A new study by British climate researchers estimates just how hot urban areas around the world could become by mid-century under a “business-as-usual” scenario in which carbon dioxide concentrations rise to 645 parts per million.

The rise in average global temperatures under that scenario is expected to be 5.4 degrees Fahrenheit. But “urban heat island” effects — the additional rise in average temperatures in cities driven by the proliferation of paved surfaces and the lack of green spaces, heat radiation from buildings and the overall density of people, cars and other human activity — will exacerbate the heating.

Under a scenario of 645 ppm, average nighttime lows in Middle Eastern cities could rise nearly 11 degrees Fahrenheit — 5.4 degrees from global warming and another 5.4 degrees from urban heat island effects, the study’s authors estimated.

Average nighttime lows in cities across South Asia, West Africa, and the western United States could rise 9 degrees Fahrenheit.

About six billion people are expected to live in cities around the world by 2050.

The study, “Climate change in cities due to global warming and urban effects,” was published May 8 in the journal Geophysical Research Letters. Researchers included Mark P. McCarthy and Richard A. Betts of the Met Office Hadley Centre, and Martin J. Best of the Joint Centre for Hydro-Meteorological Research, Met Office.

For more information, see the American Geophysical Union’s blog, Geohazards, and a news story at DiscoveryNews.