10 may 2009

Cities to sizzle as islands of heat

Jonathan Leake, Environment Editor, The Sunday Times, May 10, 2009

LONDON and other cities could see summer temperatures rise to more than 10C above those in the surrounding countryside, according to Met Office research being used to help devise the first official climate change map of Britain.

Scientists have been studying a phenomenon known as the urban heat island effect, in which cities become significantly hotter than the areas around them because of the heat they generate themselves.

Big cities such as London, Birmingham, Manchester and Glasgow already reach temperatures 2C - 3C above their surroundings in the summer. Scientists fear that difference could grow four to fivefold as hotter weather combines with soaring energy use and population growth, making such temperature gaps more frequent and more extreme.

The research is linked to a wider project aimed at helping scientists predict the impact rising temperatures will have on different parts of the country. The full results will be released next month by Hilary Benn, the environment secretary.

Vicky Pope, the head of climate change advice at the Met Office, said: “As the climate gets warmer, sweltering summer temperatures will combine with rising energy use, the heat-retaining properties of buildings, and the sheer volume of people, to push temperatures higher and higher.

“It may sometimes make life in the metropolis intolerable. Imagine the scorching conditions that commuters will face on London’s Tube network.”

The warning follows the disclosure by the Intergovernmental Panel on Climate Change that global temperatures have risen by almost 1C since preindustrial times.

The panel predicts global temperatures will have risen by 2C by 2050 with total warming of up to 5-6C possible by 2100.

Such findings are now widely accepted but questions remain, especially regarding the impact on cities, where more than half the world’s population live. New York – hotter in summer than British cities - is regularly 7C-8C hotter than nearby rural areas.

In Britain, 90% of the population lives in urban or suburban areas so the impact on people is potentially huge.

The research is based partly on data from heatwaves, such as the one in 2003, and on computer projections. It also looked at cities such as Athens and Beirut which suffer from the urban heat phenomenon.

The August 2003 heatwave saw England’s daytime temperatures top 30C for 10 days and exceed 35C in many places.

The same heatwave saw temperatures in the upper 30Cs in the centres of cities such as London, Birmingham and Manchester. This was often 6C-7C above those in rural areas.

Researchers fear central city temperatures may exceed 40C as the century progresses.

“The high temperatures of 2003 were extraordinary but may become common by 2050 and even be seen as relatively cool by 2100,” said Pope.

One of the factors that made London so hot was its inability to cool down. At night during the heatwave, the city centre was sometimes 9C warmer than its surrounding green belt.

This is because rural and suburban areas lose heat at night but in cities the materials used for hard surfaces store more solar energy and lose it more slowly. This effect is amplified by the heat from lights, electrical equipment and cars. Also, as cities get warmer, they consume more power trying to stay cool, because of air-condition-ers and fridges working harder.

Richard Betts, head of climate impacts at the Met Office, who oversaw the research, said Tokyo showed what British cities might face. Its tall, densely packed buildings and high energy use mean the Japanese capital is often 10C hotter than the surrounding countryside.

“We must change how we plan cities, to maximise green spaces and create structures that dissipate heat,” said Betts.

Urban heat islands have a serious impact on health. In 2003 there were 2,091 more deaths than normal between August 4 and 13 in Britain, most of them among elderly people in southeast England. For people aged over 75 there was a 33% increase in mortality.

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