Background
Potential changes in the frequency or intensity of extreme weather events may be of greater relevance to the biosphere and society than are changes in the mean state of earth's climate.  Hence, a large effort is underway to detect such changes - which are typically claimed to be positive, not in terms of more value but in terms of more variability (which is typically claimed to be bad) - and to attribute them to the ongoing rise in the air's CO2 content.  Analyses of observed changes in the variability of upper-air climate needed to test such claims, however, have not been plentiful; and it was the authors' intent to partially ameliorate this situation.

What was done
The authors analyzed two midtrospheric temperature data sets spanning the last forty years, calculating day-to-day variability within each month, season and year.

What was learned
Averaged over the entire Northern Hemisphere, midtropospheric temperature variability exhibited a slight upward trend since the late 1950s in one of the data sets; but as the authors note, "this trend is significant in the spring season only."  They also admit that "the robustness of this springtime trend is in doubt," because the trend obtained from the other data set was negative!

For the conterminous United States, however, the two data sets both showed "mostly small positive trends in most seasons."  But, again, none of these trends were statistically significant.  Therefore, the authors acknowledge that they "cannot state with confidence that there has been a change in synoptic-scale temperature variance in the midtroposphere over the United States since 1958."

What it means
When all is said and done, the authors' analyses indicate that all the hype of increasing climate variability over the past four decades is simply that: hyperbole without substance.