed in this model in such a limited area could have such a widespread effect,” Francis said in an email. “It’s also difficult to imagine a mechanism that would result in the large areas of cooling they show during [summer and fall].”

That doesn’t mean the new study is wrong, but it’s far too early to declare it right, either. “It should be followed up, with another that tries to identify the mechanisms involved,” Francis said.