Jane Powerson cooling our cities.
By next year, for the first time, more than half of the world's population will be living in cities, according to the United Nations Population Fund. Urban dwellers will number 3.3 billion, with the most significant increases in Africa and Asia. European cities will also grow, but not at the same rate. By 2030 the world's urban population is expected to expand to nearly five billion people, with 80 per cent living in Africa and Asia.
Cities have a profound effect on the climate and the environment. As villages develop into towns and then cities, their average temperature increases by between two and four degrees above that of the surrounding countryside.
This "urban heat island effect" is a result of buildings and paving storing and releasing heat, and also of a reduction in vegetation. Green spaces and trees help to lower the temperature when they take in water and then gradually release it to cool the air. The processes of evaporation (from soil, water and leaf surfaces) and of transpiration (where a plant exhales or perspires water from its leaves) are rather neatly combined into one word: evapotranspiration.
Lack of green space, or "surface sealing", also prevents rainwater from entering the soil, causing rapid run-off and leading to the danger of overflowing sewers and waterways. In city centres and high-density areas about 70 per cent of the ground is covered by buildings, roads or other impermeable surfaces. As climate change brings more extreme weather, and more instances of torrential rain, flooding in urban areas becomes a greater threat.
City heat begets more heat, as air conditioners - increasingly necessary to make buildings bearable - spew warm air into the immediate atmosphere. They also consume energy and produce carbon dioxide and other emissions, exacerbating global warming and contributing to air pollution.
Clearly, the galloping urbanisation of the planet could land us in the soup if something isn't done. But let's not panic. Instead let's celebrate (and pay heed to) a team at Manchester University that have come up with a simple solution. Following a research project, using Greater Manchester as their model, they have calculated that a 10 per cent increase in green space in built-up areas will reduce surface temperatures by up to four degrees. This figure, incidentally, is equivalent to the average predicted temperature rise through global warming by 2080. Hallelujah, and glory be to the boffins in Manchester.
And, just to drive a point home, let me tell you that the team also calculated that reducing green space by 10 per cent could increase surface temperatures by 8.2 degrees by 2080. Yikes.
The future health of cities is in our hands. Get them 10 per cent greener. Get them evapotranspirating.