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July 29, 2004
Urban Heat Islands Make Cities Greener
Some people think cities and nature don’t mix, but a new NASA-funded study finds that concrete jungles create warmer conditions that
cause plants to stay green longer each year, compared to surrounding rural areas.
Urban areas with high concentrations of buildings, roads and other artificial surfaces retain heat, creating urban heat islands. Satellite data
reveal that urban heat islands increase surface temperatures compared to rural surroundings.
Using information from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on the Terra satellite, Boston University,
Boston, researchers discovered that city climates have a noticeable influence on plant growing seasons up to 10 kilometers (6 miles) away from a
city’s edges. Growing seasons in 70 cities in eastern North America were about 15 days longer in urban areas compared to rural areas outside of
a city’s influence.
“If you live in a rural area and drive regularly into the city, and if you pay attention to vegetation, you will see a difference in the
growing seasons in early spring and late autumn,” said Xiaoyang Zhang, the study’s lead author and a researcher at Boston University. The
study appeared in a recent issue of the American Geophysical Union’s Geophysical Research Letters journal.
Zhang added that urban heat islands provide a very good model to assess the effects of global warming on plant growing seasons and ecosystems. As
temperatures warm due to climate change, growing seasons will likely change as well. Zhang and colleagues found that for every 1 degree Celsius (C)
or 1.8 Fahrenheit (F) that temperatures rose on average during the early springtime, vegetation bloomed 3 days earlier.
Springtime land surface temperatures in eastern North American cities were on average 2.3°C (4.1°F) warmer than surrounding rural
areas, according to the study. In late autumn to winter, the city temperatures were 1.5°C (2.7°F) higher than the surrounding areas.
These higher urban temperatures caused plants to start greening-up on average seven days earlier in spring. Similarly, in urban heat island areas,
the growing season lasted eight days longer in the fall than the rural areas.
The researchers used MODIS surface reflectance data to measure seasonal changes in plant growth for the entire year of 2001. By accounting for
angles of views from the satellite, varying sunlight, land surface temperatures, cloud cover, and the presence of snow, the scientists were able to
detect daily variations in the green color of plants.
The researchers classified urban areas using MODIS data from October 2000 to October 2001, as well as Defense Meteorological Satellite
Program’s (DMSP) nighttime lights imagery and population density data. Only eastern North American cities with urban areas larger than 10
square kilometers (4 square miles) were included in the study.
The researchers found that the effect urban heat islands have on plants’ growing seasons is exponentially weaker the further away from the
city one travels. Significant effects were seen up to 10 kilometers (6 miles) from city lines. In other words, the impact of urban climates on
ecosystems extended out 2.4 times the size of a city itself.
“Warming from global climate change will definitely impact ecosystems,” Zhang said. “Thus, urban areas provide us with some
measures of how changes in temperature might affect vegetation,” he added.
NASA is dedicated to understanding the Earth as an integrated system and applying Earth System Science to improve prediction of climate, weather,
and natural hazards using the unique vantage point of space.
For information and images about this research on the Internet, visit:
http://www.gsfc.nasa.gov/topstory/2004/ 0801uhigreen.html
For information about NASA and agency programs on the Internet, visit:
http://www.nasa.gov/home/index.html
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Contacts:
Gretchen Cook-Anderson
Headquarters, Washington
Phone: 202/358-0836
Krishna Ramanujan
Goddard Space Flight Center, Greenbelt, Md.
Phone: 607/273-2561
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Central Park, New York City
Credit: Junye Chen
Zoom Out of New York, NY: Central Park
Using data from different spacecraft and some powerful computer technology, this animation shows New York City’s Central Park from an
unusual perspective. According to this study, plants in a city park like this bloom earlier and stay green longer than plants outside the
city’s influence. Credit: NASA/Goddard
Space Flight Center Scientific Visualization Studio
Atlanta in the Heat
Urban areas with high concentrations of buildings, roads and other artificial surfaces retain heat, creating urban heat islands. Satellite data
reveal that urban heat islands increase surface temperatures compared to rural surroundings. This animation of Atlanta shows how cities retain heat
from day to night. Credit:NASA/Goddard Space Flight Center Scientific
Visualization Studio
Urban Heat Island from Aircraft
The false color image illustrates airborne remote sensing data collected by the Airborne Terrestrial and Land Acquisition Sensor (ATLAS) over
downtown Atlanta, Georgia on May 11, 1997. The ATLAS multispectral sensor is flown on a Lear jet operated by the John C. Stennis Space Center located
on the Mississippi Gulf Coast. Trees and other vegetation appear red in the color image. Buildings, streets, and other urban land covers appear white
or blue-green to almost black in color. Credit: NASA
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