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Under a dual onslaught of global warming and localized, urban heating, some of the world’s cities may be as much as eight degrees Celsius warmer by 2100, Celsius warmer by 2100, researchers have warmed. Such a temperature spike can have dire consequences for the health of city-dwellers, robbing companies and industries of able workers, and putting pressure on already strained natural resources such as water. The projection is based on the worst-case-scenario assumption that emissions of planet-warming greenhouse gases continue to rise throughout the 21st century. For the latest study, researchers used data from the world’s 1,692 largest cities for the period 1950 to 2015. The top quarter of most populated cities, in this scenario, could see the mercury rise 7 degrees Celsius or more by century’s end, said a study in the journal Nature Climate Change. For some, nearly 5 degrees Celsius of the total would be attributed to average global warming. The rest would be due to the so-called urban heat island (UHI) effect, which occurs when cooling parks, dams and lakes are replaced by heat-conducting concrete and asphalt — making cities warmer than their surrounds, the researcher said. “The top 5% (of cities per population) could see increases in temperatures of about 8 degree Celsius and larger,” said study co-author Francisco Estrada of the Institute for Environmental Studies in the Netherlands. Mr. Estrada and a team used different projections of average planetary warming, combined with the UHI effect and potential harms, to estimate the future costs of warming on cities. The median city, right in the middle of the range, stands to lose between 1.4% and 1.7% of GDP per year by 2050 and between 2.3% and 5.6% by 2100, they concluded. “For the worst-off city, losses could reach up to 10.9% of GDP by 2100,” wrote the team. UHI “significantly” increases city temperatures and economic losses from global warming, they added. This meant that local actions to reduce UHI — such as planting more trees or cooling roofs and pavements, can make a big difference in limiting warming and minimizing costs. Cities cover only about 1% of the earth’s surface but produce about 80% of gross world product and account for around 78% of energy consumed worldwide, said the researchers. They produce more than 60% of global carbon dioxide emissions from burning coal, oil and gas for fuel.