Scientists have published a paper in the Proceedings of the National Academy of Sciences, which is first of its kind paper that explores that extreme heat exposure of people in urban areas and the risk they are at from such kind of exposures.
Global warming is causing increase in average temperatures around the world and cities / urban areas are the most affected owing the increased use of vehicles, proximity to industrial areas among other factors.
The study points out that extreme heat exposure is very common in urban areas and it is increasing by the day. Researchers studied 13,000 settlements over nearly three and a half decades. The authors found that exposure to dangerous temperatures increased by 200% since the mid 1980s, with poor and marginalized people particularly at risk.
Food security is one of the major concerns of global warming and to study how global warming is impacting food security of urban residents, the researchers required to figure out how many people are even impacted by extreme heat. The startling thing that researchers found was that there was no prior research that could look at baseline understanding of where extreme heat is impacting individuals in cities at fine scales.
The authors encountered two main challenges while piecing together this baseline. The first was obtaining reasonably accurate population estimates. Researchers don’t actually know how many people live on Earth, Because there are many places where population counts aren’t feasible due to geography, infrastructure or governance. Researchers gathered population data from a global human settlement database produced by the European Commission Joint Research Centre (JRC). The commission estimates the distribution of urban populations using the finest available census data combined with Landsat remote imaging techniques.
The second major challenge was obtaining the meteorological data to characterize heat exposure around the globe. Fortunately, UC Santa Barbara’s Climate Hazards Center recently created a new dataset with this information: the Climate Hazards InfraRed Temperature with Stations (CHIRTS).
Throughout their analysis, the researchers used a metric called the wet bulb globe temperature (WBGT) to quantify extreme heat. WBGT is an index that accounts for temperature, humidity, wind speed and radiant heat. Similar to a “feels like” index, it was developed to more accurately reflect how ambient conditions affect the human body.
To calculate this researchers divided the Earth’s surface with a grid. For each cell, he used his models and datasets to calculate the maximum temperature and relative humidity for each day from 1983 through 2016. This enabled him to calculated the WBGT.
Next they overlayed this grid on the map of urban populations and chose a wet bulb globe temperature of 30° Celsius (86° Fahrenheit) as the threshold for extreme heat exposure. This value is commonly used as it’s considered to pose a high risk of occupational heat-related illness by the International Standards Organization. For each year, they counted how many days each cell exceeded a WBGT of 30° C, and then multiplied that by the population in that cell. The result was the number of person-days per year of extreme heat exposure at a resolution of 0.05° of latitude by 0.05° of longitude.
They found that in 34 years urban extreme heat exposure increased 200% globally. Economically disadvantaged and marginalized people are particularly vulnerable to extreme heat exposure, the authors state. In addition to their risk of food insecurity, these people have fewer options to mitigate their exposure to extreme heat. They also tend to live in urban areas prone to more severe and prolonged bouts of extreme temperatures.