Trees and the Public Realm - Westminster City Council

Trees and the Public Realm (Draft) Page 43
110. Generally speaking, urban areas will warm more than rural because buildings and hard
surfaces absorb heat from the sun and reradiate the stored heat energy back into the
atmosphere. The result of this ‘heat island effect’ can be higher temperatures,
particularly at night, and in enclosed areas (including under tree canopies) in
comparison to suburban and rural areas. The current maximum surface temperature of
woodlands is 18.4 o C, compared to 31.2 o C in town centres 12 .
Summary of expected climate change in the UK 13
Temperature • Annual warming by the end of the century of between 1 o C and
5 o C
• Greater warming in the Southeast than in the Northwest
• Increase in the number of very hot days
• Decrease in the number of very cold days
Precipitation • Generally wetter winters (by up to 30%) increase in winter
precipitation intensity
• Substantially drier summers
Soil Moisture • Decreases in summer and autumn, especially in the Southeast
• Increases in winter
111. Periods of unusually high temperatures (heat waves) are predicted to become more
frequent and last longer, with town centres predicted to experience a bigger
temperature rise than rural. Green space, in the form of street trees, parks, and private
gardens has a significant role to play in reducing the urban heat island effect. A 10%
increase in trees, in the right places, can reduce surface temperatures by 3-4 o C.
112. Although it is not straightforward within Westminster to plan green space at the strategic
level (as would be the case with greenfield development) there is much that can be
done at the local neighbourhood level to moderate the effects of climate change.
Shade provided by trees can keep hard surfaces cooler than unshaded areas by
several degrees, and trees, especially when located close to buildings, provide not only
shade but act as natural cooling systems through the process of evapotranspiration.
113. While it is recognised that not every street is suitable for trees both in terms of
practicality and urban design, maximising the opportunities to plant larger growing
shade trees will assist in the moderation of high summer temperatures and in so doing
make Westminster a more liveable city.
114. It is anticipated that trees will also need to be more adapted to a hotter, drier climate
and consideration should perhaps be given to tree species that are indigenous in
climates similar to those predicted for the south of England around 2040-2060. The
adaptability of our existing tree stock (eg. oak) will also be increasingly relevant.
12 Adaption Strategies for Climate Change in the Urban Environment
13 CLG, 2007, Climate Change and Urban Spaces .