How much sea ice is there?
The most commonly discussed measure is the extent of sea ice in the arctic. This is used mostly because it is easier to measure precisely.
However, one downside of measuring extent is that it’s not easy to get a feel for what’s really happening with the loss of arctic sea ice.
Each winter the arctic freezes over, and the the extent returns more-or-less to “normal.”
In the summer, the minimum extent gives a fairly good indication of how much ice we’re losing, but even there, things bounce around quite a bit, as we can see on the following graph from the Arctic Sea-Ice Monitor page.
One gets a better sense of the fairly steady loss of ice over the past decades by looking instead at sea ice volume. This takes into account the thickness of the ice and the fact that even though the arctic refreezes each winter, the resulting ice is thin and will melt more quickly in the summer.
Here’s a nice animated graph of the decrease in sea ice volume.