Michael Mann refutes John Christy and Roy Spencer's amazingly faulty research in an excerpt from "The Hockey Stick and the Climate Wars"

What about the related third pillar: that other independent evidence contradicts surface evidence of warming? The source of this claim was a set of analyses by one pair of scientists, John Christy and Roy Spencer, of one particular dataset [13]. Those data come from instruments, known as microwave sounding units (MSUs), installed on polar orbiting satellites. The MSUs measure, from space, the intensity of microwave radiation emitted by Earth’s atmosphere, which, in turn, can be related to temperatures in the atmosphere. Different channels of the MSU measure different frequencies of radiation, and an appropriate combination of information from the different channels (called 2LT) provides an estimate of temperatures in the lower atmosphere. For more than a decade, the Christy and Spencer MSU record was held up as the decisive refutation of global warming—a role that Christy, in particular, seemed to relish. NPR, for example, highlighted Christy’s role in the climate change debate thusly in 2004: “His major contribution has been to analyze millions of measurements from weather satellites, looking for a global temperature trend. He’s found almost no sign of global warming in the satellite data, and is confident that forecasts of warming up to 10 degrees in the next century are wrong” [14].

In the late 1990s, a number of problems had already been identified in the Christy and Spencer analysis. One issue—analogous to the segment length curse encountered in dendroclimatology—was the way Christy and Spencer merged what were distinct overlapping satellite records to form a single, ostensibly continuous record. The net effect of their method was to underestimate any long-term warming trend [15]. 

Still other researchers found that Christy and Spencer had failed to account for the slow decay of the orbit of the satellites, which yet again worked in the same direction, tending to impose spurious apparent cooling [16]. Christy and colleagues, however, claimed that fixing another offsetting error, related to a required correction for the diurnal heating by the Sun, left the record basically unchanged [17]. Other authors later demonstrated an additional problem: that Christy and Spencer’s claimed measurement of lower atmospheric temperatures was biased because they were actually averaging in some temperature information from the stratosphere to their estimates of lower atmospheric temperature [18]. Greenhouse warming is a kind of zero-sum game; the lower atmosphere warms at the expense of upper atmosphere (stratosphere) cooling. Erroneously averaging in stratospheric temperatures led Christy and Spencer to introduce artifi cial cooling into their estimates. 

Then, in 2005, the independent team of Carl Mears and Frank Wentz of the Remote Sensing Systems (RSS) group in California determined through reverse engineering that there must be an error in the correction that Christy and Spencer had applied back in the late 1990s, as discussed earlier, to account for diurnal heating by the Sun [19]. As it turns out, their correction had the wrong sign. This was now the fourth consecutive error identified in Christy and Spencer’s MSU analyses that had worked in the same direction, that is, that masked the true warming taking place in the atmosphere. Christy and Spencer admitted the error, albeit in a somewhat cryptic reply [20]. Using the same MSU data, Mears and Wentz produced their own estimates, which were free of these errors and which showed a warming of the lower atmosphere that was in fact entirely consistent with observed surface warming. This one is perhaps the easiest call of all. Judgment: pillar toppled.