Background
The authors write that IPCC Fourth Assessment Report (AR4) general circulation models (GCMs) of earth's climate "predict a tropical tropospheric warming that increases with height, reaches its maximum at ~200 hPa, and decreases to zero near the tropical tropopause," noting that "this feature has important implications to the climate sensitivity because of its impact on water vapor, lapse rate and cloud feedbacks and to the change of atmospheric circulations." Therefore, they say that it is "critically important to observationally test the GCM-simulated maximum warming in the tropical upper troposphere," which is what they proceed to do.

What was done
As they describe it, Fu et al. examined trends in the temperature difference (ΔT) between the tropical upper- and lower-middle-troposphere based on satellite microwave sounding unit (MSU) data, as interpreted by both the University of Alabama at Huntsville (UAH) and the Remote Sensing System (RSS) teams, comparing both sets of results with AR4 GCM ΔT simulations for the period 1979-2010.

What was learned
The three researchers report that the RSS and UAH ΔT time series "agree well with each other," and they indicate that they show little trend over the period of record. On the other hand, they state that there is "a steady positive trend" in the model-simulated ΔT results, concluding that the significantly smaller ΔT trends from both the RSS and UAH teams "indicate possible common errors among AR4 GCMs." In addition, they note that the tropical surface temperature trend of the multi-model ensemble mean is more than 60% larger than that derived from observations, "indicating that AR4 GCMs overestimate the warming in the tropics for 1979-2010."

What it means
Fu et al. state that in addition to greatly overestimating the tropical surface temperature trend, "it is evident that the AR4 GCMs exaggerate the increase in static stability between [the] tropical middle and upper troposphere during the last three decades," which findings do not bode well for the climate-modeling enterprise that is the foundational basis of the IPCC's unsupported claims of CO2-induced climate change.