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The Science Behind The Projects: the Urban Heat Island phenomenon

An article explaining the science behind the Environment 2.0 participatory mass observation experiments at Futuresonic 2009 by Carlo Buontempo and Mark McCarthy at the Met Office, and John Tweddle at Natural History Museum.

With most of the human population living and working in large towns and cities, it is becoming more and more important to study how the urban environment interacts with the surrounding countryside. ‘Mega’ cities like Beijing and London are usually taken as an example, but a phenomenon called the urban heat island effect means that even medium-sized cities like Manchester modify the way that local climate and biodiversity behaves.

From a physical point of view, cities are a peculiar and unnatural environment, mainly because of the materials that we use to build our houses and roads. Paved surfaces not only warm up more than bare soil, but also reduce evaporation and the amount of water that the ground soaks up. This directly affects the weather condition at ground level and can also result in a higher risk of flooding. As well as these direct effects, there are at least two other ways in which buildings alter local climate. Firstly, the design of streets has a variable effect on how efficient buildings are at radiating heat and secondly, we produce a lot of heat within buildings, through processes such as air conditioning, central heating and manufacturing processes. This serves to further increase temperature at ground level.

The above processes mean that built-up areas like cities are usually warmer than the surrounding countryside. This simple fact, which many cyclists must have noticed while riding, has wide reaching consequences. For example, the unnaturally warm centres of cities alter the pattern of wind and consequently the distribution of pollen and pollutants, both of which can impact the health of the people who live and work there. Plants and animals also respond to this warmth by altering the way they behave (a process called phenology). There is evidence that some urban plants flower earlier in the year than those that live in nearby countryside.

Because cities are warmer than the surrounding rural areas, they have the potential to act as a ‘test-bench’ of how climate change through global warming may affect the planet. Current research indicates that we can expect to see an increase in average global surface temperature of the order of a few degrees over the next hundred years or so. This value is very close to the difference in temperature that the urban heat island effect causes between city-centres and the surrounding countryside. Although the two processes (climate change and urban heat island) are not connected, the fact that they have similar temperature magnitudes can help us to evaluate some of the impacts that climate change will have on the natural environment, and of course how we live our lives.

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