“Urban areas are centres of wealth, and damages to urban infrastructure represent potentially large economic losses,” Vimal Mishra of the Indian Institute of Technology Gandhinagar told environmentalresearchweb. “Nonetheless, there have been surprisingly few studies that have focused on changes in climatic extremes in these areas.”

By 2050, around 70% of the world’s population is projected to reside in urban areas. “Understanding how climate and climate extremes in particular are changing in the relatively small part of the global land area [urban areas] represent is especially important,” said Mishra.

Mishra and colleagues from the University of Washington, Northeastern University and the University of California, Los Angeles, all in the US, examined data from the Global Summary of the Day stations for 217 cities with more than 250,000 inhabitants, from 1973 to 2012.

Nearly half the cities saw a significant increase in the number of extreme hot days, while almost two-thirds saw extreme hot nights more often.

Over the same time period, the frequency of periods of extreme cold decreased, and extreme windy days declined in about 60% of the urban areas. The picture for rainfall was less clear cut, with 17% of cities seeing an increase in the frequency of daily precipitation extremes but about half as many seeing a decrease.

“Our results show significant increases in heat waves and the number of hot days and warm nights, and at the same time declines in cold waves and extreme windy days in many urban areas over the last 40 years,” said Mishra. “We also find that the number of changes in precipitation extremes was modest, which is somewhat surprising as our previous work (GRL, 2011) showed a predominance of increases in precipitation extremes in major US urban areas.”

Comparing 142 pairs of urban and non-urban sites with similar characteristics revealed a difference in the trends in temperature and wind extremes between cities and rural regions. Urban areas generally saw more increases in temperature-related extremes and more decreases in wind-related extremes.

“This has important implications for the global climate and health community,” said Mishra. “For instance, increasing numbers of heat waves may lead to enhanced heat wave-related mortality. Increased warming in urban areas may also have profound implications [for] residential heating demands (reduced) and cooling demands (increased). Moreover, increasing precipitation extremes in urban areas may pose challenges for urban storm-water infrastructure.”

So what’s next? “It would be important to better understand the relative roles of climate change and urbanization in extreme weather events,” said Mishra. “We would like to extend this work to other variables, e.g. humidity, especially during heat waves, to better understand implications of changes in urban areas.”

Mishra and colleagues from Northeastern University and the University of Washington, both in the US, reported their findings in Environmental Research Letters (ERL).

Related links

Related stories