By Tim Radford
An entire continent faces lethal conditions for many of its people: by 2090, one person in three can expect African city heat in the great conurbations severe enough to expose them to potentially deadly temperatures.
That is: the number of days in which the apparent temperature – a notional balance of thermometer-measured heat and maximum humidity – could reach or surpass 40.6°C will increase dramatically, and the days when individuals could be at risk could in some scenarios multiply 50-fold.
The scientists selected this “apparent” temperature of 40.6°C because it is significantly beyond the natural temperature of the human body, which must then be kept cool by perspiration. This is possible in arid climates.
But as humidity goes up – and with each 1°C rise in temperature, the capacity of the air to hold moisture rises by 7% – cooling by perspiration becomes less efficient.
So at this notionally-defined apparent temperature, people who cannot retreat to air-conditioned or cooler, shadier places could die. Heat kills: researchers recently counted 27 ways in which extreme temperatures could claim lives.
“If we follow the Paris Agreement, we’ll halve the number of people at risk in 2090, which is encouraging”
And more, and more intense and prolonged heat waves are on the way, and with them episodes of potentially extreme humidity. By 2100, according to some studies, certain regions of the planet could become dangerous habitat.
European scientists report in the journal Earth’s Future that they considered the hazard for just one, rapidly-growing continent: Africa. They selected 173 cities of more than 300,000 people in 43 nations across a range of climates, from Algiers on the Mediterranean to the burgeoning monsoon cities of the equatorial west coast, such as Lagos and Kinshasa, the drier east African states, and the relatively mild townships of Southern Africa.
They then considered how much cities might grow, by migration or birth-rate increases, and how they might develop. Then they factored in a range of climate scenarios and looked at possible forecasts for the years 2030, 2060 and 2090.
They found that because of population growth, the numbers of days on which people could be at risk – measured in person-days (one person working for one full day) – would in any case increase.
“In the best case, 20 billion person-days will be affected by 2030, compared with 4.2bn in 2010 – a jump, in other words, of 376%” said Guillaume Rohat, of the University of Geneva, who led the study. “This figure climbs to 45bn in 2060 (up 971%) and reaches 86bn in 2090 (up 1947%).
And that is the best-case scenario. When the researchers factored in the steepest population increases, the most rapid growth of the cities and the worst disturbances in climate, the figures rose more sharply. By 2030, 26 billion, a fivefold increase, could be at risk, 95bn in 2060 and 217 bn in 2090. This is an increase of 4967%, or nearly 50-fold.
The researchers assumed that not everybody in their 173 cities would be exposed to dangerous levels of heat. Were that to happen, the number of person-days could hit 647 billion. But the researchers made a conservative estimate of one in three people who would be exposed to a minimum temperature of 40.6°C.
Research of this kind makes assumptions about how the climate is going to change, and separately about how nations are going to develop, how populations are going to grow and change, and how governments are going to respond to the climate emergency, and the authors recognise the problems.
The sample is biased towards the larger cities. Their calculations don’t include predictions for capital investment. But the researchers say their conclusions are if anything conservative. They do not, for instance, factor in the notorious urban heat island effect that tends to make cities 3°C or more hotter than the surrounding countryside, and therefore even more dangerous.
The good news to emerge from the study is that concerted action, by governments and civic authorities, can reduce the risk. Were nations to stick to an agreement made by 195 of them in Paris in 2015, and keep global temperature rise to “well below” 2°C, the final exposure hazard would be reduced by 48%.
“This proves that if we follow the Paris Agreement, we’ll halve the number of people at risk in 2090, which is encouraging,” said Rohat.
“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 on.”