World’s megacities may influence global weather

Heat generated by the Earth’s major cities has influenced global weather patterns and is probably responsible for winter warming in parts of North America and northern Asia, according to scientists.

So-called waste heat produced by human activities in major urban centers has altered aspects of the jet stream and other atmospheric systems, causing significant warming in some regions and cooling in others, according to a study published recently in Nature Climate Change.

“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 lead author Guang Zhang, a research meteorologist at Scripps Institution of Oceanography in La Jolla. “This is accomplished through atmospheric circulation change.”

This heating, according to study authors, is separate from the planetary warming caused by greenhouse gases, as well as the so-called urban heat island effect. The heat island phenomenon occurs when heat is stored and re-radiated by expanses of asphalt, concrete and other building materials, making urban areas warmer than rural areas.

Overall, the waste heat produced by the globe’s cities is small. However, the heat is highly concentrated, and in many cases, positioned directly beneath major atmospheric troughs and ridges, according to study authors.

In Russia and northern Asia, the effect can increase temperatures by almost 2 degrees Fahrenheit during the winter. In the northeastern U.S. and southern Canada, the effect has raised winter temperatures by more than a degree, authors say.

Zhang, along with colleagues Ming Cai, a meteorology professor at Florida State University, and Aixue Hu, an oceanographer at the National Center for Atmospheric Research, used computer models to map the effects of waste heat. Among the effects they observed in models was a widening of the jet stream.

They suggest that their findings be incorporated into climate warming models.

The idea that heat produced by daily urban activities could influence weather across the planet is not new; it was introduced about 50 years ago. However, authors said the theory was largely forgotten until recently.

The authors also noted that their estimates were conservative.

“We consider only megacities with large energy consumption,” the authors wrote. “The climate response to this anthropogenic energy source may be in the lower bound of the plausible climate impact.”