Review and Critical Analysis of International UHI Studies

5.2.2 Negative Radiative Forcing Studies
There was only one study that provided a review of the global potential of negative radiative
forcing. This study originates from the Lawrence Berkley National Laboratory Urban Heat Island
Group ‐ “Global cooling: increasing world‐wide urban albedos to offset CO2” [35]. It is the first
study of its kind that attempts to examine the mass scale up effects of both roof and pavement
albedos in urban areas on a global basis in order to induce a negative radiative forcing and
equate it to a carbon offset value.
The study takes inputs from previous estimates based on other studies and available databases to
establish estimates for urban areas, the percentage of urban/roof/pavement coverage, potential
net albedo change of an urban space and thus the overall Earth’s albedo change. The study then
generates an estimate for the increase in the radiative forcing due to an increase in atmospheric
CO2 by one tonne. Then a simple comparative analysis enables the authors to evaluate the change
in radiative forcing due to the urban surface albedo modifications to the changes in the
atmospheric CO2. Then an estimate for the radiative forcing associated with CO2 is established
based on several different sources and thus methodologies which broadly yield similar results.
This study therefore examines the hypothetical example where the urban albedo of the world’s
pavements and roofs is increased through material modification. It is estimated that increasing
the albedo of pavements by 0.15 and roofs by 0.25, where they are estimated to constitute 35%
and 25% of the world’s urban areas respectively, would increase the total albedo of urban areas
by 0.1 ‐ this yields an Earth albedo increase of approximately 3×10 ‐4 . The global urban land
surface was conservatively estimated at 1% of the Earth’s surface area and with these
modifications was estimated to induce a negative radiative forcing of 4.4 ×10 ‐2 Wm ‐2 . The
emerging global CO2 offset approximation for cool roofs and pavements combined was found to
be 44Gt with 24Gt attributed to cool roofs and 20Gt to cool pavements. This 44Gt CO2 emission
offset value is over one year of the 2025 estimated world‐wide emission of 37Gt of CO2 per year –
a figure from the 2003 International energy outlook.
Such estimates are the first of their kind and an important indicator of the potential of UHI
mitigation measures; they do however, as the authors recognize in some elements of the
methodology and data suffer from some shortfalls:
� Global Urban Area Estimates: there are several methods that could be used to estimate
the magnitude of global urban areas and they all demonstrated slightly different
outcomes; however, this study ended up using an estimate of 1% which is thought likely
to produce a conservative estimate of the effect of increasing the albedo of urban areas in
terms of the global CO2‐equivalent emissions offset;
� Surface Modification Estimates: the fractions of urban areas subject to modification are
estimated based on aerial color photography, statistical data sampling methods and land‐
use/cover data from the US Geological Survey from limited major city types in the US
and then adjusted to account for metropolitan urban areas from across the world which
are often have lower vegetative cover than the US. These figures are therefore not
necessarily representative of the city makeup across the globe and are a gross
aggregation;
Review and Critical Analysis of International UHI Studies
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