Trees in our towns and cities - the urban heat island
Planting trees in our towns and cities can help reduce the ‘urban heat island effect’. This is where buildings, concrete and other hard surfaces such as roads act as giant storage heaters, absorbing heat during the day and releasing it at night. The effects can be dramatic; on some days there is a difference of as much as 10°C between city centres and the surrounding areas (British cities ‘could be up to 10oC hotter than countryside by 2100’, Daily Telegraph online)
Projections for our changing climate suggest urban heat island effect will get markedly worse, a problem compounded by an increasing lack of green space. Trees and plants provide direct cooling from shade and reduce the ambient temperature through the cooling effect of evaporation and transpiration from the soil and leaves.
The impact on health of urban heat islands is two-fold; firstly higher temperatures increase ground level ozone, exacerbating the symptoms of chronic respiratory conditions. Secondly, exposure to prolonged high temperatures can result in cardiovascular or respiratory failure, or even dehydration. This is particularly dangerous for the elderly, very young or chronically ill (Bhattachary 2003). In the 2003 summer heat wave over 2,000 people died in Britain alone and more than 50,000 died across Europe.
Preliminary results from the itrees project, a collaboration between Red Rose Community Forest and Manchester University, suggest that;
• Concrete surfaces in tree shade can be up to 20°C cooler than concrete in sun.
• Tree shade can cool people by up to 7°C on hot days
Research using computer modelling has shown how increasing green spaces within Manchester can help mitigate urban heat island effect. Without any increase in green space, by 2050 the temperature in Manchester is projected to rise by 3°C. However if the amount of green space increases by just 10 per cent then the effects of climate change on increasing surface temperatures could potentially be mitigated. However, reducing tree cover by the same percentage could lead to an increase of 8.2°C under some scenarios (Handley and Carter, 2006).
Trees for shade, shelter and privacy
Planting a tree in your garden for shade can reduce the risks from UV radiation (Heisler and Grant, 2000). Children’s’ skin is more sensitive to UV damage and the amount of sun exposure during childhood is thought to increase the risk of developing skin cancer in adult life. Being out of the direct glare of the sun is also more important for comfort than the temperature.
Trees can form a living screen or windbreak providing privacy and shelter, though you must be mindful of potentially blocking a neighbour’s view or reducing daylight. With careful planting you can hide unsightly buildings or service poles, frame a view or create shapes for the sun to slip through.