Almost a quarter of the world’s population is now affected: exposure to urban heat has tripled worldwide since the 1980s, according to data analyzes for 13,000 cities. The trend is due to the combination of the increased proportion of city dwellers in the world population and the increased temperatures in the urban environment. Which role the two factors play in each case varies greatly from region to region. The detailed information from the study can now be used to develop specifically tailored strategies to reduce heat stress on city dwellers, say the researchers.
Sweaty, paralyzing and worse: In addition to the strain on quality of life and performance, heat also endangers human health, many studies show. Periods of oppressive scorching heat are also reflected in lower economic output and lower mortality rates for the affected population. Due to the so-called heat island effect, city dwellers are exposed to heat stresses particularly intensely: In urban areas, temperatures are usually higher than in the countryside because there is hardly any cooling vegetation, but instead many concrete surfaces and other sealed areas absorb and concentrate the heat. Earlier studies have already shown that this is a problem that has increased in importance in the course of the rapid urbanization of the world population and the consequences of climate change in recent decades. However, the researchers working with Cascade Tuholske from Columbia University have now documented the development of global urban heat stress in more detail than ever before.
Global view of the urban embers pollution
They combined infrared satellite images and readings from thousands of ground gauges to determine the daily maximum heat levels and humidity in 13,115 cities around the world between 1983 and 2016. As extreme heat, they defined temperatures in combination with the air humidity, which result in a “perceived” heat load of over 40 degrees Celsius. Under these conditions, even most healthy people find it difficult to be outdoors for a long time – for weak people, however, they can even be life-threatening. In order to generate a measure for assessing the development of urban heat load, the researchers compared the weather data with the population statistics of the cities for the same period.
The analysis found that the number of days per person exposed to city dwellers increased from 40 billion annually in 1983 to 119 billion in 2016 – an increase of three times. In 2016, 1.7 billion people were exposed to the stressful conditions on several days. The increase in pollution is due to a combination of population growth and temperature increases in the cities, the researchers explain. As you emphasize, the proportions of the two aspects vary greatly from region to region and from city to city.
Detailed information with practical potential
Overall, it was mostly the cities that were most severely affected by an increase in personal heat stress, which is in the low latitudes. Dhaka – the rapidly growing capital of Bangladesh – was at the forefront. With an 80 percent share in the development, the population increase was the main cause. Other major cities that showed similar patterns are Shanghai and Guangzhou in China, Yangon in Myanmar, Bangkok, Dubai, Hanoi, Khartoum, and various cities in Pakistan, India, and the Arabian Peninsula. In some other major cities, however, almost half or more of the burden was caused by global warming alone and not by population growth. These include Baghdad, Cairo, Kuwait City, Lagos, Calcutta, Mumbai and other large cities in India and Bangladesh.
As far as Europe is concerned, the researchers report: Since the population development in the cities is relatively static here, the increase in exposure was almost exclusively due to the increasing heat development in the urban environment. In the United States, pollution has risen particularly sharply in around 40 major cities, primarily Texas and the Gulf Coast. In some cases, however, population growth is largely responsible for the effect, the scientists report.
As they emphasize, it is precisely in this level of detail that the special significance of their study lies: They show how many people in each location are affected by heat exposure and to what extent they can be traced back to population developments or climate changes. This information could now help city planners to develop more targeted strategies to contain the pollution. In some cases, for example, extensive tree planting, the use of cooling building coatings and green roofs could already make an important contribution to improving the stand climate.
Source: The Earth Institute – Columbia University, Technical article: PNAS, doi: 10.1073 / pnas.2024792118