Ice cover can be determined directly by satellite measurement.
Volumes have to be modeled, and denialists reject the models, so I normally don't use them. They are really scary.
As the Arctic warms, the proportion of "old" ice (more than one season's freezing) is diminishing. The total volume goes down dramatically, but the area loss is more gradual. Still, area is important, and a valuable figure, because even a thin layer of ice reflects sunlight in summer, keeping the underlying water cool. In winter, a thin layer of ice also insulates the water from heat loss just as well as a thick one.
But here is an animation of the PIOMASS model.
[youtube id="YgiMBxaL19M" /]