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

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 HandleySchool of Environment and Development

University of Manchester

Roland Ennos

Faculty of Life Sciences Susannah Gill

John HandleySchool of Environment and Development

University of Manchester

BackgroundBackground

• 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.

• 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.

UKCIP02 GM Mean Summer Temperature

Produced with data from the UK Met Office and UKCIP

UKCIP02 GM Average Maximum Summer Temperature

Produced with data from the UK Met Office and UKCIP

UKCIP02 GM Summer Precipitation

Produced with data from the UK Met Office and UKCIP

UKCIP02 GM Winter Precipitation

Produced with data from the UK Met Office and UKCIP

Effects of UrbanisationEffects 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.

• 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.

Aims of the ASCCUE ProjectAims 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

• 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

UMT Mapping of ManchesterUMT 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)

• 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)

UMT Map of Greater Manchester UMT Map of Greater Manchester

Measuring Surface CoverMeasuring 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

• 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

UMT Surface CoverUMT Surface Cover

Surface Cover in Residential UMTsSurface Cover in Residential UMTs

Evapotranspiring Surfaces Evapotranspiring Surfaces

Quantifying the Cooling Potential of Greenspace

Quantifying the Cooling Potential of Greenspace

• We investigated the Surface Temperature on a hot summer’s day using the model of Tso (1990, 1991)

• We investigated the Surface Temperature on a hot summer’s day using the model of Tso (1990, 1991)

Quantifying the Cooling Potential of Greenspace

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

• 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

Maximum Surface TemperatureMaximum Surface Temperature

The Effect of Altering Greenspace in Town CentresThe Effect of Altering Greenspace in Town Centres

0

5

10

15

20

25

30

35

40

45

1970s 2020sLow

2020sHigh

2050sLow

2050sHigh

2080sLow

2080sHigh

Time period and emissions scenario

Ma

x s

urf

ac

e t

em

p (

de

g C

)

town centre town centre -10% green town centre +10% green

Quantifying the Potential of Greenspace to Reduce Runoff

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)

• We investigated the Runoff after a once a year precipitation event using the model of the Soil Conservation Service (1972)

Quantifying the Potential of Greenspace to Reduce Runoff

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

• 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

Surface Runoff PatternSurface Runoff Pattern

Change in Total Runoff Change in Total Runoff

0

5,000,000

10,000,000

15,000,000

20,000,000

25,000,000

30,000,000

35,000,000

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34

Precipitation (mm)

Ru

no

ff (

m3)

Total GM

'Urbanised' GM

• Total runoff will increase by 82%

• Excluding farmland, runoff will increase by 78%

Potential for Greenspace to Reduce Runoff

Potential for Greenspace to Reduce Runoff

High density residential

0

200,000

400,000

600,000

800,000

1,000,000

1,200,000

1,400,000

1,600,000

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34

precipitation (mm)

run

off

(m

3)

Low density residential

0

100,000

200,000

300,000

400,000

500,000

600,000

700,000

800,000

900,000

1,000,000

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34

precipitation (mm)ru

no

ff (

m3)

0

100,000

200,000

300,000

400,000

500,000

600,000

700,000

800,000

900,000

1,000,000

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34

current dev

-10% green

+10% green

-10% trees

+10% trees

Conclusions on the Environmental Effectiveness of Greenspace

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?

Quantifying the Likelihood of Drought

Quantifying the Likelihood of Drought

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

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

Quantifying the Likelihood of Drought

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

• 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

Drought (months when water will limit evapotransipration)

Drought (months when water will limit evapotransipration)

Conclusions of the Drought Model

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

• Grasslands will suffer from progressively longer periods of drought

• This will compromise the ability of grasslands to cool urban areas

Implications of the Work for Urban Planning

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

• 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

Potential Further ResearchPotential 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

• 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