It's a matter of energy, Antarctica is less of a problem as it is ice several kilometers thick a little surface melt will be covered by snow again. In the Arctic it's a different matter, it's sea ice only which is on average 2m thick, when it melts it's sea water snow has no base to land on and hitting liquid water it melts at once.

Water has one of the lowest albedo of any surface on the Earth

http://en.wikipedia.org/wiki/Albedo

"but for half the year the poles have no sun and for the other half they have precious little"

This is simply not true, total darkness lasts about 3 months on the reverse, in Summer you have pretty much 24 hour sunlight. The amount depends on how far from the poles you are.

Glaciers grow and retreat to these cycles, but in recent decades the retreat has out weighed the growth, giving glacial trends like this.

http://climate.nasa.gov/key_indicators#l...

For the Arctic it has resulted in much less ice extent at the peak of the summer melt, while denier post fraudulent pics of ice extent in winter and try to pretend it shows nothing has changed, they post images like this.

http://igloo.atmos.uiuc.edu/cgi-bin/test...

To try to fool the poor and misguided, when this is what the Arctic looks like at the melt peak.

http://igloo.atmos.uiuc.edu/cgi-bin/test...

The albedo effect is playing a part in this, with effects like a 3-5c warming across the Arctic, well above the global average and a shortening of the Winter season by weeks as ice now melts sooner and takes longer to reform, the polar bear (in a number of regions) comes ashore in Summer and basically starves for several months, it is adapted to do that but it is now having to do it for weeks longer, but then deniers pretend this is not happening either.

"Then there are endless days of cloud cover"

Again not correct, you get many days of clear blue sky's in Antarctica, it is after all classed as a desert, it certainly like any place has storms and bad weather and operates at a much cooler temperature than most of the planet but low water vapor means many days a clear and cloudless, China has installed an optical telescope there for that very reason.

http://zeenews.india.com/news/space/chin...

You've almost answered your own question. Cloud cover albedo is the dominant albedo effect with the tropics being most important due to the high angle of incidence of solar radiation.

Atmospheric aerosols are another dominant albedo factor. Unfortunately, cloud cover effects and aerosols are two of the least well understood aspects of climate.

It is springtime when albedo is most powerful, a progressive increase in early springs is an indication of global warming, albedo multiples temperature change a slight warming reduces albedo and allows more warming and vice versus.

彼は、クラーケンの水中を移動せずに戦ったモンスターを止めて、彼はケルベロスを飼い...

Ok the snow reflects more of the sun's energy back into space but for half the year the poles have no sun and for the other half they have precious little. Then there are endless days of cloud cover, what is there to reflect back.