Green roofs and the urban heat island (Edited from the EarthObservatory
newsletter.)

Stuart Gaffin studies urban heat islands.
"About half the world's population -3 billion people, now live in
cities. In a couple decades, it's going to be 5 billion." he says.
"Right now, we average about 14 days above 90 degrees [in New York].
In a couple of decades, we could be experiencing 30 days or more. How
do we make cities more habitable in the future?"

On a hot, sunny afternoon, put your hand on the asphalt and then on the
grass. Notice the difference in temperature.

To make room for buildings and roads, cities squeeze out vegetation
that would otherwise cool its surroundings by evaporating water.
Coupled with vehicles, air conditioners, clothes dryers, and chimneys,
an urban area can see air temperatures up to 6 degrees Celsius hotter
than the nearby suburban and rural areas

The team estimated how much cooling the city could achieve by planting
trees, replacing dark surfaces with lighter ones, and installing
vegetation-covered "green roofs" in New York City. They predicted
that a combination of urban forestry and light roofs could reduce New
York City's overall temperature by 0.67 degrees Celsius throughout
the day.

At 3 p.m., when temperatures typically peak, these mitigation
strategies could reduce New York City's temperature by 0.89 degrees
Celsius. Results varied for the smaller areas under study, but
Mid-Manhattan West realized the greatest temperature alleviation
throughout the day, at -0.94 degrees Celsius and Lower Manhattan East
saw the best reduction at 3 p.m., at -1.33 degrees Celsius.

Temperature reductions might not sound like much but they can have a
big impact. Power usage, is very sensitive to even slight variations in
temperature. "If you're near the maximum power availability of your
grid, each degree can make a significant difference.

Also:
A lot of [regional climate] models were originally designed for natural
land surfaces.

But the 2002 study provided enough evidence to allow Gaffin and his
colleagues to make some recommendations for mitigating the heat:

An especially effective mitigation strategy involved planting trees to
cool their immediate area, and cast shade onto nearby buildings. Gaffin
and his colleagues estimated that 17 percent of NY city's surface
could be planted with trees, 64 percent of the city area consists of
dark, impervious surfaces such as roads and roofs.

About 14 percent of the impervious surface area consists of rooftops,
most of them dark, heat-generating surfaces, typically tar, and
sometimes overlain with gravel. Cooling them is like turning off an
open oven.

The study in New York confirmed that white roofs absorb much less of
the Sun's energy than asphalt roofs, and they are fairly inexpensive
and easy to install. But they trap heat and in urban settings, get
dirty quickly.

Even when they're kept clean, white roofs reflect sunlight, heating
up the immediate area. And in winter, light roofs may cool buildings,
increasing heating demands.

Another issue is storm-water runoff. To deal with runoff from heavy
rains, cities have storm sewers, but many cities use the same systems
to handle both the overflow from rainstorms and the water flushed out
of toilets, pushing raw sewage into waterways. It's the major source
of pathogens in the New York Harbor. It's a major problem in Europe.

If cities don't have much room for lots of additional trees, and if
light-colored roofs only partially reduce urban heat and in no way
reduce runoff, just one option remains: vegetation-covered roofs.

Pennsylvania State University assessed the cooling capabilities of
roofs covered with vegetation and found the peak temperatures on the
roofs planted with Sedum were 30 degrees Celsius lower than the
temperatures on standard roofs.

They also found that the low-maintenance Sedum plants thrived without
any supplemental watering. Sedum is a desert-adapted plant with shallow
root systems. The plants can tolerate long periods of drought. In the
arid American Southwest, this cactus-family succulent is a popular
landscaping choice for those looking to minimize water use in lawns.

There are green roofs in Europe that last 30, 40, 50 years, or more but
they are expensive. Vegetation-covered roofs typically include the
following layers: a waterproof membrane at the bottom, a layer of
drainage materials, a root-repellant and filter layer, a lightweight
soil-like growing medium, and finally the plants.

Green roofs have more mass but thin systems of only 3 to 4 inches are
sufficient. When they are saturated with rainwater, they may create a
load of 25 pounds per square foot, which is often feasible for many
city buildings.

Panayoti Kelaidis, the director of outreach for Denver Botanic Gardens
doubts green roofs could succeed in an arid climate without
supplemental watering. Switching to less thirsty plants wouldn't
necessarily solve the problem. "Some plants take even less water than
Sedum. Some plants grow in saltpans. They could practically grow on the
surface of the Moon," Kelaidis says.

Such plants respond to protracted dry spells, however, by going
dormant. "The downside is that their biotic activity is so minimal
when they are dormant, they're no better than asphalt." That
doesn't mean he thinks green roofs are doomed to failure, but he does
think irrigation systems would be required. "Even modest irrigation
would work. Just a mist of water would be enough."

The Chicago heat wave of 1995 claimed more than 700 lives. The
record-warm European summer of 2003 claimed between 22,000 and 45,000
lives. "Can we air condition our way out of these heat waves? Not
always," Gaffin says. "So how can we cool these cities down? There
aren't many strategies we can choose. But green roofing looks like a
great way to alleviate these problems."

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