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April 21, 2004
Satellites Act as Thermometers in Space, Show Earth has a Fever
Like thermometers in space, satellites are
taking the temperature of the Earth’s
surface or skin. According to scientists, the
satellite data confirm the Earth has had an
increasing “fever” for decades.
For the first time, satellites have been used
to develop an 18- year record (1981-1998) of
global land surface temperatures. The record
provides additional proof that Earth’s
snow-free land surfaces have, on average, warmed
during this time period, according to a NASA
study appearing in the March issue of the
Bulletin of the American Meteorological Society.
The satellite record is more detailed and
comprehensive than previously available ground
measurements. The satellite data will be
necessary to improve climate analyses and
computer modeling.
Menglin Jin, the lead author, is a visiting
scientist at NASA’s Goddard Space Flight
Center, Greenbelt, Md., and a researcher with the
University of Maryland, College Park, Md. Jin
commented until now global land surface
temperatures used in climate change studies were
derived from thousands of on-the- ground World
Meteorological Organization (WMO) stations
located around the world, a relatively sparse set
of readings given Earth’s size. These
stations actually measure surface air temperature
at two to three meters above land, instead of
skin temperatures. The satellite skin temperature
dataset is a good complement to the traditional
ways of measuring temperatures.
A long-term skin temperature data set will be
essential to illustrate global as well as
regional climate variations. Together with other
satellite measurements, such as land cover,
cloud, precipitation, and sea surface temperature
measurements, researchers can further study the
mechanisms responsible for land surface
warming.
Furthermore, satellite skin temperatures have
global coverage at high resolutions, and are not
limited by political boundaries. The study uses
Advanced Very High Resolution Radiometer Land
Pathfinder data, jointly created by NASA and the
National Oceanic and Atmospheric Administration
(NOAA) through NASA’s Earth Observing
System Program Office. It also uses recently
available NASA Moderate Resolution Imaging
Spectroradiometer skin temperature measurements,
as well as NOAA TIROS Operational Vertical
Sounder (TOVS) data for validation purposes. All
these data are archived at NASA’s
Distributed Active Archive Center.
Inter-annually, the 18-year Pathfinder data in
this study showed global average temperature
increases of 0.43 Celsius (C) (0.77 Fahrenheit
(F)) per decade.
By comparison, ground station data (2 meter
surface air temperatures) showed a rise of
0.34°C (0.61°F) per decade, and a
National Center for Environmental Prediction
reanalysis of land surface skin temperature
showed a similar increasing trend in global and
land surface temperature, in this case 0.28°C
(0.5°F) per decade. Skin temperatures from
TOVS also prove an increasing trend in global
land surface temperatures. Regional trends show more temperature
variations.
“Although an increasing trend has been
observed from the global average, the regional
changes can be very different,” Jin said.
“While many regions were warming, central
continental regions in North America and Asia
were actually cooling.”
One issue with the dataset is that it cannot
detect surface temperatures over snow. In winter,
most of the land areas in the mid to upper
latitudes of the Northern Hemisphere are covered
by snow. Of Earth’s land area, 90 percent
of it is snow free in July, compared to only 65
percent in January. For this reason, the study
only focused on snow free areas. Still, in
mountainous areas that are hard to monitor, like
Tibet, satellites can detect the extent of snow
coverage and its variations.
The satellite dataset allows researchers to
also look at daily trends on global and regional
scales. The largest daily variation was above
35.0 C (63 F) at tropical and sub-tropical desert
areas for a July 1988 sample, with decreasing
daily ranges towards the poles, in general. Daily
changes were also closely related to vegetation
cover. The daily skin temperature range showed a
decreasing global mean trend over the 18-year
period, resulting from greater temperature
increases at night compared to daytime.
Things like clouds, volcanic eruptions, and
other factors gave false readings of land
temperatures, but scientists factored those out
to make the skin temperature data more accurate.
Scientists are considering extending this 18-year
satellite- derived skin temperature record up to
2003. The mission of NASA’s Earth Science
Enterprise is to develop a scientific
understanding of the Earth system and its
response to natural or human-induced changes to
enable improved prediction capability for
climate, weather, and natural hazards. NASA
funded the study. For more information and images
about the research, visit:
http://www.gsfc.nasa.gov/topstory/2004/ 0315skintemp.html
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Contacts:
Gretchen Cook-Anderson/Etta Jane Pagani
Headquarters, Washington
Phone: 202/358-0836/1034
Krishna Ramanujan
Goddard Space Flight Center, Greenbelt, Md.
Phone: 607/273-2561
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Global land surface temperature, July
2003
This image shows land surface temperature for
the entire month of July 2003, one of the warmest
months on record throughout much of Europe. This
image was derived using data from NASA’s
Moderate Resolution Imaging Spectroradiometer
(MODIS) sensor. Credit: NASA
High-Resolution
Image
July Temperatures
1982-1998
This map shows averaged land surface
temperature for the month of July from the years
1982 through 1998. The image was derived using
data from the National Oceanographic and
Atmospheric Association’s (NOAA) Advanced
Very High Resolution Radiometer sensor.
Temperatures are in degrees Kelvin. Credit:
NASA/NOAA
Land surface temperature
trends
These three panels show trends in global
monthly mean skin temperature from 1982 to 1998,
derived from three different observational
techniques; a) AVHRR land surface temperature, b)
Ground observations of air temperature, c)
National Centers for Environmental Prediction
(NCEP) reanalysis of land surface temperature.
Data are the average of January and July. The
straight line in each panel is linear regression.
Credit: NASA/NOAA
European Heat Wave, July
2003
This image shows the differences in daytime
land surface temperatures collected between July
2001 and July 2003 by the Moderate Resolution
Imaging Spectroradiometer (MODIS) on NASA’s
Terra satellite. A blanket of deep red across
southern and eastern France (left of image
center) shows where temperatures were 10 degrees
Celsius (18 degrees Fahrenheit) hotter this
summer. White areas show where temperatures were
similar, and blue shows where temperatures were
cooler in 2003 than 2001. Credit: NASA
High-Resolution
Image
Seasonal temperature variations in
Africa, 2000
These two images show temperatures on the
African continent in the first week of January
(top) and the first week of July (bottom) 2000,
respectively. These images were created using
data from the National Oceanic and Atmospheric
Administration’s NOAA-14 AVHRR Land Surface
Temperature product. Credit: NASA/NOAA
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