The world’s urban areas have experienced significant increases in deadly heat waves over the past 40 years, new research reveals.
These prolonged periods of extreme hot days have significantly increased in over 200 urban areas across the globe between 1973 and 2012, and have been most prominent in the most recent years on record.
The new findings, which were published in the journal Environmental Research Letters, show that over the same time period, more than half of the studied areas showed a significant increase in the number of individual extreme hot days, while almost two-thirds showed significant increases in the number of individual extreme hot nights.
The study, undertaken by researchers at the Indian Institute of Technology (IIT) Gandhinagar, Northeastern University, University of California, Los Angeles, and the University of Washington, is one of the first to focus solely on the extent of extreme weather on a global scale, as well as examining disparities between urban and non-urban areas.
Heat waves increasing in frequency across the globe
In their study, the researchers obtained daily observations for rain, air temperature and wind speed from the Global Summary of the Day (GSOD) data set produced by the National Climatic Data Center (NCDC).
They identified all urban areas globally with a population greater than 250,000 (around 650 areas) and then refined the list based on the area’s proximity to a GSOD station and the availability of complete weather records. They were left with 217 stations with complete records for the period 1973-2012, most of which were located at airports close to urban areas.
Once the data was obtained for the 217 urban areas, the researchers identified extremes for temperature, precipitation and wind and calculated heat waves, cold waves as well as individual extreme hot days and nights. Heat waves were defined as periods where the daily maximum temperature was hotter than 99 percent of days for the period 1973-2012, for a consecutive period of six or more days.
The results showed that there were statistically significant increases in the number of heat waves per urban area during the last four decades. Of the five years with the largest number of heat waves, four were the most recent years on record (2009, 2010, 2011 and 2012).
Results also showed a general decline in cold waves, and around 60 percent of urban areas experiencing a significant decline in extreme windy days. Around 17 percent of urban areas experienced a significant increase in daily precipitation extremes, and around 10 percent experienced a significant increase in annual maximum precipitation.
“Over half of the world’s population now live in urban areas; hence, it is particularly important to understand how the climate and climate extremes, in particular, are changing in these areas,” said lead author Dr. Vimal Mishra, of IIT Gandhinagar.
“Urban areas make up a relatively small part of the global land area; however, they are the center of wealth, so damage to urban infrastructure could result in potentially large economic losses,” he said. “Surprisingly, there have been few studies that have focused on changes in climatic extremes in these areas.”
Urban areas particularly vulnerable to heat waves
Using a separate data set in which 142 pairs of urban and non-urban areas were selected, the researchers found disparate changes for temperature and wind related extremes, with generally more increases in temperature-related extremes, and more decreases in wind-related extremes in urban areas compared to non-urban areas.
The team is now examining the impacts of climate and weather extremes in urban regions on critical lifeline infrastructures, as well as on urban and coastal ecosystems and marine life.
According to the Centers for Disease Control and Prevention, extreme temperatures are the deadliest of all weather-related phenomenon. From 1999 through 2009, extreme heat exposure caused or contributed to more than 7,800 deaths in the United States. Deaths generally result from heat stroke and related conditions, but also from cardiovascular disease, respiratory disease, and cerebrovascular disease. Heat waves are also associated with increased hospital admissions for cardiovascular, kidney, and respiratory disorders.
Extreme heat waves cause the most harm among elderly people and young children. City dwellers are at particular risk because of elevated temperatures in cities, known as the “urban heat island effect” due to the magnifying effect of paved surfaces and the lack of tree cover.
Average temperatures around the world have become warmer since the start of the 20th century, and they have risen at a particularly fast rate during the last 50 years. The ten warmest years on record have all occurred since 2000, with 2014 ranking as the hottest year ever recorded.
As the effects of climate change persist, experts project that the frequency of extreme heat events — and the health threats associated with them — will continue to rise. Based on our current trajectory, average temperatures in the United States are expected to rise between 5 and 9°F (3-5°C) over the next century.