NASA Satellites Act As Thermometers in Space

Like thermometers in space satellites are taking the
temperature of the Earth’s surface or skin. According to
scientists, the satellite data confirms 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 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 trend of increasing temperatures, in this case 0.28 C
(0.5 F) per decade. Skin temperatures from TOVS also prove an
increasing trend in global land surfaces. Regional trends show
more 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