Cool Cities Hold Promise to Reduce Climate Change
Research shows light-colored roofs and pavements are cost-effective ways
to save energy and reduce atmospheric pollution.
By MICHAEL TOTTEN
Michael Totten is chief
adviser on climate,
water and ecosystem
services at Conservation
him at m.totten@
roofs and cool
For a quarter century, working at the Lawrence
Berkeley National Laboratory, the inimitable Dr.
Art Rosenfeld (“Art of Efficiency”), and his research
partner Dr. Hashem Akbari (“Mr. Cool”), have been accu-
mulating evidence on the immense value cities can accrue
by increasing the albedo — that is, the reflectivity — of
road and rooftop surfaces. Incredibly, transforming a city
into a solar-reflecting mirror saves money, while achieving
significant CO mitigation at a negative cost. In the recent
issue of the journal Climatic Change, Akbari, Rosenfeld
and their colleague Surabi Menon estimate cumulative
worldwide economic savings could exceed $1 trillion while
effectively displacing the equivalent of 44 gigatons of CO
from the atmosphere.
Sound too good to be true? That’s what Los Angeles
officials thought back in the 1980s. Rosenfeld and Akbari
estimated that if the city could reduce its heat island effect
with higher albedo surfaces, it might save half a billion dollars a year in energy and pollution-abatement costs. This
would include a 12 percent reduction in lung-searing ozone
pollutants. Their research paid off: Cool roof requirements
were integrated into South Coast Air Quality Management
District plans, EPA State Implementation Plans, California
Title 24 Building Standards and several other state standards, as well as in ASHRAE building standards. Akbari
and Rosenfeld offer a refreshing dose of fiscal frugality
while leveraging global-scale benefits. The following Q&A
captures the pioneering spirit of innovative research that
moved their insights from the lab to the marketplace.
rates. And it can
save 44 billion
metric tons of
MT: Art, what prompted the idea of cooler cities?
AR: Back in 1985 my LBNL colleagues Hashem Akbari,
Haider Taha and I realized that hot, dark roofs and pave-
ments were half of the cause of summer urban heat islands,
which in turn increased the smog (ozone, O ) in Los Ange-
les and many other large cities. We already disliked hot
roofs because they raise air-conditioning demand by 20
percent, and we had long been trying to get building energy
codes to give credit for cool roofs. Throughout the world,
cities are summer heat islands. They are 3º to 10º F hotter
than their surroundings, and as cities grow, they typically
add 1º F each decade. A few percent of this heating is man-
made (from cars and air conditioners, for instance), but
overwhelmingly it comes from two roughly comparable
sources: air blows over dark-colored roofs and pavements
and warms by conduction, and trees, which cool the air by
evapotranspiration, are disappearing.
MT: As you further analyzed these scientific observations, what emerged in terms of economic opportunities
for changing this urban heat island effect?
AR: The saving of electricity and avoidance of smog costs
little. At the time of roof replacement, a new light-reflecting
roof costs little more than a dark one, but will last longer.
Pavements can be cooled two different ways: retain asphalt
as the binder, but use white aggregate that will show as the
dark asphalt wears down to the light aggregate color, or
“white top” with concrete, which is stronger and actually
cheaper in the long run. In Los Angeles, trees shading a
lawn actually save water because the trees, after a few years
of watering, survive on natural ground water, whereas the
cooler lawn requires less municipal water.
MT: That was nearly a quarter century ago. Hashem,
what is your current assessment given the extensive
research, testing and cumulative evidence?
HA: These two simple technologies, cool roofs and
cool pavements, which have been around for thousands of
years, should be the first geoengineering techniques used
to combat global warming. This is due to a combination
of the direct and indirect effects of light-colored surfaces.
Directly, light-colored roofs reflect solar radiation, reducing
air-conditioning use. For highly absorptive roofs, the difference between the surface and ambient air temperatures
may be as high as 90° F, while for highly reflective roofs with
similar insulative properties, the difference is only about
10° F. For this reason, “cool” roofs are effective in reducing
cooling energy use.
Indirectly, the light-colored surfaces in a neighborhood
alter the surface radiative energy balance, resulting in lower
ambient temperature. The higher albedo (heat-reflecting)
surfaces directly cool the world by 0.01 K, quite independent of avoided CO .
MT: Can you further explain the benefit of altering the
surface radiative energy balance?
HA: Albedo is defined as a number between 1 and 0
indicating the fraction of incident radiation that is reflected,
including the invisible ultraviolet and near-infrared parts of
the spectrum. Planet Earth now has an average albedo of
0.3 — that is, it reflects about 30 percent of the sunlight that
lands on it. There is great potential in the United States and
worldwide for cool roofs. Currently more than 90 percent
of the roofs in the United States are dark colored, with an
average albedo of approximately 0.15. The higher albedo of
a cool roof instantly reduces the amount of heat that can be
trapped by the Earth’s greenhouse gases.