In September 1984, there were more than a million square miles of thick, older sea ice in the Arctic—an area roughly the size of Alaska, Texas, and California combined. By September 2016, multi-year ice had shrunk to only around 68,000 square miles (a little smaller than Missouri).
All Arctic ice has been steadily disappearing, but the loss of older ice is especially problematic because—as one NASA researcher puts it—it's an "insurance policy" for the rest of the pack. The less older sea ice remains, the more likely it is that the Arctic could be completely ice-free in the summer.
This older, thicker ice is like the bulwark of sea ice: a warm summer will melt all the young, thin ice away but it can’t completely get rid of the older ice. That bulwark is not as good as it used to be. The older ice is becoming weaker because there is less of it, and the remaining ice is more broken up and thinner.
A NASA animation shows the shrinking ice. The oldest ice is white, while darker colors depict newer ice.
Unlike the massive sheets of freshwater ice in Greenland and Antarctica, melting sea ice won't make sea levels higher on its own (the ice displaces the same amount of water as it would if it were liquid). But as it melts, and can't reflect as much sunlight, the more heat the planet absorbs. Rising Arctic temperatures make even more ice melt (including the ice in Greenland), and make the permafrost melt, which releases even more greenhouse gases.
Some researchers predict that the Arctic could be essentially ice-free by the summer of 2017 or 2018.