Transcript presentation - Treework Environmental Practice

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
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.
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 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.
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
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)
UMT Map of Greater Manchester
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
building
other impervious
tree
shrub
mown grass
rough grass
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Proportional cover
UMT Surface Cover
Surface cover in UMTs (400 pts sampled from each)
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
UMT
water
bare soil / gravel
Low Density Residential
Surface Cover in Residential UMTs
Hig h D ensit y R esid ent ial
Lo w D ensit y R esid ent ial
M ed ium D ensit y R esid ent ial
building
other impervious
tree
shrub
mow n grass
rough grass
cultivated
w ater
bare soil / gravel
Evapotranspiring Surfaces
Quantifying the Cooling Potential of
Greenspace
• We investigated the Surface
Temperature on a hot summer’s day
using the model of Tso (1990, 1991)
Quantifying the Cooling Potential of
Greenspace
•
•
•
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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 Temperature
Max surface temp (deg C)
The Effect of Altering Greenspace
in Town Centres
45
40
35
30
25
20
15
10
5
0
1970s
2020s
Low
2020s
High
2050s
Low
2050s
High
2080s
Low
2080s
High
Time period and emissions scenario
town centre
town centre -10% green
town centre +10% green
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)
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
Surface Runoff Pattern
Change in Total Runoff
35,000,000
30,000,000
Runoff (m3)
25,000,000
20,000,000
Total GM
'Urbanised' GM
15,000,000
10,000,000
5,000,000
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
Precipitation (mm)
•
Total runoff will increase by 82%
•
Excluding farmland, runoff will increase by
78%
Potential for Greenspace to
Reduce Runoff
Low density residential
High density residential
1,000,000
1,600,000
900,000
1,400,000
800,000
700,000
current dev
runoff (m3)
runoff (m3)
1,200,000
1,000,000
800,000
600,000
400,000
200,000
600,000
-10% green
500,000
+10% green
400,000
-10% trees
300,000
+10% trees
200,000
Low density residential
100,000
0
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
precipitation (m m )
precipitation (m m )
current dev
-10% green
+10% green
-10% trees
+10% trees
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
• We investigated the water availability in
the rooting zone of grasses using the
Bucket soil water model (Rowell, 1994)
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
Drought (months when water will
limit evapotransipration)
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
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
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