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Exposure to deadly urban heat has TRIPLED worldwide since the 1980s

Exposure to deadly urban heat has tripled worldwide since the 1980s, a new study has warned.

Researchers at Columbia University’s Earth Institute said a quarter of the world’s population is now affected as a result of both rising temperatures and booming urban population growth.

They studied more than 13,000 cities across the globe and based their calculations on the number of ‘person-days’ that inhabitants were exposed to extreme combinations of heat and humidity. 

These went from 40 billion per year in 1983 to 119 billion in 2016 — a threefold increase. By 2016, 1.7 billion people were being subjected to such conditions on multiple days, the researchers said.

Researchers at Columbia University’s Earth Institute said a quarter of the world’s population is now being exposed to deadly urban heat as a result of both rising temperatures and booming urban population growth. This graphic shows the annual increases in the rate of urban population exposure to extreme heat in more than 13,000 cities worldwide from 1983 to 2016

Experts studied more than 13,000 cities across the globe and based their calculations on the number of 'person-days' that inhabitants were exposed to extreme combinations of heat and humidity. Pictured is the sun beating down on downtown Phoenix in Arizona

Experts studied more than 13,000 cities across the globe and based their calculations on the number of ‘person-days’ that inhabitants were exposed to extreme combinations of heat and humidity. Pictured is the sun beating down on downtown Phoenix in Arizona

Which cities have had the biggest increase in urban heat exposure?

The worst-hit city in terms of person-days was Dhaka, the fast-growing capital of Bangladesh; it saw an increase of 575 million person-days of extreme heat over the study period. 

Its ballooning population alone — 4 million in 1983, to 22 million today — caused 80 percent of the increased exposure. 

This does not mean that Dhaka did not see substantial warming — only that population growth was even more rapid. 

Other big cities showing similar population-heavy trends include Shanghai and Guangzhou, China; Yangon, Myanmar; Bangkok; Dubai; Hanoi; Khartoum; and various cities in Pakistan, India and the Arabian Peninsula.

On the other hand, some other major cities saw close to half or more of their exposure caused by a warming climate alone rather than population growth. 

Among them were Baghdad, Cairo, Kuwait City, Lagos, Kolkata, Mumbai, and other big cities in India and Bangladesh. 

The populations of European cities have been relatively static, so increases in exposure there were driven almost exclusively by increased warmth. 

Urban population growth accounted for two-thirds of the exposure spike, while rising temperatures caused by global warming contributed a third. 

The worst-hit city was Dhaka, the fast-growing capital of Bangladesh, which has seen an increase of 575 million person-days of extreme heat over the past 40 years, mainly due to its ballooning population from 4 million in 1983 to 22 million today.

This caused 80 per cent of the increased exposure, the researchers said, although they cautioned that it doesn’t mean it hasn’t seen rising temperatures as well, it’s just this has been overshadowed by the population increase.

Over recent decades, hundreds of millions of people have moved from rural areas to cities, which now hold more than half the world’s population. 

There, temperatures are generally higher than in the countryside because of sparse vegetation and abundant concrete and asphalt that tend to trap and heat—the so-called urban heat island effect.

‘This has broad effects,’ said the study’s lead author, Cascade Tuholske, a postdoctoral researcher at Columbia University’s Earth Institute. 

‘It increases morbidity and mortality. It impacts people’s ability to work, and results in lower economic output. It exacerbates pre-existing health conditions.’

Other big cities showing similar population-heavy trends include Shanghai and Guangzhou, China; Yangon, Myanmar; Bangkok; Dubai; Hanoi; Khartoum; and various cities in Pakistan, India and the Arabian Peninsula.

On the other hand, some other major cities saw close to half or more of their exposure caused by a warming climate alone rather than population growth. 

Among them were Baghdad, Cairo, Kuwait City, Lagos, Kolkata, Mumbai, and other big cities in India and Bangladesh. 

The populations of European cities have been relatively static, so increases in exposure there were driven almost exclusively by increased warmth. 

‘A lot of these cities show the pattern of how human civilisation has evolved over the past 15,000 years,’ said Tuholske, pointing out that many are located in warm climates where humidity is delivered by big river systems. This made them attractive for farming and eventually urbanisation. 

‘The Nile, the Tigris-Euphrates, the Ganges. There is a pattern to the places where we wanted to be,’ he said.

‘Now, those areas may become uninhabitable. Are people really going to want to live there?’ 

Urban population growth accounted for two-thirds of the exposure spike, while rising temperatures caused by global warming contributed a third (stock image)

Urban population growth accounted for two-thirds of the exposure spike, while rising temperatures caused by global warming contributed a third (stock image)

What is the ‘wet-bulb globe temperature’ scale?

