5 June 2019

One third of the African urban population exposed to extreme heat by 2090

Posted by llester

Locations of the 173 cities analyzed in the new study, with 2015 population (millions). Credit: AGU.

Locations of the 173 cities analyzed in the new study, with 2015 population (millions). Credit: AGU.

By Aurélie Kuntschen 

Climate change, population growth and urbanization are instrumental in increasing exposure to extreme temperatures. A new study in AGU’s journal Earth’s Future assessed a range of possible scenarios regarding the rate of climate change and socio-economic development in 173 African cities for the years 2030, 2060 and 2090. The results show a third of African city-dwellers could be affected by deadly heat waves in 2090. The projections also highlight the influence of socio-economic development on the impact of climate change.

The effects of climate change are felt specifically in countries with tropical climates, which are characterized by high humidity and very high temperatures. Furthermore, countries in these regions – especially in Africa – are experiencing heavy urbanization and socio-economic development, leading to an explosion in the size of urban populations. A combination of these two factors is having a major impact on the living conditions of city-dwellers in Africa, especially in terms of exposure to extreme – or even lethal – temperatures.

“We consider the critical threshold to be 40.6 degrees Celsius in apparent temperature, taking humidity into account,” said Guillaume Rohat, a researcher at UNIGE’s Institute for Environmental Sciences (ISE). High outdoor humidity levels disrupt our ability to thermoregulate, with potentially fatal consequences. The scientists based their research on scientific climate projections and future urban demographics, rather than on current demographic data, to calculate the risk in the years ahead – which was in itself a first.

“The idea was to factor in all possible scenarios regarding climate change and urban population growth, the best and the worst, so we could find out what the future holds,” Guillaume explained. The scientists then combined five scenarios based on socio-economic projections and three climate change projection scenarios carried out by the Intergovernmental Panel on Climate Change (IPCC) for the years 2030, 2060 and 2090.

“This gave us twelve different plausible combinations for each of the years. It also meant we could calculate the number of people per day exposed to apparent temperatures above 40.6 degrees Celsius (105 degrees Fahrenheit) in cities in Africa on an annual basis. The same individual can be counted several times, because he or she may be exposed to these heat waves several days a year,” Guillaume said. Based on these twelve models, the scientists analyzed the demography, urbanization and climate in 173 cities with at least 300,000 inhabitants in 43 countries across Africa.

Sharp rise in the number of people at risk

The initial results of the new study show regardless of the scenario selected, a drastic increase in the number of people affected by extreme temperatures on an annual basis is inevitable.

“In the best case, 20 billion person-days will be affected in 2030, compared to 4.2 billion in 2010 – a jump, in other words, of 376 percent,” Rohat said. “This figure climbs to 45 billion in 2060 (up 971 percent) and reaches 86 billion in 2090 (up 1947 percent).”

When the researchers modeled the worst-case scenario for each of these three years –  a very steep population increase, an explosion in urbanization and a climate badly disturbed by a continuous increase in CO2 – the figures rose even more sharply: 26 billion in 2030 (up 519 percent compared to 2010), 95 billion in 2060 (up 2,160 percent) and 217 billion in 2,090 (up 4,967 percent. If every inhabitant in the 173 cities studied was exposed every day of the year in 2090, the figure would rise to 647 billion.

“We see that the worst scenario for 2090 affects 217 billion people – that’s a third of Africa’s urban population potentially exposed on a daily basis!” Rohat said. This means that one third of the population would be exposed every day to a minimum temperature of 40.6 degrees Celsius (105 degrees Fahrenheit) or that every African city would experience this heat for four months of the year. The figure falls to 10 percent in the best possible scenario for 2030.

The Paris Agreement and Sustainable Development Goals

The team of scientists investigated whether it was possible to reduce the exposure to extreme heat. They performed the calculations a second time using the best possible climate scenario combined with the different socio-economic models, and found exposure was reduced by 48 percent for the year 2090.

“This proves that if we follow the Paris Agreement, we’ll halve the number of people at risk in 2090, which is encouraging!” Rohat said. Under the best socio-economic scenario for each of the climate models, the number of people exposed to extreme temperatures drops by 51 percent, according to the researchers. “We can see the importance of the UN Sustainable Development Goals: access to education, a drop in the number of children per woman, developments in the standard of living, and so forth.”

The study makes it clear exposure to extreme temperatures is going to rise sharply. But it also shows, if we act quickly, the increase can be at least partially curbed.

“That’s why we’re currently in contact with several cities that we studied,” Rohat said. “The local actors are interested in the results for 2030 and 2060 so they can adapt to the inevitable and take measures to restrict urbanization, especially by improving the quality of life in rural areas or promoting the development of other cities of more modest size.”

—Aurélie Kuntschen is an attachée de presse at the Université de Genève in Switzerland. This story was originally published as a press release in French and English on the Université de Genève website.