(PARIS) – Longer and more frequent heat waves, increased demand for air conditioning, even more dangerous viruses: the world’s megacities and their billions of inhabitants will face a crisis if global warming continues on its path toward 3 degrees Celsius, a report warned Thursday.
“The difference between 1.5°C and 3°C is a matter of life and death for billions of people around the world,” says Roger van den Berg, of the American think tank WRI, which published this study that shows a potential increase in the hardships faced by people around the world, including residents of Dakar, Rio or Padang (in Indonesia).
The study — which was originally scheduled for spring but was postponed to review the data — looks at potential climate risks in nearly 1,000 major cities that now house 2.1 billion people, or 26% of the world’s population. Combined, more than half of humanity lives in urban centers.
The authors show the key difference for city dwellers between global warming of 1.5°C compared to pre-industrial times—the most ambitious limit in the Paris Agreement—and closer to 3°C, our current world.
Climate commitments made by countries so far put the planet on track for warming of up to 2.9 degrees Celsius this century, according to the latest UN data, published last November.
Heat waves and diseases
“At a 3°C rise in temperature, many cities could experience month-long heat waves, increasing energy demand for air conditioning, and shifting risks from insect-borne diseases – sometimes all at once,” the researchers concluded.
“People living in low-income cities will be the hardest hit,” they stress.
The stakes are high because of the strong growth in urban populations, especially in developing countries: by 2050, an additional 2.5 billion people will join cities, which will house two-thirds of humanity.
The Intergovernmental Panel on Climate Change, the UN-mandated climate experts, will also devote its next special report to the topic. Robert Wootard, co-chair of the IPCC’s working group, notes that cities have “very specific climate problems” and “most cities haven’t been built yet, so there’s real potential for radical transformation.”
The World Resources Institute estimates that the longest annual heatwave experienced by urban residents will be 16.3 days on average at +1.5°C, a figure that could rise to 24.5 days at +3°C. Their frequency is also likely to increase from 4.9 heatwaves per year to 6.4.
power failure
These extreme heat events will in turn encourage demand for air conditioning and therefore energy.
For example, in Johannesburg, air conditioning demand will be 69% higher at +3°C than at +1.5°C, even though the city already occasionally experiences water shortages and power outages when temperatures are high.
The heat will also lead to an increase in arboviruses, transmitted in particular by mosquitoes: dengue, chikungunya or Zika.
Thus, the peak activity of these arboviruses in eleven of Brazil's largest cities could last at least six months a year. In Rio de Janeiro, the number would increase by 71%, from 69 to 118 days of maximum activity per year.
On the other hand, peak malaria exposure days will decline worldwide – although they will likely increase in temperate regions, in Europe or North America.
In general, the poorest cities – those with the least means to adapt – are most at risk. In a +3°C world, sub-Saharan Africa will be the hardest hit by increased frequency of heatwaves and peak arbovirus activity days.
Thus, Freetown or Dakar “may experience heatwaves lasting more than a month”, with an average of seven waves per year.
“It is time for cities to prepare for a warmer world by doing everything they can to significantly reduce greenhouse gas emissions,” concludes Rogier van den Berg.
