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On Assessing Surface Air Temperature Trends
Volume 6, Number 23: 4 June 2003

Almost anyone who has even a passing interest in the global warming debate knows about the dichotomy that exists between the satellite-derived record of tropospheric air temperature and the ground-based instrumental record of surface air temperature. The latter data base suggests there has been what climate alarmists typically describe as unprecedented global warming over the past century (and especially the last quarter-century), which they attribute to anthropogenic CO2 emissions and which they claim will produce catastrophic consequences for life on earth if left unchecked. The former data base, on the other hand, depicts but a slight run-of-the-mill increase in temperature, which so-called climate skeptics, such as we, consider to be of non-human origin and of little consequence to man and nature.

In our Editorial of 7 May 2003, we describe the recent attempt of Santer et al. (2003) to discredit the satellite-based tropospheric temperature record, which recently was rigorously tested for potential errors by Christy et al. (2003) and found to be very robust when compared with tropospheric temperature observations obtained from balloon datasets created by independent organizations, some of which possess data from as many as 400 different balloon stations (see our Editorial of 28 May 2003). The approach taken by Santer et al. was to (1) cite the unpublished results of another group of scientists -- the Remote Sensing Systems (RSS) group of Santa Rosa, California -- who claim to have produced a better representation of the satellite data that gives a greater tropospheric warming trend more akin to that of the surface air temperature record, (2) derive a lower-stratospheric and mid-to-upper-tropospheric temperature history for the past quarter-century by means of a state-of-the-art coupled atmosphere-ocean climate model, which also produces a greater warming trend than the Christy et al. data set, and then (3) claim that because the larger satellite-derived warming trend of the RSS group corresponds more closely to the theoretical temperature history produced by the climate model, the RSS satellite record is to be preferred over the satellite record of Christy et al., which would thus suggest that the extreme warming trend of the surface air temperature data base is likely to be correct and not contaminated by any warming effects of intensifying urban heat islands and land use changes of the past century and quarter-century.

This line of reasoning suggests that the output of a theoretical model can be used to adjudicate between two slightly different sets of real-world data. The danger associated with this assumption is cogently articulated by Christy in the preface to a University of Alabama in Huntsville press release dated 1 May 2003: If you have reliable data that disagree with a computer model, it's time to find out what's wrong with the model. To do anything else might lead you to conclude that your theories are correct and the real world is wrong.

Fortunately, a new analysis of real-world temperature data has just been published that accomplishes what the theoretical climate model of Santer et al. could not possibly do, i.e., help to evaluate the instrumental surface air temperature record that climate alarmists use as the basis for their claim of recent unprecedented global warming.

The new study is the brainchild of two meteorologists at the University of Maryland -- Eugenia Kalnay and Ming Cai (2003) -- who report in Nature how they used "the difference between trends in observed surface temperatures in the continental United States and the corresponding trends in a reconstruction of surface temperatures determined from a reanalysis of global weather over the past 50 years [the NCEP-NCAR 50-year Reanalysis (NNR) project], which is insensitive to surface observations, to estimate the impact of land-use changes on surface warming." This reconstruction of surface air temperature, it must be emphasized, does not employ surface observations. Rather, it uses atmospheric vertical soundings derived from both satellites and balloons, so that "surface temperatures are estimated from the atmospheric values."

What did the two scientists discover when they applied this new approach to evaluating surface air temperature trends? In contrast to what is suggested by surface observations, they say they could find "no statistically significant difference in the NNR estimation of urban and rural station trends," which suggested to them, in their words, that "we could attribute the differences between monthly or annually averaged surface-temperature trends derived from observations and from the NNR primarily to urbanization and other changes in land use."

Kalnay and Cai thus went on to evaluate those differences for the city of Baltimore, Maryland, where they found that over the past 50 years there was "a growing trend in the difference between the surface observations and NNR," which increased to 1.4C during the 1990s. As described above, the two scientists attributed this difference "to urbanization and other surface changes that do not affect the NNR," which is assumed to be a pure and unadulterated manifestation of the natural climatic state of earth's atmosphere in each particular part of the world where it is derived.

Applying this technique to the entire coterminous Unites States, where the mean surface air temperature data imply a warming of 0.088C per decade over the past 50 years, Kalnay and Cai calculated a mean NNR-derived warming of only 0.061C per decade, demonstrating thereby that the surface air temperature data yield a century-long warming trend that is 0.27C too high. This real-world observation, if applicable to the rest of the planet, suggests that the "unprecedented" global warming of the past century, which has been derived from the surface air temperature record, is significantly inflated. From the Global Historical Climatology Network data base and the data base of Jones et al., for example (both of which can be accessed via our World Temperatures link), we calculate the mean global warming of the 20th century to have been 0.69 and 0.66C, respectively, for a mean global warming of 0.675C. Now, however, it appears that that figure should be reduced to something on the order of 0.4C, or 40% less than what climate alarmists have long claimed it to be.

On the basis of these observations, we can only conclude that the warming of the past century or so was nothing more nor less than the natural recovery of the earth from the global chill of the Little Ice Age, and that as recently suggested by the comprehensive literature review of Soon et al. (2003), the planet is not nearly as warm yet as it was approximately one millennium ago during the peak heat of the Medieval Warm Period, when, of course, the atmosphere's CO2 concentration was about 100 ppm less than it is today.

Sherwood, Keith and Craig Idso

Christy, J.R., Spencer, R.W., Norris, W.B., Braswell, W.D. and Parker, D.E. 2003. Error estimates of version 5.0 of MSU-AMSU bulk atmospheric temperatures. Journal of Atmospheric and Oceanic Technology 20: 613-629.

Kalnay, E. and Cai, M. 2003. Impact of urbanization and land-use change on climate. Nature 423: 528-531.

Santer, B.D., Wigley, T.M.L., Meehl, G.A., Wehner, M.F., Mears, C., Schabel, M., Wentz, F.J., Ammann, C., Arblaster, J., Bettge, T., Washington, W.M., Taylor, K.E., Boyle, J.S., Bruggemann, W. and Doutriaux, C. 2003. Influence of satellite data uncertainties on the detection of externally forced climate change. Sciencexpress/ /1 May 2003.

Soon, W, Baliunas, S., Idso, C., Idso, S. and Legates, D.R. 2003. Reconstructing climatic and environmental changes of the past 1000 years: A reappraisal. Energy & Environment 14: 233-296.