‘This study shows that the urban heat island effect is a relatively minor contributor to warming, contrary to what climate skeptics have claimed,’ says Mark Z. Jacobson, a professor of civil and environmental engineering. Photo: Denis Tangney Jr./iStock
release more heat to the atmosphere than the rural vegetated areas around them,
but how much influence these urban “heat islands” have on global
warming has been a matter of debate. Now a study by Stanford University
researchers has quantified the contribution of the heat islands for the first
time, showing that it is modest compared with what greenhouse gases contribute
to global warming.
2 and 4% of the gross global warming since the Industrial Revolution may be due
to urban heat islands,” says Mark Z. Jacobson, a professor of civil
and environmental engineering who led the study. He and graduate student John
Ten Hoeve compare this with the greenhouse gas contribution to gross warming of
about 79% and the black carbon contribution of about 18%.
is a component of the soot created by burning fossil fuels and biofuels and is
highly efficient at absorbing sunlight, which heats the atmosphere.
Gross global warming is the total amount of warming that has taken place
from all sources, mainly greenhouse gases, black carbon particles, and heat
islands. Net global warming is gross global warming minus the cooling effect of
light-colored atmospheric particles that reflect sunlight back into space,
which offsets about half of global warming to date. Net, or observed, global
warming is what is typically reported in the media.
Responding to skeptics
Jacobson and Ten Hoeve are authors of a paper describing the research that will
be published in Journal of Climate.
The study modeled climate response from 2005 to 2025.
warming skeptics have claimed that the urban heat island effect is so strong
that it has been skewing temperature measurements that show that global warming
is happening. They have argued that urban areas are a larger contributor to
global warming than the greenhouse gases produced by human activity, and thus
drastic measures to reduce greenhouse gases are not needed.
study shows that the urban heat island effect is a relatively minor contributor
to warming, contrary to what climate skeptics have claimed,” Jacobson says.
“Greenhouse gases and particulate black carbon cause far more
Jacobson’s study, claims about the importance of heat island to global warming
could not be addressed directly. The few previous modeling studies by other
researchers that had examined the effect of urban heat islands on regional
scales did not calculate global impacts.
high-resolution study was the first study of the impact of urban heat islands
on global sea-surface temperatures, sea ice, atmospheric stability, aerosol
concentrations, gas concentrations, clouds, and precipitation. He characterized
urban surfaces around the world at a resolution of one kilometer, making his
simulation both extremely detailed and globally comprehensive.
study accounted not only for local impacts of the heat island effect, but also
feedbacks of the effect to the global scale,” he says.
study showed that urban heat islands are not major contributors to global
warming, Jacobson says reducing the effect of heat islands is still important
for slowing the rise of global temperatures.
heat island effect is caused mostly by replacing soil and vegetation with paved
roads, sidewalks, and buildings. Paving prevents evaporation of water from the
soil and plant leaves. Since evaporation is a cooling process, reducing
evaporation warms cities. Additionally, the darker colors of some roads and
buildings absorb more sunlight, heating a city further.
“geoengineering” proposal for reducing the impact of urban heat
islands is to paint roofs worldwide a reflective white. Jacobson’s computer
modeling concluded that white roofs did indeed cool urban surfaces. However,
they caused a net global warming, largely because they reduced cloudiness
slightly by increasing the stability of the air, thereby reducing the vertical
transport of moisture and energy to clouds. In Jacobson’s modeling, the
reduction in cloudiness allowed more sunlight to reach the surface.
sunlight reflected back into the atmosphere by white roofs in turn increased
absorption of light by dark pollutants such as black carbon, which further
increased heating of the atmosphere.
study did not examine one potential benefit of white roofs—a reduced demand for
electricity to run air conditioning in hot weather. But a recent study done at
the National Center for Atmospheric Research showed
that the decrease in air conditioning use, which occurs mostly in the summer,
might be more than offset by increases in heating during winter months.
“There does not seem to be a benefit from investing in white
roofs,” says Jacobson. “The most important thing is to reduce
emissions of the pollutants that contribute to global warming.”
Photovoltaic panels helpful
One way to reduce emissions while simultaneously reducing summer air
conditioning demand is to install photovoltaic panels on roofs. Such panels not
only generate electricity, reducing emissions of fossil fuels from
electricity-producing power plants, but they also reduce sunlight absorbed by buildings
because they convert sunlight to electricity. Because photovoltaic panels do
not reflect the sunlight back to the air, unlike white roofs, reflected light
is not available to be absorbed again by pollutants in the air, creating heat.
“Cooling your house with white roofs at the expense of warming the
planet is not a very desirable trade-off,” Jacobson says. “A warmer
planet will melt the sea ice and glaciers faster, triggering feedbacks that
will lead to even greater overall warming. There are more effective methods of
reducing global warming.”