This guest post from Skeptical Science is written by Dana Nuccitelli.
A skeptic is a person who keeps an equally open mind to all information and evidence, whereas many self-proclaimed "climate skeptics" only seem to consider evidence which appears to undermine the man-made global warming theory. A recent example of how climate skeptics have spun and distorted climate science involves satellite measurements of the temperature of the lower troposphere (TLT), i.e., the lowest layer of the Earth's atmosphere.
In November of 2011, we reached the 33rd anniversary of the satellite temperature record, celebrating nearly one-third of a century of these very valuable measurements. Roy Spencer and John Christy at the University of Alabama at Huntsville (UAH) compile one of the main atmospheric temperature records, compiled from data generated by NASA satellites. Christy and Spencer are also two of the most prominent "skeptic" climate scientists.
Christy in particular is rather vocal about an apparent discrepancy between what climate models have forecast for TLT changes and what he and Spencer compute it to be from their analysis of satellite data. Climate models predict, that as a consequence of global warming, the TLT will warm about 20% faster than the Earth's surface temperature. On the contrary, TLT as computed by Spencer and Chrtisty is warming about 20% more slowly than the surface, as measured by groups like NASA Goddard Institute for Space Studies (GISS) and the National Oceanic and Atmospheric Administration (NOAA). There are three possible explanations for this model-data discrepancy:
- The models are incorrect and the lower atmosphere should not warm faster than the surface.
- The surface temperature estimates are biased high, showing more warming than is actually occurring.
- The TLT estimates are biased low, showing less warming than is actually occurring.
The answer may also involve a combination of these three possibilities. But which is most likely?
The climate model expectation of greater warming in the lower atmosphere is based on fundamental atmospheric physicsbased on fundamental atmospheric physics, so this may be the least likely explanation for the discrepancy. There are also several surface temperature data sets which are all in very close agreement, and whose accuracy was recently independently confirmed by the Berkeley Earth Surface Temperature (BEST) project, so the second possible explanation also appears rather unlikely.
However, UAH is not the only group using satellite data to measure atmospheric temperatures. There are several other groups who have performed similar analyses of the data, and who generally estimate greater atmospheric warming than UAH. Moreover, atmospheric temperature estimates from instruments on weather balloons also indicate more warming than has been computed from the UAH record.
Estimating atmospheric temperatures from satellite data is a tricky business. The microwave sounding units (MSU) aboard the satellites don't actually measure air temperature, but rather the intensity of microwave radiation given off by oxygen molecules in the atmosphere, from which the scientists estimate the temperature. The satellites sensors face down toward the Earth and radiation therefore reaches the satellites having travelled upwards through a warming lower atmosphere and cooling upper atmosphere. This influences any warming signal received by the satellites, and because the lower atmosphere is what is being measured. creates a cooling bias that has to be accounted for. But it doesn't end there; bias also exists between the various instrument sensors on each satellite, and the satellite orbits decay over time. These and a number of other obstacles mean a lot of careful and painstaking analysis is required. As a result of all this complexity and data correction, there's much that can go wrong.
Considering these challenges, it's not a surprise that there have been a number of major corrections to the satellite temperature data over the years. Groups outside of UAH identified two major errors in the UAH analysis, both of which had caused Spencer and Christy to significantly underestimate TLT warming. Despite the difficulties in the available data, and the numerous adjustments made to their analysis, Spencer and Christy have all along insisted that their data set is correct, and they continue with this overconfidence today.
In a press release celebrating the satellite temperature record's 33rd anniversary, Spencer and Christy made a number of misleading statements, including some about the model-data discrepancy previously discussed. For example, Christy remarked:
The climate models produce some aspects of the weather reasonably well, but they have yet to demonstrate an ability to confidently predict climate change in upper air temperatures.
In short, Christy blamed the discrepancy entirely on the models, even though as discussed above, this is probably the least likely explanation, because the model expectation is based on solid fundamental atmospheric physics. Most notably, Christy did not even consider the possibility that his UAH record could be biased low, and hence it the source of the discrepancy.
The UAH scientists' blind spot towards this most likely explanation has spread to other self-proclaimed "climate skeptics," including James Taylor of the conservative, fossil fuel-funded Heartland Institute think tank. Taylor recently published an error-riddled op-ed in Forbes business magazine, making similar arguments as those in the UAH press release. While Christy only considered the possibility that climate models are wrong, Taylor considered three possibilities: (1) the surface temperature record is biased high, (2) a factor other than human greenhouse gas emissions is causing global warming, or (3) the "assumptions about greenhouse gas theory are wrong."
The possibility that the UAH satellite temperature record could be wrong is notably absent from Taylor's proposed explanations. He only seems interested in considering possibilities which would undermine the man-made global warming theory. This is not skepticism. It is certainly possible that our understanding of atmospheric physics is wrong, or that the surface temperatures are biased high. However, a true skeptic would also consider the (arguably most likely) possibility that the discrepancy is at least in part, if not primarily due to the UAH data being biased low. After all, UAH has a history of being biased low, and other measurements of atmospheric temperatures estimate higher warming rates than in the UAH record.
Failing to even consider such a plausible explanation is not skepticism. In order to be considered a true skeptic, one must consider all lines of evidence with an open mind, including those which we may find inconvenient.