Background
Over the final two decades of the 20th century, the surface air temperature record that is officially recognized by the Intergovernmental Panel on Climate Change depicts what climate alarmists call unprecedented global warming.  Over the same period, satellite measurements of tropospheric temperature show little change.  The difference between the two records - a relative surface warming on the order of 0.12°C per decade - is significant and cries out for explanation.  The authors of this intriguing paper attempt to provide one.

What was done
The approach of the authors was to see if trends in recognizable atmospheric modes of variability account for all or part of the observed trend in surface-troposphere temperature differential (lapse rate), based on observations of surface and tropospheric temperatures obtained from satellites, radiosondes, and land-surface air and sea-surface data.

What was learned
The authors say that "modes of variability that affect surface temperature cannot explain trends in the observed lapse rate," and that "no mechanism with clear spatial or time structure can be found that accounts for that trend."  In addition, they state that "all attempts to explain all or a significant part of the observed lapse rate trend by modes of climate variability with structured patterns from observations have failed," and that "an approach applying model data to isolate such a pattern has also failed."  Nor does it seem, they say, "that interdecadal variations in radiative forcing, such as might be caused by volcanic eruptions, variations in solar output, or stratospheric ozone depletion alone, offer a compelling explanation."  Hence, they ultimately conclude "there remains a gap in our fundamental understanding of the processes that cause the lapse rate to vary on interdecadal timescales."

What it means
There are two different ways of interpreting the findings of this study.  First, if Hegerl and Wallace's final conclusion is correct, and there is indeed a significant gap in our fundamental understanding of important meteorological and/or climatological processes related to the lapse rate of the lower troposphere, we are probably not justified in placing much trust in the predictions of climate models that do not incorporate that fundamental knowledge.  A logical extension of this interpretation further suggests that if state-of-the-art climate models are deficient in this one respect, they could well be deficient in other respects, which would make their predictions even more untrustworthy.

On the other hand, the reason why no explanation can be found for the ever-increasing difference between the surface and satellite temperature trends of the past 20-plus years may be that one of the temperature records is incorrect.  Faced with this possibility, one would logically want to determine which of the records is likely to be erroneous and then assess the consequences of that determination.

Although this task may seem daunting, it is not that difficult to make the determination.  One important clue comes from the incredibly good correspondence that exists between the satellite and radiosonde temperature trends that are portrayed in Fig. 1 of Hegerl and Wallace's paper, which leaves little reason for doubting the veracity of the satellite results, since this comparison essentially amounts to an in situ validation of the satellite record.  A second important clue comes from the realization that it would be extremely easy for a spurious warming of 0.12°C per decade to be introduced into the surface air temperature trend as a consequence of the order-of-magnitude greater anthropogenic-induced (heat-island-type) warming that occurs in most of the places where land-surface air temperature measurements are made, due to increases in human population and urban development that occurred over the final two decades of the 20th century (see Urban Heat Island in our Subject Index).

We tend to favor the latter of these two perspectives, although we realize that both of them could well be true at one and the same time.  In either case, there appear to be three good reasons for not believing climate model predictions of future, or replications of past, CO2-induced global warming: (1) the internal consistency of past satellite and radiosonde temperature measurements over areas where both were operative validates the satellite record of global tropospheric temperature, which shows essentially no upward trend over the last two decades of the 20th century, (2) the models do not replicate the two-decade negligible trend of the satellite-measured tropospheric temperature, and (3) there are good reasons for believing the surface air temperature trend is falsely inflated by urban-heat-island-type warming that is next to impossible to remove to the degree of accuracy that is required to confidently assess the magnitude of, or even detect, non-urban warming that might possibly be driven by anthropogenic emissions of greenhouse gases.