Urban areas experiencing intense heat waves globally

Washington: For the past four decades, global warming has increased dramatically in cities, shows a new study led by an assistant professor of civil engineering Vimal Mishra at IIT-Gandhinagar.

The trend appears to be growing faster in urban areas than in less-populated areas.

Only two percent of those urban areas experienced a significant decline in heat waves.

"Urban areas make up a relatively small part of the global land area, but over half the world's populations now live in them, so the trend is troublesome," said Mishra.

"The combination of higher temperatures and lower wind in particular is not a good combination for human health and well-being. This should concern everyone," he pointed out.

The increase in precipitation could damage cities' infrastructure, which could also mean large economic losses, Mishra noted.

"Our findings suggest that urban areas are experiencing a kind of double whammy -- a combination of general climatic warming combined with the heat island effect, wherein human activities and the built environment trap heat, preventing cities from cooling down as fast as rural areas," said Dennis Lettenmaier, a geography professor at University of California, Los Angeles and co-author.

Lettenmaier and his co-authors studied 217 urban areas across the globe and found that prolonged periods of extreme heat increased significantly in 48 percent of them between 1973 and 2012.

The study is one of the first to focus solely on the extent of extreme weather in urban areas globally.

The team obtained daily observations for rain, air temperature and wind speed from the National Oceanic and Atmospheric Administration (NOAA).

The researchers identified about 650 urban areas with populations greater than 250,000.

For each of the locales in the study, the researchers identified extremes for temperature, precipitation and wind, calculated heat and cold waves, and pinpointed extremely hot days and nights.

"In urban areas, buildings are disrupting the air flow, which affects not only the immediate area of buildings, but apparently the larger regional wind fields," Lettenmaier explained.

The study was published in the journal Environmental Research Letters.