Please allow me to respond to an article written by Sigbjørn Grønås appearing in forskning.no on December 23rd 2003. The article speaks at length about the global satellite temperature data I and Dr. Roy Spencer generate at the University of Alabama in Huntsville (UAH).

Though referring to our data several times, it is clear Grønås has not read our scientific publications because his article contains several basic and fundamental misunderstandings of the issue.

First of all, Grønås apparently does not understand how these satellite data are created. We do not use, as he states, any form of a “radiative transfer model” which is then iterated to a solution. The microwave brightness temperatures are rather calculated from atmospheric emissions measured by the spacecraft based on the known temperature relationship between emissions and temperature through on-board and deep-space calibration targets (i.e. empirical external calibration). This method is simple, direct and robust as noted in our several publications.

Secondly, when Grønås writes “Consequently, in forecasting weather, such data are given little weight, except where no other data is available.”, he may give the readers the impression that these data are actually used in forecasting weather. The UAH data products are not the data used in forecasting models. Our data are produced directly from the instrument’s digital counts and are not influenced in anyway by a forecasting algorithm, which as Grønås correctly implies, introduces errors.

Thirdly, Grønås writes that the temperature data are based on “heat radiation” (varmestråling), a term most often used to mean infrared radiation. It should be stressed that the tropospheric temperatures are derived from microwave radiation only.

Fourthly, our criticism of a paper appearing in Science is clearly the responsible task of those scientists who have experience on such issues. Grønås suggests it is improper to criticize such papers (especially when the result agrees with his beliefs).

The Science paper referred to, which describes a dataset with greater atmospheric warming than UAH’s, is one that ignored a calibration error published in two independent papers and which leads to erroneous warming. Readers should be aware that such independently discovered calibration errors cannot be dismissed, as in this case Science magazine allowed, simply because they produce a result which agrees better with climate models and some surface temperature datasets.

Fifthly, Grønås indicates balloon measurements are quite useful for understanding atmospheric temperature changes. I agree. In fact, UAH satellite data are the only such atmospheric data which have been compared in several scientific publications with balloon data. The result? The UAH atmospheric data are completely consistent with the independent balloon data, while the other, “warmer” satellite dataset recently published in Science is not. These are comparisons done on a point by point basis, so geographic coverage is not a factor. Grønås is either unaware of these many studies, or elected to keep this important information from the readers.

Finally, to be called a skeptic on any issue should be considered an honor for a scientist, so I am grateful to Grønås for such a designation. To be sure, Grønås has a view about climate change policy which he wants to promote. Readers should recognize this. I encourage the readers to separate such policy beliefs from the hard numbers of science.

John R. Christy
University of Alabama in Huntsville
United States of America
January 5th 2004