Satellite Data
 

Since 1979 satellites have been used to observe temperature changes in the earth's atmosphere. Favored for their complete surface coverage of global temperature trends, and ability to record fluctuations throughout various parts of the atmosphere in a relatively short amount of time (each satellite takes approximately 40,000 readings globally each day), satellites have quickly become an invaluable method for collecting temperature change data. Dr. Roy W. Spencer, a scientist at NASA/Marshall and Dr. John R. Christy, of the University of Alabama at Huntsville's Earth System Science Laboratory, reported the observed temperature trends of a series of TIROS-N National Oceanic and Atmospheric Administration (NOAA) satellites in a March 30, 1990 issue of the scientific journal Science, stating that the ten-year observed temperature record does not support any rising temperature trends over the period extending from 1979 to 1988 (Spencer and Christy 1990). Furthermore, the team reported that more recent observations over the past ten-years (1989 to present) do not indicate any temperature trend either.

The twenty-year period of temperature change observations has been the subject of much debate, as scientists have expressed concern over the accuracy of the satellites, as well as data compiling and processing techniques. Among those critics are scientists James W/. Hurrell and Kevin E. Trenberth of the National Center for Atmospheric Research, who in an essay published in Nature, reported that satellite observations are unreliable due to "high noise levels" resulting from variations in radiation emitted from oxygen molecules located on the surface of the Earth. When merging the recorded observations from the different satellites, this noise could explain the discrepancies between satellite data and observed sea-surface-temperatures. Moreover, as the current satellite record utilizes eight different satellites with their own individual orbits, each satellite samples a different portion of the atmosphere. This could serve to contaminate the record even further, and for Trenberth and Hurrell, is hardly satisfactory.

Indeed the science and understanding underlying satellite observations are far from settled as it has been shown that temperature trends within the various layers of the Earth's atmosphere are not uniform; the lower troposphere and lower stratosphere show a marked cooling trend over the 20-year period of satellite monitoring while observations in the upper troposphere reveal a slight warming trend.

More recently, there has been controversy over the impact of orbital decay on satellite observations. A study conducted by Frank Wentz and Matthias Schabel of Remote Sensing Systems, in California, revealed that satellites go through a period in which the orbit of the satellite decreases in altitude due to atmospheric drag (approximately 15 km from mean altitude of 850 km) over time. The slow fall of the earth-orbiting satellites, in effect, creates a false cooling trend, as Spencer and Christy reported an apparent decrease in temperature of 0.07 deg C/decade over the twenty-year observation period. The adjusted data, according to Wentz and Schabel, showed an upward temperature trend of 0.08 deg C/decade, still only one-third of the warming rate as predicted by the IPCC's 1995 estimate of +0.23 deg C/decade and in direct conflict with Spencer's and Christy's adjusted value of -0.01 deg C/decade. Table 1 shows the adjusted and unadjusted satellite data as according to Wentz/Schabel and Spencer/Christy, in relation to the atmospheric temperature trends as recorded by weather balloons over the same period:

Weather balloon trend (Angell/NOAA)

-0.07 deg. C/decade

Unadjusted satellite trend

-0.04 deg. C/decade

Weather balloon trend (Parker, UK Met Office)

-0.02 deg. C/decade

Spencer/Christy adjusted satellite trend

-0.01 deg. C/decade

Wentz/Schabel estimated adjusted satellite trend

+0.08 deg. C/decade
Table 1: Adjusted and Unadjusted temperature trends for lower troposphere in relation to weather balloon observation record for same period (taken from Spencer's Measuring the Temperature of Earth From Space, 14 August, 1998).

In defense of their calculated cooling trend, Christy and Spencer point out that their processed data corresponds closely with that of weather balloons covering the lower troposphere over the same period of time. Furthermore, they argue that the disparity in the calculated temperature trends is on the order of a tenth of a degree C/decade or less, too small to produce any significant climatic impact.

What is to be thought of the disparity between sea-surface temperature trends and the observations derived from satellites? Spencer and Christy believe that their satellites are the most accurate method for measuring global temperature change to date. The surface data are obtained from thermometers covering less than half the earth's surface, with most stations existing in the Northern Hemisphere. The Northern Hemisphere consists mostly of land which has been shown to warm up more rapidly than oceans, thus revealing a possible false warming trend. Furthermore, surface temperature readings are influenced by the "urban heat island" effect, in which calculated temperatures show false warming trends due to the warming impact of increased urbanization on concentrated areas. The ways in which surface temperature data have been gathered is also the subject of controversy, as it has been argued that a variety of methods have been used to gather the data (e.g. temperature determined from bucket of sea water, thermometers attached to buoys, or in the intake port of ships) thus producing inaccurate results when merging the data from their respective sources.

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