Totten Column Cool Cities Solar TodayJan-Feb 09 Final Version

8/6/2019 Totten Column Cool Cities Solar TodayJan-Feb 09 Final Version

http://slidepdf.com/reader/full/totten-column-cool-cities-solar-todayjan-feb-09-final-version 1/3

Totten Sustainable Worldview Column, Solar Today magazine, Jan/Feb 2009

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

For a quarter century, working at the Lawrence Berkeley National Laboratory, the inimitable Dr.

Art Rosenfeld (now Commissioner on the California Energy Commission), and his research

partner Dr. Hashem Akbari ("Dr. Cool"), have been accumulating evidence on the immense value

cities can accrue by increasing the albedo — that is, the reflectivity — of road and rooftop

surfaces. Incredibly, the cost of transforming a city into a solar‐reflecting mirror saves money,

while achieving significant CO2 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 CO2 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. The rest, one

could say, was making history. In the current times when politicians are rushing to subsidize

ridiculously costly and risky solutions to society's energy and climate problems, Akbari and

Rosenfeld offer a refreshing dose of fiscal frugality while leveraging global‐scale benefits. The

following Q&A captures their pioneering spirit of innovative research that moved their insights

from the lab to the marketplace.

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 pavements were half of the cause of summer urban heat islands, which in turn

increased the smog (ozone, O3) in Los Angeles 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 manmade (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 (0.01 Kelvin) quite independent of avoided CO2.

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.

MT: What do you estimate in savings by shifting to a higher albedo cool roof?

HA: Most existing flat roofs are dark and reflect only 10 to 20 percent of sunlight. Resurfacing

the roof with a cool white material that has a long‐term solar reflectance of 0.60 or more

increases its solar reflectance by at least 0.40. We estimate that retrofitting 100 square meters

(1,076 square feet) of roof offsets 10 metric tonnes of CO2 emission. That would be worth $250

at the current European Trading System price.

It is fairly easy to persuade (or to require) the owners of buildings to select white materials for

flat roofs, and in California this has been required since 2005. However, the demand for white

sloped roofs is limited in North America, so California compromises by requiring only “cool

colored” surfaces for sloped roofs. This rule takes effect in July 2009. Use of cool‐colored

surfaces increases solar reflectance by about 0.20 and yields a CO2 offset of about five tonnes

per 100 m², or about half that achieved with white surfaces. The solar reflectance of pavement

can be raised on average by about 0.15, offsetting about four tonnes of CO2 per 100 m².

The solar reflectance of a freshly installed asphalt pavement is about 0.05. Aged asphalt

pavements have a solar reflectance between 0.10‐0.18, depending on the type of aggregate



used in the asphalt mix. A light‐color (low in carbon content) concrete can have an initial solar

reflectance of 0.35‐0.40 that will age to about 0.25‐0.30.

Over 50 percent of the world population now lives in urban areas, and by 2040 that fraction is

expected to reach 70 percent. Pavements and roofs comprise over 60 percent of urban surfaces

(roofs 20 to 25 percent, pavements about 40 percent). The global roof area is about 380,000

square kilometers, and paved surface area about 530,000 km². So, the cumulative total of global

atmospheric CO2 equivalency potentials for cool roofs and cool pavements are 12 gigatonnes

and 10 Gt of CO2, respectively. Given that only 52 percent of the emitted CO2 stays in the

atmosphere, the total global emitted CO2 equivalent potential for cool roofs and pavements is

actually 44 Gt of CO2.

Permanently retrofitting urban roofs and pavements in the tropical and temperate regions of

the world with solar‐reflective materials, and offsetting 44 billion tonnes of emitted CO2, is

worth $1.1 trillion at $25 per tonne.

MT: How can such opportunities be captured in cities around the world?

HA: Installing cool roofs and cool pavements in cities worldwide does not require delicate

international negotiations about capping CO2 emission rates. An international cool cities

program can be used as a model to organize the world’s efforts to mitigate global warming. We

propose an international campaign to use solar reflective materials when roofs and pavements

are initially built or resurfaced in temperate and tropical regions. Let developed countries offer

$1 million per large city in a developing country, to trigger a cool roof and pavement program in

that city. Such an international “cool cities” program is a win, win, win proposition.

Pull quote: Installing cool roofs and cool pavements in cities worldwide does not require delicate

international negotiations about capping CO2 emission rates. And it can save 44 billion metric

tonnes

of

CO2

annually.

Caption: [Editor: check with Dr. Hashem Akbari ‐ email is [email protected] ‐ for high‐quality

photos of several large white‐surfaced buildings] This Los Angeles warehouse, roofed in white,

saves about (TKK) in air‐conditioning costs and offsets roughly TKK tons of CO2 annually.

Documents

Totten Column Cool Cities Solar TodayJan-Feb 09 Final Version