Background
The authors report that studies of observational temperature records over the last 50-100 years have found evidence for increases in both mean and extreme (maximum and minimum) near-surface air temperatures; but they note that the increase in maximum temperature has been of smaller magnitude than the increase in minimum temperature, which state of affairs has led to a decrease in the diurnal temperature range.

What was done
In further exploring this intriguing subject, Morak et al. compared "observed and climate model-simulated trends in mean values of temperature extreme indices, splitting the year into the dynamically active boreal cold (ONDJFM) and warm (AMJJAS) seasons." This they did using "modeled daily minimum and maximum surface temperature data derived from simulations with the Hadley Centre Global Environmental Model, version 1 (HadGEM1)."

What was learned
Among a number of other things, several unfortunate findings came to the fore, namely, the three UK researchers report that the model: (1) "significantly underestimates changes in some regions, particularly in winter across large parts of Asia," (2) "has a tendency to overestimate changes in the frequency of hot days in both the [a] winter and [b] summer seasons over [c] most regions, and in the [d] global and [e] hemispheric mean," (3) "also overestimates changes in the frequency of warm winter days on larger scales," while with respect to changes in cold extremes the model (4) "does underestimate them in some regions," while (5) "there are some regions with trends of the opposite sign." In addition, they say that (6) "the particular regional trend pattern, often also referred to as the 'warming hole,' is not evident in the simulated trend pattern," citing Pan et al. (2004), Kunkel et al. (2006), Portmann et al. (2009) and Meehl et al. (2012). And they indicate that (8) "the model shows a tendency to significantly overestimate changes in warm daytime extremes, particularly in summer."

What it means
Although the HadGEM1 does some things well, there are a number of other things that it has yet to satisfactorily accomplish.

References
Kunkel, K.E., Liang, X.-Z., Zhu, J. and Lin, Y. 2006. Can CGCMs simulate the twentieth-century "warming hole" in the central United States? Journal of Climate 19: 4137-4153.

Meehl, G.A., Arblaster, J.M. and Branstator, G. 2012. Mechanisms contributing to the warming hole and the consequent U.S. east-west differential of heat extremes. Journal of Climate 25: 6394-6408.

Pan, Z., Arritt, R.W., Takle, E.S., Gutowski, J.W., Anderson, C.J. and Segal, M. 2004. Altered hydrologic feedback in a warming climate introduces a warming hole. Geophysical Research Letters 31: 10.1029/2004GL020528.

Portmann, R.W., Solomon, S. and Hegerl, G.C. 2009. Spatial and seasonal patterns in climate change, temperatures, and precipitation across the United States. Proceedings of the National Academy of Sciences 106: 7324-7329.