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August 21, 2003
In Tropics, Forests Are Cool but Croplands Are Hotter
While croplands may provide more food than forests, they don't offer
much relief from hot tropical climes, a new study finds.
A study of Santa Cruz, Bolivia, which used NASA satellites and computer
models, reports that cutting down tropical forests and converting grasslands
to crops may inadvertently warm those local areas. According to the research,
forest canopies create wind turbulence that cools the air, and native
grasslands are better adapted to the tropics than crops, in ways that
also have a cooling effect.
Lahouari Bounoua, a researcher at the University of Maryland (UMD),
College Park, Md., and NASA's Goddard Space Flight Center (GSFC), Greenbelt,
Md., used a computer model to show that temperatures in January may have
warmed on average by about 1 degree Fahrenheit in the last 25 years,
solely because native forests and grasslands in Santa Cruz were replaced
with crops. Co-authors in this interdisciplinary study of land cover
and climate changes included University of Maryland researcher, Ruth
DeFries, NASA GSFC/UMD scientist Marc Imhoff and NASA GSFC researcher
Marc Steininger.
"It's important to understand the effects of changing land cover in
the tropics, because unlike the past, future deforestation is likely
to occur in tropical and sub-tropical regions," said Bounoua.
The Santa Cruz region has one of the highest rates of concentrated deforestation
in the world over the last 20 years, a recent study by co-author Steininger
found.
According to the computer model, in places where tropical forest species
were replaced by crops, nighttime temperatures dropped slightly, while
daytime local temperatures rose by 3.6 degrees Fahrenheit (F) or 2 degrees
Celsius. Forests have high canopies with varied surfaces, and the movement
of winds over these rough surfaces creates turbulence and cools the air.
Low, even croplands on the other hand create less turbulence from winds
and don't cool the air as much, the researchers found.
When grasslands were replaced by crops in the model, warming occurred
because crops were only about half as efficient with water as the drought-adapted
local grasses and therefore transpired less. Transpiration is a daytime
process where water evaporates from the leaves during photosynthesis
and cools the air.
Bounoua's simulated 1 degree (F) rise in average monthly temperature
agreed with historical records acquired from a weather station in Santa
Cruz over the same time period.
The paper, appearing in the current issue of the Journal of Meteorology
and Atmospheric Physics, used NASA's Landsat images acquired between
1975 and 1999 to provide data of the steady conversion of forest and
grassland to cropland. Data from the Advanced Very High Resolution Radiometer
aboard the Polar Orbiting Environmental Satellite were used to account
for seasonal changes of the vegetation.
Between 1975 and 1999 the area of agricultural land over the study site
expanded by about 8155 square kilometers (5067 sq. miles), or about 466
percent of its 1975 value. Results show that deforestation around Santa
Cruz increased between 1975 and 1996, and then declined between 1996
and 1999.
The researchers entered these and other data of the tropical forest
into the Simple Biosphere (SiB2) model, which was developed by NASA.
Climate conditions of the Santa Cruz area were obtained from a separate
model. This separate model was not influenced by temperature changes
that occurred in the SiB2 simulations. Bounoua and colleagues then kept
all the conditions the same but changed only the quantity and type of
plants as they were converted to crops over time.
NASA's mission is to understand and protect our home planet by studying
the primary causes of climate variability, including what changes are
occurring in global land cover and land use, and their causes and impacts.
This research was funded by NASA.
For more information and images on the Internet, visit:
http://www.gsfc.nasa.gov/topstory/2003/0715bolivia.html
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Contacts:
Krishna Ramanujan
Goddard Space Flight Center, Greenbelt, Md.
301-286-3026
kramanuj@pop900.gsfc.nasa.gov |
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Satellite View of Santa Cruz, Bolivia (May 2003)
This image from the LANDSAT 7 satellite (Enhanced Thematic Mapper instrument)
shows the Santa Cruz region, Bolivia, on May 6, 2003. Credit: NASA LANDSAT
7 ETM+
Satellite View of Santa Cruz, Bolivia (2000)
This image from the LANDSAT 7 satellite (Enhanced Thematic Mapper instrument)
shows the Santa Cruz region, Bolivia, in 2000. The forested areas appear
reddish-purple, and cleared areas are green and bright red. Credit: NASA
LANDSAT 7 ETM+
High-Resolution
Image (11.1 MB)
Satellite View of Santa Cruz, Bolivia (1986)
This image from the LANDSAT 5 satellite (Thematic Mapper instrument)
shows the Santa Cruz region, Bolivia, in 1986. The forested areas appear
reddish-purple, and cleared areas are green and bright red. Credit: NASA
LANDSAT 5 TM
High-Resolution
Image (8.8 MB)
Animation of Changes to Landcover, Santa Cruz, Bolivia (1975-2000)
This animation shows a time series of changing landcover in Santa Cruz,
Bolivia from 1975 to 2000. Forested areas appear reddish-purple and areas
cleared for crops are green and bright red. The data are from LANDSAT
1, 5 and 7. Credit: Animation by Jesse Allen, NASA GSFC Earth Observatory,
based on data provided by Compton Tucker, NASA GSFC (8.8 MB download)
High-Resolution
Animation (23.3 MB)
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