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Much of the world is sweltering under near record-breaking temperatures. Future heatwaves are set to be more frequent, intense and prolonged. The impact will be particularly felt in heat-trapping cities. At what temperature might they become unlivable?
Humans can only survive for at most six hours at a wet-bulb temperature — a measure that combines air temperature and humidity — of 35°C, according to Nasa researchers. High temperatures combined with high humidity are particularly dangerous as it is harder to cool down by sweating.
Nasa’s climate models suggest that South Asia, the Gulf, and the Red Sea could hit the critical wet-bulb temperature by around 2050. Some Midwestern states such as Arkansas, Missouri, and Iowa might hit that limit within 50 years.
Even less intense heatwaves can be fatal if they strike a region that is unprepared. During the deadly European summer heatwave of 2003 that killed between 35,000 and 70,000 people the wet bulb temperatures were no greater than 28°C. Poor countries are also vulnerable, despite long experience of coping with heat. Excess mortality increased by 43 per cent during a 2010 heatwave in Gujarat.
Heatwaves cost the global economy an estimated $16tn in the 21 years to 2013, according to researchers at Dartmouth College. The damage, as a share of per capital GDP, was more than four times greater in low income regions than rich ones.
Global investors may need to discount property values in hot southern cities and raise them in cooler northern conurbations. Unbearable summer temperatures will contribute to northward migration both within countries and across national boundaries.
City leaders can prepare by studying places accustomed to hotter temperatures. Madrid’s climate in 2050 is likely to resemble Marrakech’s climate today, according to Zurich-based scientists. Similarly Stockholm might ape Budapest; London could take after Barcelona.
Cities can adapt. Qatar, one of the world’s hottest countries, is a eye-catching example. It has banned outdoor work between 10am and 3.30pm in summer, campaigners say that does not go far enough. Extensive — and environmentally damaging — air conditioning — extends to a 1.1km air-conditioned jogging track in a city park.
Heat-proofing poorer cities will be harder. As temperatures rise, there is a real risk that some become uninhabitable.