This map shows afternoon temperature changes in the three summer months resulting from increasing the surface reflectivity in urban areas. As expected, urban areas in California and the northeast experienced cooling. Some rural areas in parts of the southeast saw temperatures go up due to surface feedback. |
While
cool roofs and pavements have been found to cool the planet by
preventing energy from being radiated back into the atmosphere, previous
studies have not accounted for atmospheric feedbacks that may result
from changing the surface reflectivity of urban areas. A new study from
Lawrence Berkeley National Laboratory (Berkeley Lab) breaks new ground
by using a high-resolution model of the continental United States that
incorporates land-surface feedback to probe the effects of deploying
light-colored roads and rooftops.
Berkeley
Lab researchers Dev Millstein and Surabi Menon found that atmospheric
feedback—such as changes in cloud cover or precipitation—does have an
important effect, resulting in different amounts of cooling in different
cities, but that cool roofs and pavements are still beneficial for
combating global warming. Their results were published in Environmental Research Letters.
“Although
further studies based on varying assumptions are required to validate
our results, our modeling indicated cool roofs are not necessarily as
effective in a city like Dallas as in a city like Los Angeles,” says
Millstein, a postdoctoral fellow in the Atmospheric Sciences Department. “In places near Dallas and parts of the southeast, the absence of
summer cooling is associated with less rainfall and more sun reaching
the surface—fewer clouds and more sun, basically. Still, no major urban
area saw any significant warming due to feedback effects.”
This
study used the same assumptions as that of a previous Berkeley Lab
study, in which the average albedo (solar reflectance) of all roofs was
increased by 0.25 and of pavements by 0.15. However, the model used in
this study had a higher degree of complexity than that used in previous
studies of cool roofs thanks to continental scale, fine spatial
resolution, feedback effects, and more years of data. The researchers
used the Weather Research and Forecasting model, with a domain that
spans the continental United States and has a resolution of 25 square
kilometers, allowing it to calculate changes in individual cities. It
was run over a 12-year period using weather data from 1998 to 2009.
They
then used the model to investigate an opposite scenario, darkening the
albedo in southern California to simulate the installation of 1 terawatt
of photovoltaic arrays in the Mojave Desert, enough capacity to power
the entire country at noontime. Although such a deployment is several
orders of magnitude larger than current solar developments in the United
States, Millstein notes that a project of similar scale has been
considered in the Sahara Desert to power Europe.
Again,
the researchers found significant and consistent feedback effects to
the solar arrays, including changes in wind patterns several hundred
kilometers away. However any changes to climate at the continental scale
were obscured by year-to-year variability.
“Some
years it decreased the amount of radiation reflected back to space and
some years it increased, and that’s because we had the feedback
effects,” Millstein says. “Without the feedback, you’d always see a
penalty, or heating. That doesn’t mean it’s not there. We could see the
benefits of cool roofs, but it’s not easy to see the penalties of
desert-based photovoltaics.”
More
reflective surfaces, such as cool roofs and pavements, reflect
radiation back into the atmosphere and into space and thus help cool the
planet in two ways. At the scale of individual cities, they can combat
the urban heat island effect, and at a continental scale, they can
combat global warming. Of course, in air-conditioned buildings, cool
roofs can also help lower energy bills by decreasing the need for air
conditioning.
On
the local scale, this study validated previous studies finding
California and the greater northeast of the U.S. as good candidates for
cool roofs. Cities such as Los Angeles, Detroit, and New York saw summer
temperatures drop by 0.30 to 0.53 C. “Half a degree
Celsius makes a big difference in terms of air quality,” Millstein says.
As
for the southeast, some rural areas in Oklahoma, northern Texas, and
parts of Louisiana and Florida saw increases in temperature whereas
cities either stayed the same or cooled slightly. But because
temperature affects the chemistry of the atmosphere, causing higher
ozone levels and more smog, cool roofs can still play an important role
in improving air quality.
“Even
when you take feedback into account, cool roofs are still beneficial
for most places,” Millstein says. “With the exceptions, there may be
more study needed. The southeast is certainly not ruled out as a
candidate for cool roofs.”
On
the global or continental scale, the findings also confirmed the
benefit of brightening roofs and pavements. “Even with the feedbacks
from decreasing clouds in certain locations, we still had more
reflection overall,” Millstein says.
For
each square meter of cool roof surface deployed, the increased
reflectivity is equivalent to offsetting 175 kilograms of carbon
dioxide. For the continental U.S., it would achieve a one-time offset of
3.3 gigatons of carbon dioxide, or about half of total U.S. emissions in 2009.
The
researchers’ next step will be to study the role of air pollution in
weather patterns and investigate how photovoltaics and other forms of
renewable energy may be used to reduce air pollution.
Previous Berkeley Lab cool roof study
Regional climate consequences of large-scale cool roof and photovoltaic array deployment
