Climate Change Raises Danger for Urban Heat Islands

Scientists warn that densely-packed cities face even greater health hazards as greenhouse gases continue to push up temperatures.

However hot the weather gets, it will be worse in the cities – and scientists in the US have just worked out how much worse by measuring the notorious urban heat island effect.

They planted 150 sensors in and around Madison, the Wisconsin state capital, in time for the heatwaves that hit the US in 2012, and found that the city experienced twice as many hours at temperatures above 90° Fahrenheit (32°C) than the surrounding rural areas.

Sealed roads and pavements, bricks, tiles, concrete and slate all absorb heat.  But densely-packed cities also generate their own heat − from traffic exhausts, lighting, central heating, and air conditioning.

Greater investment in air conditioning will make things worse, as all the heat inside buildings will be dumped into the streets, adding to the stress.

Cooling mechanism
Furthermore, water – which, when it evaporates, cools the grass and the trees – runs off the urban pavements into subterranean drains, so cities surrender another natural cooling mechanism.

The consequence is that any urban area is likely to be several degrees warmer than the surrounding countryside.  Half of the world’s billions now live in cities; by 2050, it will be two-thirds.

Extremes of heat are likely to become more pronounced and more frequent as global average temperatures climb in step with greenhouse gas discharges from the combustion of fossil fuels, so the crowded cities could become a serious health hazard, especially in the tropics.

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