From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change
Is there a method that can distinguish anthropogenic global warming from natural warming? The IPCC (IPCC-SAR, 1996, p. 411; IPCC, 2007-I, p. 668) and many scientists believe the “fingerprint” method is the only reliable one. It compares the observed pattern of warming with a pattern calculated from greenhouse models. While an agreement of such fingerprints cannot prove an anthropogenic origin for warming, it would be consistent with such a conclusion. A mismatch would argue strongly against any significant contribution from greenhouse gas (GHG) forcing and support the conclusion that the observed warming is mostly of natural origin.
Climate models all predict that, if GHG is driving climate change, there will be a unique fingerprint in the form of a warming trend increasing with altitude in the tropical troposphere, the region of the atmosphere up to about 15 kilometers. (See Figure 3.4.1.) Climate changes due to solar variability or other known natural factors will not yield this pattern; only sustained greenhouse warming will do so.
The fingerprint method was first attempted in the IPCC’s Second Assessment Report (SAR) (IPCC-SAR, 1996, p. 411). Its Chapter 8, titled “Detection and Attribution,” attributed observed temperature changes to anthropogenic factors—greenhouse gases and aerosols. The attempted match of warming trends
with altitude turned out to be spurious, since it depended entirely on a particular choice of time interval for the comparison (Michaels and Knappenberger, 1996). Similarly, an attempt to correlate the observed and calculated geographic distribution of surface temperature trends (Santer et al. 1996) involved making changes on a published graph that could and did mislead readers (Singer, 1999, p. 9; Singer, 2000, pp. 15, 43-44). In spite of these shortcomings, IPCC-SAR concluded that “the balance of evidence” supported AGW.
With the availability of higher-quality temperature data, especially from balloons and satellites, and with improved GH models, it has become possible to apply the fingerprint method in a more realistic way. This was done in a report issued by the U.S. Climate Change Science Program (CCSP) in April 2006—making it readily available to the IPCC for its Fourth Assessment Report—and it permits the most realistic comparison of fingerprints (Karl et al., 2006).
The CCSP report is an outgrowth of an NAS report “Reconciling Observations of Global Temperature Change” issued in January 2000 (NAS, 2000). That NAS report compared surface and troposphere temperature trends and concluded they cannot be reconciled. Six years later, the CCSP report expanded considerably on the NAS study. It is essentially a specialized report addressing the most crucial issue in the global warming debate: Is current global warming anthropogenic or natural? The CCSP result is unequivocal. While all greenhouse models show an increasing warming trend with altitude, peaking around 10 km at roughly two times the surface value, the temperature data from balloons give the opposite result: no increasing warming, but rather a slight cooling with altitude in the tropical zone. See Figures 3.4.2 and 3.4.3, taken directly from the CCSP report.
By contrast, observed temperature trends versus latitude and altitude; this is figure 5.7E from CCSP 2006, p. 116. These trends are based on the analysis of radiosonde data by the Hadley Centre and are in good agreement with the corresponding U.S. analyses. Notice the absence of increased temperature trends in the tropical mid-troposphere. The CCSP executive summary inexplicably claims agreement between observed and calculated patterns, the opposite of what the report itself documents. It tries to dismiss the obvious disagreement shown in the body of the report by suggesting there might be something wrong with both balloon and satellite data. Unfortunately, many people do not read beyond the summary and have therefore been misled to believe the CCSP report supports anthropogenic warming. It does not.
The same information also can be expressed by plotting the difference between surface trend and troposphere trend for the models and for the data (Singer, 2001). As seen in Figure 3.4.4 and 3.4.5, the models show a histogram of negative values (i.e. surface trend less than troposphere trend) indicating that atmospheric warming will be greater than surface warming. By contrast, the data show mainly positive values for the difference in trends, demonstrating that easured warming is occurring principally on the surface and not in the atmosphere.
This mismatch of observed and calculated fingerprints clearly falsifies the hypothesis of anthropogenic global warming (AGW). We must conclude therefore that anthropogenic greenhouse gases can contribute only in a minor way to the current warming, which is mainly of natural origin. The IPCC seems to be aware of this contrary evidence but has tried to ignore it or wish it away. The summary for policymakers of IPCC’s Fourth Assessment Report (IPCC 2007-I, p. 5) distorts the key result of the CCSP report: “New analyses of balloon-borne and satellite measurements of lower- and mid-tropospheric temperature show warming rates that are similar to those of the surface temperature record, and are consistent within their respective uncertainties, largely reconciling a discrepancy noted in the TAR.” How is this possible? It is done partly by using the concept of “range” instead of the statistical distribution shown in Figure 12a. But “range” is not a robust statistical measure because it gives undue weight to “outlier” results (Figure 12b). If robust probability distributions were used they would show an exceedingly low probability of any overlap of modeled and the observed temperature trends.
If one takes greenhouse model results seriously, then the greenhouse fingerprint would suggest the true surface trend should be only 30 to 50 percent of the observed balloon/satellite trends in the troposphere. In that case, one would end up with a much-reduced surface warming trend, an insignificant AGW effect, and a minor greenhouse-induced warming in the future.
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