Urban heatwaves getting worse

PARIS: Urban heatwaves have become more frequent over the last 40 years, scientists reported on Friday.
A weather database of cities around the world reveals “significant” increases in periods of extremely hot days and falls in the number of cold days, they found.
Previous research found that, in the four decades covered in the study, man-made global warming stepped up a gear.
But, in urban heatwaves, additional factors can play a role, the authors cautioned.
These include local climate variability, the design and spread of a city, and land cover beyond it.
“Over half of the world’s population now live in urban areas,” lead author Vimal Mishra, a professor at the Indian Institute of Technology (IIT) Gandhinagar, said.
“It is particularly important to understand how the climate and climate extremes, in particular, are changing in these areas.”
The data trawl covered around 650 cities, defined as areas with a population greater than 250,000, with weather stations that reported to a US-run meteorological report called Global Summary of the Day (GSOD).
The researchers were left with 217 cities for which there were complete records for 1973-2012. Heatwaves were defined as periods lasting six days consecutively or more — their daily maximum had to be greater than 99 percent of the temperatures recorded at that time of the year, as measured over the whole 40 years.
From 1973-2012, the number of heatwaves per urban area rose by 0.3 of an event, a “statistically significant” increase, the researchers said.