"The volume of sea ice – that is, how thick the Arctic ice is..."
Er, no, "how thick the ice is" is called "thickness". Volume is thickness times area (or more precisely, thickness integrated over area).
That said, two data points (last year's area and this year's area) do not a trend make. I can't believe how many people don't get that (or enjoy telling lies so much that they don't care that it contradicts reason).