Scientists Present 1998 Earth-Temperature Trends

January 12, 1999

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-Niño/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-Niños occurred in the past 20 years, this is the first one that occurs without a simultaneous volcanic eruption." El-Niño 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 Niños 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 Niño 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 Niño event of '98.

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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