Measuring the Temperature of the Earth From Space

The paper published by Frank Wentz and Matthias Schabel in Nature this week (August 14, 1998) is bound to generate controversy about the satellite measurements of global tropospheric temperatures. These measurements, for the period since 1979, have been made with the TIROS-N satellite Microwave Sounding Units (MSUs) by myself and Dr. John Christy (The University of Alabama in Huntsville). We are grateful to Wentz and Schabel for discovering the first convincing evidence for needed corrections to our satellite-based global temperatures.

However, we believe that there are a few important points that should be considered when reporting on this paper.

1) The spurious cooling in the satellite record due to the orbital decay (“downward drift”) effect was only estimated by Wentz and Schabel as an average adjustment to our processed satellite data. The effect, which will have different values for the eight different satellites in the record, should instead be removed one satellite at a time before the satellites in the record are intercalibrated. We (John Christy and Roy Spencer) have performed this adjustment, with the results given below.

2) The effect reported by Mr. Wentz had been partly offset by an east-west drift in the satellites’ orbits. The valuable discovery of the downward drift effect by Wentz and Schabel allowed us to separately quantify two consequences of the east-west drift (MSU instrument temperature change, and observation time-of-day change). We have now performed these adjustments as well (below).

3) The global decadal temperature trends, for the period 1979-1997, from the various satellite, weather balloon, and surface temperature measurements are as follows, in order of increasing temperature trend:

DEEP LAYER MEASUREMENTS


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


Our Adjusted Satellite Trend:

-0.01 deg.  C/decade


Wentz-estimated adjusted satellite trend:

+0.08 deg. C/decade


SURFACE MEASUREMENTS


Sea surface and land surface temperatures (U.K. Met Office):

+0.15 deg. C/decade

It can be seen that the adjustment by Wentz and Schabel does not agree with our (more complete) adjustments, or to the weather balloon data. Instead, their adjustment comes closer to the surface thermometer measurements, and herein lies a temptation to jump to conclusions. 

4) The adjusted satellite trends are still not near the expected value of global warming predicted by computer climate models. The Intergovernmental Panel on Climate Change’s (IPCC) 1995 estimate of average global warming at the surface until the year 2100 is +0.18 deg. C/decade.

Climate models suggest that the deep layer measured by the satellite and weather balloons should be warming about 30% faster than the surface (+0.23 deg. C/decade). None of the satellite or weather balloon estimates are near this value.

5) 1998 UPDATE: The last six months of our adjusted satellite record (February through July 1998) were the warmest in the 20 year record. The updated trend is now +0.04 deg. C/decade (which is still only 1/6th of the IPCC-expected warming rate). The current demise of El Nino, and the possibility of a La Nina forming, will likely cause significant cooling in the coming months.