The ‘wet-bulb global temperature’ scale (WBGT) is ‘a measure of heat stress in direct sunlight, which is based on temperature, humidity, wind speed, sun angle, and cloud cover (solar radiation).’

This differs from the heat index, which is based only on temperature and humidity and is calculated for shady areas.’

The WBGT measurement takes into account the multiplier effect of high humidity on human physiology.

A wet-bulb reading of 30 is the rough equivalent of 106 degrees Fahrenheit on the so-called ‘real feel’ heat index —the point at which even most healthy people find it hard to function outside for long, and the unhealthy might become very ill or even die.

The researchers combined infrared satellite imagery and readings from thousands of ground instruments to determine maximum daily heat and humidity readings in 13,115 cities, from 1983 to 2016. 

They defined extreme heat as 30°C on the so-called ‘wet-bulb globe temperature’ scale, a measurement that takes into account the multiplier effect of high humidity on human physiology. 

A wet-bulb reading of 30 is the rough equivalent of 106 degrees Fahrenheit on the so-called ‘real feel’ heat index — the point at which even most healthy people find it hard to function outside for long, and the unhealthy might become very ill or even die.

To come up with a measure of person-days spent in such conditions, the researchers matched up the weather data with statistics on the cities’ populations over the same time period.  

In the US, about 40 large cities have seen rapidly growing heat exposure, mainly clustered in Texas and the Gulf Coast. 

In many, the causes of the rises have been varying combinations of both increasing population and increasing heat. 

These include Houston, Dallas-Fort Worth, San Antonio and Austin, Texas, along with Pensacola and other cities in Florida. In some, population growth is the main driver. 

These include Las Vegas; Savannah, Georgia; and Charleston, South Carolina. In others, it is almost exclusively fast-rising heat: Baton Rouge, Louisiana; Gulfport, Mississippi; and Lake Charles and Houma, Louisiana. 

One major outlier is the city of Providence, Rhode Island, where rising exposure was 93 per cent due to warmer, more humid weather.

Temperatures are generally higher in cities than in the countryside because of sparse vegetation and abundant concrete, asphalt and other impermeable surfaces that tend to trap and concentrate heat—the so-called urban heat island effect (stock image)

Temperatures are generally higher in cities than in the countryside because of sparse vegetation and abundant concrete, asphalt and other impermeable surfaces that tend to trap and concentrate heat—the so-called urban heat island effect (stock image)

Because the study period only ran through to 2016, the data did not include the record heatwaves that hit the U.S. Northwest and southern Canada this summer, killing hundreds of people. 

The authors say their research should be used to help urban planners come up with better-targeted strategies to help citizens adapt to increasing city heat.

Kristina Dahl, a climate researcher at the Union of Concerned Scientists, said the study ‘could serve as a starting point for identifying ways to to address local heat issues,’ such as planting trees and modifying rooftops with lighter colours or vegetation so they don’t trap so much heat. 

‘This study shows that it will take considerable, conscientious investments to ensure that cities remain livable in the face of a warming climate,’ she added.

The research has been published in the Proceedings of the National Academy of Sciences.

WHAT IS AN URBAN HEAT ISLAND?

In urban heat islands, solar radiation and hot air from vehicles and buildings get trapped between high-rises. 

There aren’t enough trees to provide shade and evaporative cooling that can bring down temperatures.

The phenomenon pushes up temperatures in areas covered in heat-retaining asphalt and concrete.

Heat ripples blur the downtown skyline as a jet lands in Phoenix, Ariz., as temperatures exceed 100 degrees in the morning hours. Already devilishly hot for being in the Sonoran desert, Arizona's largest city is also an 'urban heat island,' a phenomenon that pushes up temperatures in areas covered in heat-retaining asphalt and concrete.

Heat ripples blur the downtown skyline as a jet lands in Phoenix, Ariz., as temperatures exceed 100 degrees in the morning hours. Already devilishly hot for being in the Sonoran desert, Arizona’s largest city is also an ‘urban heat island,’ a phenomenon that pushes up temperatures in areas covered in heat-retaining asphalt and concrete.

The heat island effect prevents a city from peeling off the day’s furnace-like heat after the sun sets, driving up temperatures over time. 

Long noted in cities as far flung as Cairo, Mumbai, Johannesburg, Mexico City and Nanjing, China, the heat island effect can add more than 10 degrees Fahrenheit to nighttime temperatures, climate scientists say. 

 

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