Review and Critical Analysis of International UHI Studies

o The magnitude of the costs incurred from implementing such measures on an
global basis;
o The time it would take to instigate and implement such a mitigation program;
o That existing CO2 markets are currently not designed to support CO2 equivalent
offsets from cool material technologies;
o Furthermore, the study assumes that albedo of urban surfaces remains constant
i.e. at the installed value throughout the lifetime of the scenario – studies have
shown that this is not the case due to dirt/pollution, use, age, weathering and
microbial growth particularly in humid climates which cause the material
performance to deteriorate.
The authors seek to find the upper and lower boundaries for the potential emission offset from
radiative forcing in considering a selection of the above issues. They estimated that the global
potential of increasing the reflectivity of urban surfaces would yield between 30–100Gt of CO2
emission offset over one year of the 2025 projected world‐wide emissions of 37 Gt of CO2 per
year. Furthermore, this study recognized the potential gains from implementing high albedo
solutions in terms of direct/indirect effects on cooling/heating energy use of buildings and cities
but these benefits were not account for in these estimates. It is important to note that these
estimates although preliminary in nature indicate a colossal mitigation potential but evidently
there remain a number of issues that need to be addressed to establish more accurate global
estimates not least how to translate them into realizable financial gains.
5.2.3 Direct/Indirect Effect Studies
The use of urban trees, green roofs and high albedo surfaces were reported to provide both
practical and in most cases financially feasible solutions to reducing the temperatures both within
buildings and the ambient temperatures across the city. The vast majority of studies examined
the urban modifications with respect to the type of contribution the countermeasure made i.e.
direct or indirect contributions. The direct effect was classed as a countermeasure that would
alter the energy balance of the individual building i.e. the effect on energy consumption of an
individual building by installing a high albedo roof and/or shade trees. The indirect effects were
viewed as the reduction on temperature upon a greater area due to large scale modification
through a combination of modifying albedo of surfaces (roofs and pavements) and the use of
urban trees and vegetation. These factors were reduced to be dependent upon size and time
frame with direct effect providing instantaneous benefits to the individual building modified and
indirect effect benefits attained with mass scale implementation.
The majority of studies reviewed used a combination of experimentation, modeling and
simulations to evaluate the direct/indirect effects on energy, peak power savings and air quality
from implementation of UHI countermeasures predominantly on case specific examples at the
building and city scales – most were conducted in a limited set of climate types, namely:
equatorial, arid and warm climates. Some studies that evaluated the direct effect on the building
energy use found that increasing the roof albedo by 40% could yield up to 20% cooling energy
savings – in scaling up these effects to the rest of the United States this would yield savings
greater than $1 billion/year in the net annual energy bills. Another study indicated that the
energy savings were greater where countermeasures that provide both direct and indirect energy
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