FOR IMMEDIATE RELEASE: 12 JANUARY 1999
Contact: Dr. Roy Spencer
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NASA/Marshall Space Flight Center--Space Sciences Laboratory
Scientists Present 1998 Earth-Temperature Trends
Since late 1978, polar-orbiting satellites have monitored the
microwave emissions from oxygen in the Earth's atmosphere. These
emissions, measured with the Microwave Sounding Unit (MSU) instruments
aboard TIROS-N weather satellites, are proportional to the temperature
of the atmosphere, and have allowed scientists to build a 20-year
record of the temperature of the Earth as measured from space.
Today at the 10th Symposium on Global Change Studies, Dr. John Christy
of the University of Alabama in Huntsville, Dr. Roy Spencer of
NASA/Marshall Space Flight Center, and Dr. W. D. Braswell of Nichols
Research Corp. will present and discuss the addition of 1998 data to
the two-decade global temperature trend they have assembled. Each
works at the Global Hydrology and Climate Center in Huntsville,
Alabama, and their paper is part of the 79th annual meeting of the
American Meteorological Society in Dallas, TX.
The global satellite temperature measurements are obtained from nine
different satellites, and provide a record of the temperature in two
regions of the Earth's atmosphere: the lower troposphere (the lowest 5
miles of the atmosphere) and the lower stratosphere (covering an
altitude range of about 9-12 miles).
The resulting time series shows the temperature trend in the lower
troposphere is zero for 1979-97. At the same time, the temperature of
the lower stratosphere has declined at a rate of -0.6 degrees C per
decade. The warm El-Niņo/Southern Oscillation of 1998 caused the
warmest monthly temperature anomalies of any observed to date, with
April and May 1998 near +0.7 C above the base period mean of 1982-91.
"Every year, of course, we add another 12 months to the temperature
trend," said Christy. "But 1998 was particularly interesting. While
two previous strong El-Niņos occurred in the past 20 years, this is
the first one that occurs without a simultaneous volcanic eruption."
El-Niņo warming events and the eruptions of Mt. Pinatubo in 1991 and
El Chichon in 1983 have been the most influential events on the
temperature trend to date.
"Obviously, El Niņos are part of the natural weather cycle, and
shouldn't be discounted," said Christy. "When we look at long-term
trends, however, we shouldn't assign excess importance to individual
unusual or extreme short-term events, such as this El Niņo or the
cooling that followed the eruption of the Pinatubo volcano in 1991."
The lower tropospheric data are often cited as evidence against global
warming, because they have as yet failed to show any significant
warming trend when averaged over the entire Earth. The lower
stratospheric data show a significant cooling trend, which is
consistent with ozone depletion. In addition to the recent cooling,
large temporary warming perturbations may be seen in the data due to
two major volcanic eruptions: El Chichon in March 1982, and Mt.
Pinatubo in June 1991.
"We've incorporated some essential corrections to the data," noted Roy
Spencer. Last August, a paper published in the Journal Nature
demonstrated that these data required corrections to account for drift
of the satellites' orbits. "When the need for some of the corrections
was first noticed, people applied them to the entire dataset.
However, this isn't correct, as the data are compiled from nine
different satellites, each with it's own necessary adjustments."
Christy, Spencer, and Braswell have accounted for orbital decay,
diurnal drift, and instrument-body temperature feedback individually
from each of the nine spacecraft instruments, and then merged the
datasets to derive the twenty-year temperature trend.
"We can also check the satellite data against other independent
measurements of temperature, in particular balloon-borne
instrumentation that measure the temperature in-situ," noted Christy.
The figure below compares the MSU satellite temperature measurements
(in red) against two datasets in the tropics (between 20 degrees south
latitude and 20 degrees north latitude). The first independent
comparison dataset, compiled by the UK Meteorological Office and known
by the name "HadRT2.1" is shown in green, consisting of independent
balloon observations. The second comparison dataset is from weather
maps of temperature at about 5.8 km altitude, from the National Center
for Environmental Prediction, National Weather Service, NOAA, and is
shown in blue.
"The tropical region was the region criticized in the past year as
being the region of greatest errors in the MSU. However, a direct
comparison of the data shows that the agreement is astounding between
these different tropical temperature datasets," Christy said.
Surface temperature measurements for 1998 show this to be the warmest
year this century. In constrast to the gradual warming of the surface
over the past 20 years, the tropospheric measurement from MSU showed
no trend until the major warm El Niņo event of '98.
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