Major cities will be hit hard if global warming reaches 3°C

"The difference between 1,5°C and 3°C represents a life-or-death issue for billions of people around the world," underlines Rogier van den Berg, from the American think tank WRI, which published this study showing a possible multiplication of difficulties for the inhabitants of Dakar, Rio or Padang (in Indonesia).

The study — originally scheduled for this spring but delayed due to data revisions — looks at potential climate risks in nearly 1.000 major cities that are now home to 2,1 billion people, or 26 percent of the world's population. In fact, more than half of humanity 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 are in fact placing 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 say.

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 house 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 heatwaves to 6,4.

power outages

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 a 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, peak days of exposure to malaria would decrease worldwide - despite a possible increase in temperate regions, in Europe or in North America.

Overall, the poorest cities - those with the least means to adapt - are the most exposed. In a +3°C world, 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 heat waves lasting more than a month", with an average of seven episodes per year.

"It is time to prepare cities for a much warmer world while doing everything possible to drastically reduce greenhouse gas emissions," concludes Rogier van den Berg.