Review and Critical Analysis of International UHI Studies

heating and cooling systems if present, building insulation levels (addressed by some
studies), and the human tolerance and comfort levels;
� Complete Costing: The studies that produced financial estimates for the cost of
implementation did not always account for the complete lifecycle costs e.g. design,
maintenance, thermal cyclical effects upon lifespan, changes in water quality, night time
illumination, project size, design life and intended end‐use. These effect are complex but
could counter or enhance the financial savings attributed to the direct/indirect effect of
any particular countermeasure but they require further investigation and integration into
any potential estimates;
� Performance Effects on Cool Roofs/Pavements: Although most studies acknowledged
that soot, dirt, microbial growth and end‐use affect the performance of cool roofs and
pavements there was no attempt to incorporate such performance diminishing
characteristics in the direct/indirect energy savings that resulted – these factors therefore
require further investigation as they can give rise to errors in the potential energy and
CO2 savings attributed to such direct/indirect effect countermeasures;
� Isolation of Indirect Effects: A select number of studies did examine the benefits
attributed to the indirect effect of countermeasures such as cool pavements. This was
examined using a modified ambient temperature vs. base case scenario model to develop
the estimate. Further investigation in a range of different climate types accounting for
vast array of different cool pavement types is required. Such research should take the
form of theoretical and empirical studies that seek to quantify the cooling effect in
isolation to other indirect countermeasures (e.g. vegetation) – this will enable not only
more accurate savings potential estimates but also the evaluation of the performance
range for the different cool pavement solutions.
The benefits of an increased surface albedo and the use of green roofs/urban areas to directly and
indirectly reduce urban temperatures (thus the building/city energy use, peak power and thus
CO2 emissions) were proven to be significant. There are however, as illustrated, several obstacles
that remain for the present methodologies and data sets to address before they can be used to
accurately evaluate the savings at both the micro and macro scales.
5.2.4 Conclusion
If we were to standardize the key findings from the literature to evaluate the potential of the
direct and indirect effect vs. the negative radiative effect at a global scale in terms of Gt of CO2
per year, then we obtain the following offset estimates:
� 44 GtCO2 emissions per year from the negative radiative forcing effect if the albedo of all
the world’s existing urban roofs and pavements was increased by 0.1;
� 30 MtCO2 emissions per year from the direct energy reduction effect of cool roofs,
assuming that the US scales to the global level in proportion to its share of energy
consumption;
� 12 MtCO2 emissions per year from the direct energy reduction effect of shade trees, this
again assumes that the US scales to global in proportion to its share of energy
consumption;
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