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. We estimate that retrofitting 100 square meters (1,076 square feet) of roof offsets
10 metric tons of CO emissions. [A metric ton equals
about 2,205 pounds.] That would be worth $250 at the
current European Trading System price.
The solar reflectance of pavement can be raised on
average by about 0.15, offsetting about four metric tons
of CO per 100 m2.
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 global atmospheric
CO equivalency potentials for cool roofs and cool
pavements are 12 gigatonnes and 10 Gt of CO , respec-
tively. Given that only 52 percent of the emitted CO
stays in the atmosphere, the total global emitted CO
equivalent potential for cool roofs and pavements is
actually 44 Gt of CO annually.
Permanently retrofitting urban roofs and pavements
in the tropical and temperate regions of the world
with solar-reflective materials, and offsetting 44 billion
metric tons of emitted CO , is worth $1.1 trillion at
$25 per metric ton. ST