What was done
The authors analyzed distributional changes, broadly spread over the past century, for non-migratory species of butterfly whose northern boundaries were in northern Europe and whose southern boundaries were in southern Europe or northern Africa.

What was learned
A northern boundary analysis of the ranges of 52 species revealed that their northern boundaries shifted northward for 65% of them, remained stable for 34% of them, and shifted southward for 2% of them.  A southern boundary analysis of the ranges of 40 species revealed that their southern boundaries shifted northward for 22% of them, remained stable for 72% of them, and shifted southward for 5% of them.

What it means
In the words of the authors, "nearly all northward shifts [of butterfly ranges] involved extensions at the northern boundary with the southern boundary remaining stable."  As we note in our Journal Review Global Warming and Shifts in British Bird Ranges, this type of behavior is precisely what we would expect to see given the fact that increases in atmospheric CO2 concentration tend to ameliorate the effects of heat stress in plants and actually induce an upward shift in the temperature at which they function optimally.  These phenomena tend to cancel the impetus for poleward migration at the warm edge of a plant's range, but provide an opportunity for significant poleward expansion at the cold edge of its range.  It is thus possible that the observed changes in butterfly ranges over the past century of concomitant warming and rising atmospheric CO2 concentration are related to matching changes in the ranges of plants upon which the butterflies depend for food.  Or, this similarity could be due to some more complex phenomenon, possibly even some direct physiological effect of temperature and atmospheric CO2 concentration at work on the animals themselves.

In any event, in the face of the amount of "dreaded" global warming that has occurred so far (0.8°C this century in Europe), the consequences for European butterflies have been either beneficial or benign.  Since "nearly all northward shifts involved extensions at the northern boundary with the southern boundary remaining stable," according to the authors, "most species effectively expanded the size of their range when shifting northwards."