April 7, 2004

Nothing’s Changed

New research further demonstrates that the lower atmosphere during the past 25 years has not warmed at the rate predicted by global climate models.

Major systematic problems in general circulation models (GCMs) are apparent in the discrepancy between observed temperature trends in the lower atmosphere and the trend predicted by models. As long as these problems persist, GCMs cannot provide reliable estimates of future climate conditions.

The longstanding benchmark time series of temperatures in the middle to lower atmosphere is that developed by University of Alabama- Huntsville (UAH) scientists Dr. John Christy and Dr. Roy Spencer. It employs measurements made by microwave sounding units (MSUs) carried aloft on NASA satellites. As the satellites orbit the earth, the MSUs observe microwave emissions from oxygen molecules in the earth’s atmosphere. The emissions vary depending on the temperature of the emitting molecule. Thus, they provide an accurate indication of atmospheric temperature and are real-world observations.

The UAH record indicates that the temperature of the middle atmosphere (at an altitude of about 15,000 feet) has warmed only at a rate of +0.03°C (±0.05°C) per decade in the 25 years since inception of high-quality, global satellite temperature measurements. The absence of much warming in the UAH record contrasts with temperature measurements from the earth’s surface which have warmed at about 0.17ºC warming per decade during the same period. The UAH record also contrasts with GCM projections of how the middle atmosphere should behave. Climate models project the middle to lower atmosphere generally will warm a bit faster than the surface under conditions of an enhancing greenhouse effect.

Because of these contrasting discrepancies, the UAH temperature record has been subjected to intense scrutiny. Advocates of the need to “do something” about global warming seemingly prefer climate model output to actual observations because GCMs, one and all, project a much more scary future climate than observations suggest is likely.

Several researchers developed alternative satellite temperature histories because they believed the methods employed by the UAH researchers in processing the microwave data are inadequate or in error. Carl Mears, Frank Wentz and Matthias Schabel of Remote Sensing Systems (RSS) developed one such alternative history. Konstantin Vinnikov and Norman Grody (VG) are authors of another. The RSS temperature record shows four times the warming of that in the UAH record at +0.12 °C (±0.02°C) per decade. The rate of warming in the VG record is greater still: +0.24°C (±0.02°C). We compare these histories in Figure 1.

Satellite Temperatures

Figure 1. Annual MSU temperature anomalies (departures from the long-term mean) as calculated by UAH, RSS, and VG. (adapted from Christy and Norris, 2004).

Trends in the RSS and VG records bring middle atmospheric warming more in line with surface measurement and, consequently, closer to climate model forecasts of what should be happening in the lower to middle atmosphere. The question is: Which of the three MSU records most accurately reflects true atmospheric temperature? They can’t all be right because each purports to measure precisely the same quantity (temperature in the mid- to lower atmosphere) and yet result in significantly different records.

Each research group begins with identical raw data collected by the same NASA satellites. The problem is that the average lifetime of the instruments aboard the individual satellites is only about 3-4 years. This has resulted in measurements being made by eleven different satellites over the course of the past 25 years. What is in contention is how each individual satellite’s record is combined with others to create one continuous time series.

Satellites’ orbits degrade. So do the instruments onboard. These changes lead to measurement errors. Before each research group can compile an accurate history sufficiently robust for trend calculations, they must compensate for such errors. Each research group has its own correction scheme and each staunchly defends its methods. Yet they are so different as to result in the discrepancies shown in Figure 1.

There does, however, exist a possible external referee to help resolve these differences. That referee is the independent measurement of temperature in the lower atmosphere made by thermometers carried aloft twice daily by weather balloons released from locations around the world.

It would seem any research team eager to demonstrate that its MSU temperature realization is the best would carefully compare its history with that of the weather balloons. However only one group does so — the UAH scientists. They’ve published their most recent result in Geophysical Research Letters .

John Christy and William Norris selected high-quality weather balloon observations (from records kept at the U. S. National Climatic Data Center) from locations around the world between January 1979 through July 2001 and compared them with the UAH satellite data. They perform a series of comparisons designed to account for disparities in the weather-balloon data that can arise from missing data or changing instruments. In all cases, they find very close correspondence between the UAH satellite record and the independent weather balloon record. In fact, the differences between the records are statistically indistinguishable.

Data appropriate and necessary for Christy and Norris to extend their detailed comparison to the other two MSU satellite records is not available. However, they conclude generally that because the three MSU records differ from one other, and because the independent weather balloon data closely match one of the three, then the other two (by definition) must not enjoy close correspondence. This finding implies that the RSS and VG records are in error.

This is not the only research published by UAH scientists comparing their satellite record with the record of weather balloons. This is just the latest in a series of such comparisons, that has included weather balloon data from a variety of sources including the U. K.’s Hadley Centre, the U. S. National Center for Environmental Prediction, the U. S. National Climatic Data Center, and the Russian Research Institute for Hydrometeological Information (see http://www.co2andclimate.org/climate/previous_issues/vol8/v8n18/feature.htm for the results from these other comparisons). No matter where the data comes from though, the results are still the same—an extremely close match between the weather balloons and the UAH satellite temperature record.

The evidence presented by the UAH researchers seems as conclusive as any that exists. It demonstrates that the most accurate MSU record is that with the least amount of warming during the past 25 years and the one that most differs from predictions generated by climate models — when independent weather balloon temperature data serves as referee. It seems to us that these basic facts can stand until and unless the other MSU research groups provide appropriate data and analyses to demonstrate otherwise.

References

Christy, J. R., and W. B. Norris, 2004: What may we conclude about global tropospheric temperature trends? Geophysical Research Letters, 31, L06211, doi:10.1029/2003GL019361.

Christy J. R. et al., 2003: Error estimates of version 5.0 of MSU-AMSU bulk atmospheric temperatures. Journal of Atmospheric and Oceanic Technology, 20, 613-629.

Mears, C. A., M. C. Schabel, and F. J. Wentz, 2003: A reanalysis of the MSU channel 2 tropospheric temperature record. Journal of Climate, 16, 3650–3664.

Vinnikov, K. Y., and N. C. Grody, 2003. Global warming trend of mean tropospheric temperature observed by satellites. Science, 302, 269–272.




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