NSF PR 97-20 - March 12, 1997
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When Satellites Mislead: Scientists Prescribe Caution
Temperature-gleaning satellites are useful tools in
the quest to diagnose global change, but only when
their limitations are well understood. This is the
message conveyed by scientists from the National Center
for Atmospheric Research (NCAR) in Boulder, Colorado,
in an article appearing in the journal Nature
on March 13. NCAR is managed by the University Corporation
for Atmospheric Research under sponsorship of the
National Science Foundation.
The article provides new findings on an ongoing controversy
involving the reliability of global temperature trends
obtained via satellite.
NCAR's James Hurrell and Kevin Trenberth analyze a
puzzling discrepancy between global temperature trends
ascertained by surface instruments versus satellites
in an article "Spurious trends in satellite microwave
sounder units (MSU) temperatures from merging different
satellite records."
The difference in trends has been a subject of spirited
debate because of its implications for the projection
and measurement of global warming. Since 1979, microwave
sounder units (MSUs) have been deployed aboard polar-orbiting
satellites. MSUs measure the brightness of oxygen
in the earth's atmosphere and thus infer the temperature
across the globe at various heights.
In their Nature article, Hurrell and
Trenberth argue that the MSU data, while useful for
many purposes, are poorly suited for gauging long-term
surface temperature trends. MSUs monitor the globe
more thoroughly than surface reports, which are concentrated
over land and approximated over oceans. However, each
MSU lasts only a few years, to be replaced by another
deployment on a different satellite. According to
the NCAR scientists, the transitions between satellites
may be producing spurious temperature drops that mask
an actual rise in global readings. "The surface and
MSU records measure different physical quantities,"
write Hurrell and Trenberth, "so that decadal trends
should not be expected to be the same." However, they
add, "unreconciled discrepancies among the different
records remain."
To study the matter further, the scientists focused
on the tropics between 20 degrees N and S, where "noise"
from short-term weather variations is lower than it
is in temperate and polar zones. Hurrell and Trenberth
compared simultaneous MSU records to each other, to
sea-surface temperatures (SSTs), and to air temperatures
simulated by an NCAR climate model using SSTs. They
found that most of the difference between MSU and
surface trends could be explained by two significant
drops in MSU data for 1981 and 1991, years when satellite
transitions took place.
Some analyses now cite only the MSU or only the surface
data in reporting on global temperature trends, without
noting the counterpart to each. Hurrell and Trenberth
stress that both data sets are needed to unravel the
mysteries of global climate. "The MSU data are excellent
for analyzing year-to-year changes, but not necessarily
for longer-term trends," says Hurrell.
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