Longer and more frequent heatwaves, an exploding demand for air conditioning and even more threatening viruses: the world’s major cities and their billions of inhabitants would be in crisis if global warming continued on its path towards 3°C, warns a report published this Thursday, September 19.
“The difference between 1.5°C and 3°C is a matter of life and death for billions of people around the world,” says Rogier van den Berg of the American think tank WRI, which publishes this study showing a possible multiplication of difficulties for the inhabitants of Dakar, Rio or Padang (in Indonesia). The study – initially planned for the spring but delayed for a revision of the data – looks at the potential climate risks in nearly 1,000 large cities which today are home to 2.1 billion people, or 26% of the world’s population. All cities combined, this is even more than half of humanity that lives in urban centers.
The authors show the major difference for city dwellers between a global warming of 1.5°C compared to the pre-industrial era – the most ambitious limit of the Paris Agreement – and almost 3°C, towards which the world is currently heading. The climate commitments made so far by States in fact place the planet on a trajectory of warming of up to 2.9°C during this century, according to the latest UN data, published last November.
Heat waves and diseases
“At 3°C of warming, many cities could face month-long heatwaves, skyrocketing energy demand to power air conditioning, and transforming risks from insect-borne diseases—sometimes all at once,” the authors conclude. “People living in low-income cities will be hit hardest,” they note.
The stakes are high because of the strong growth of the urban population, especially in developing countries: by 2050, 2.5 billion additional inhabitants will have joined the cities, which will then be home to 2/3 of humanity. The IPCC, the climate experts mandated by the UN, will devote their next special report to this subject. Cities have “very specific climate problems” and “most cities are not yet built, so there is real potential for transformation at the root”, observes Robert Vautard, co-chair of an IPCC working group. The WRI calculates that the longest annual heatwave that the population of large cities would experience would be 16.3 days on average at + 1.5°C, a figure that would rise to 24.5 days at + 3°C. Their frequency is also likely to increase, from 4.9 annual heat waves to 6.4.
These extreme heats will in turn encourage demand for air conditioning and therefore energy. For example, in Johannesburg, the demand for air conditioning at +3°C would be 69% higher than at +1.5°C, even though the city already sometimes experiences water shortages and power cuts when temperatures are high.
The heat will also result in an increase in arboviruses, which are transmitted in particular by mosquitoes: dengue, chikungunya or zika. The peak activity of these arboviruses in eleven of the largest Brazilian cities could thus last at least six months in the year. In Rio de Janeiro, it would increase by 71%, going from 69 to 118 days of maximum activity per year. On the other hand, the days of peak exposure to malaria would decrease in the world – despite a possible increase in temperate regions, in Europe or in North America.
Overall, the poorest cities – which have the least means to adapt – are the most exposed. In a world at +3°C, sub-Saharan Africa would be the hardest hit by the increase in the frequency of heatwaves and days of peak arbovirus activity. Thus Freetown or Dakar “could experience heatwaves lasting more than a month”, with an average of seven episodes per year. “It is time to prepare cities for a much hotter world while doing everything possible to drastically reduce emissions” of greenhouse gases, concludes Rogier van den Berg.