Ramez hits the nail on the head:

“We usually hear about Arctic sea ice in terms of the area it covers. Every winter almost the whole Arctic freezes over. That’s changed very little. But in summers less and less is left. But what’s even scarier is that the ice is also thinner by about half. When we look at it in terms of volume, in the height of summer, three quarters of the ice volume from the 1980s is gone.  Three quarters.  There’s ice still covering water, but it’s thin, and fragile.”

(Via Arctic Sea Ice: Less in December 2013 than Summers Before 2007 | Ramez Naam.)