NASA Study: 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
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