Over at RealClimate, Dirk Notz has written a good post about climate change and Arctic sea ice. In it, he highlights the importance of aggregated data, pointing out how, despite variations in regional weather, the mean temperature of the entire globe during the last 12 months is the highest it has been since the beginning of the records 130 years ago.
The figure that is most watched, when it comes to Arctic sea ice, is the extent of ocean covered by ice. This isn’t a spectacularly good measure, however, as Notz explains:
The reason for this is mostly that sea ice in the Arctic has become very thin. Hence, in contrast to the much thicker ice of past decades, the ice now reacts very quickly and very sensitively to the weather patterns that are predominant during a certain summer. This currently limits the predictability of sea-ice extent significantly.
A better measure is ice volume, which has been falling consistently as the planet warms.