Trees and human security: the research basis for the claim that trees can climate proof our cities

1. Trees and human security: the research basis for the claim that trees can climate proof our cities Roland Ennos Faculty of Life Sciences Susannah Gill John Handley School of Environment and Development University of Manchester

2. Background • Ecologists make many claims for the environmental benefits of trees. • In particular it is claimed they provide cooling and soak up rainfall, reducing storm runoff. • These effects should become more important as climate change will bring hotter, drier summers and wetter winters. • These effects need to be quantified to influence policy-makers.

3. UKCIP02 GM Mean Summer Temperature Produced with data from the UK Met Office and UKCIP

4. UKCIP02 GM Average Maximum Summer Temperature Produced with data from the UK Met Office and UKCIP

5. UKCIP02 GM Summer Precipitation Produced with data from the UK Met Office and UKCIP

6. UKCIP02 GM Winter Precipitation Produced with data from the UK Met Office and UKCIP

7. Effects of Urbanisation • Loss of greenspace results in reduced evaporational cooling and increased heat storage. This causes raised summer temperatures. • Loss of greenspace results in reduced rainfall interception and infiltration. This causes increased runoff. • Therefore increasing urban greenspace has the potential to cool cities and reduce runoff, adapting them to climate change. • However, vegetation may also be vulnerable to climate change.

8. Aims of the ASCCUE Project • To investigate the pattern of greenspace in a British city (Greater Manchester) • To model the effect of greenspace on its current environmental performance • To model the effectiveness of increasing greenspace at climate-proofing the city • To assess the vulnerability of the urban green space to climate change

9. UMT Mapping of Manchester • Using aerial photographs Greater Manchester was split into 29 different Urban Morphology Types (UMT’s) • The UMT’s were mapped onto the ArcView Geographical Information System (GIS)

10. UMT Map of Greater Manchester

11. Measuring Surface Cover • The next stage was to work out the surface cover in each UMT • For each UMT, 400 points were randomly chosen • The surface cover at each point was identified from aerial photographs (Cities Revealed) and classified as one of 9 cover types

12. UMT Surface Cover

13. Surface Cover in Residential UMTs

14. Evapotranspiring Surfaces

15. Quantifying the Cooling Potential of Greenspace • We investigated the Surface Temperature on a hot summer’s day using the model of Tso (1990, 1991)

16. Quantifying the Cooling Potential of Greenspace • Greenspace has high evaporative cooling • Bare ground has no evaporative cooling • Buildings and roads store heat • We calculated the mean maximum and minimum surface temperature for each UMT

17. Maximum Surface Temperature

18. The Effect of Altering Greenspace in Town Centres

19. Quantifying the Potential of Greenspace to Reduce Runoff • We investigated the Runoff after a once a year precipitation event using the model of the Soil Conservation Service (1972)

20. Quantifying the Potential of Greenspace to Reduce Runoff • Greenspace intercepts water before it reaches the ground and stores it in the soil • Buildings and roads direct rainfall directly to drains • Runoff also depends on soil permeability

21. Surface Runoff Pattern

22. Change in Total Runoff • Total runoff will increase by 82% • Excluding farmland, runoff will increase by 78%

23. Potential for Greenspace to Reduce Runoff

24. Conclusions on the Environmental Effectiveness of Greenspace • Greenspace can be effective at reversing the predicted increases in temperature • Greenspace will be ineffective at reversing the predicted increase in runoff • But what effect will climate change have on greenspace?

25. Quantifying the Likelihood of Drought • We investigated the water availability in the rooting zone of grasses using the Bucket soil water model (Rowell, 1994)

26. Quantifying the Likelihood of Drought • We assumed soil was saturated at the start of April • Water input was added according to the UKCIP02 monthly precipitation predictions • Water loss was equal to the Potential Evapotranspiration until the water deficit was 200 kPa and fell linearly until 1500 kPa • We found the number of months when water deficit fell below 200kPa

27. Drought (months when water will limit evapotransipration)

28. Conclusions of the Drought Model • Grasslands will suffer from progressively longer periods of drought • This will compromise the ability of grasslands to cool urban areas

29. Implications of the Work for Urban Planning • Greenspace has great potential for moderating temperatures in cities • Greenspace is less effective in preventing increased surface runoff • Grasslands will become more prone to drought • A potential solution to these problems is to increase storm water storage and use it for irrigation or to plant trees rather than grass

30. Potential Further Research • To validate the temperature, runoff and drought models using experimental plots set up in the ITree Project. • To investigate the efffectiveness of trees to cool cities by direct shading as well as by evaporative cooling