Urban Heat Has Large-scale Climate Effects

The heat generated by everyday activities in metropolitan areas has a significant enough warming effect to influence climate even thousands of miles away from its source during winter months, according to a trio of climate researchers.

Led by Guang Zhang, a research meteorologist at Scripps Institution of Oceanography, UC San Diego, the scientists report in the journal Nature Climate Change that the extra heat given off by Northern Hemisphere urban areas causes as much as 1 degree C (1.8 degrees F) of warming in winter. They added that this effect helps explain the disparity between actual observed warming in the last half-century and the amount of warming that computer models have been able to account for.

“What we found is that energy use from multiple urban areas collectively can warm the atmosphere remotely, thousands of miles away from the energy consumption regions,” said Zhang. “This is accomplished through atmospheric circulation change.”

The study, “Energy consumption and the unexplained winter warming over northern Asia and North America,” appeared in online editions of the journal Jan. 27. The National Science Foundation, the U.S. Department of Energy, and NOAA supported the research.

Zhang, along with Ming Cai of Florida State University and Aixue Hu of the National Center for Atmospheric Research in Boulder, Colo., considered the energy consumption that generates waste heat release. The world’s total energy consumption in 2006 was 16 terawatts (one terawatt equals 1 trillion watts). Of that, 6.7 TW were consumed in 86 metropolitan areas in the Northern Hemisphere.

“The burning of fossil fuel not only emits greenhouse gases but also directly effects temperatures because of heat that escapes from sources like buildings and cars,” Hu said.

The release of waste heat is different from energy that is naturally distributed in the atmosphere, the researchers noted. The largest source of heat, solar energy, warms Earth’s surface and atmospheric circulations distribute that energy from one region to another. Human energy consumption distributes energy that had lain dormant and sequestered for millions of years, mostly in the form of oil or coal. Though the amount of human-generated energy is a small portion of that transported by nature, it is highly concentrated in urban areas. In the Northern Hemisphere, many of those urban areas lie directly under major atmospheric troughs and jet streams.

“The world’s most populated metropolitan areas, which also have the highest rates of energy consumption, are along the east and west coasts of the North American and Eurasian continents, underneath the most prominent atmospheric circulation troughs and ridges,” Cai said. “The concentrated and intensive release of waste energy in these areas causes a noticeable interruption to normal atmospheric circulation systems, leading to remote surface temperature changes far away from the regions where the waste heat is generated.”

Zhang said the effect his team studied is distinct from the so-called urban heat island effect, an increase in the warmth of cities compared to unpopulated areas caused by human activities.

The authors report that the influence of urban heat can widen the jet stream and strengthen atmospheric flows at mid-latitudes. They add that the warming is not uniform. Partially counterbalancing it, the changes in major atmospheric systems cool areas of Europe by as much as 1 degree C, with much of the temperature decrease occurring in the fall.

Overall, these changes have an insignificant effect on global temperatures, increasing them worldwide by an average of about 0.01 degree C.

The study does not address whether the urban heating effect plays a role in accelerating global warming, though Zhang said drawing power from renewable sources such as solar or wind provides a societal benefit in that it does not add net energy into the atmosphere.

 The authors also contend that the urban heat effect accounts for the discrepancy between observed warming and winter warming simulated in the models used by the climate science community for analysis and prediction of climate. They suggest that the influence of energy consumption accompany heat-trapping gases and aerosols as necessary variables in computer models.

– Robert Monroe