Here's an interesting site I just learned about: a measure of Arctic sea ice volume over time.
Of course, the true measure of a warming Arctic is sea ice volume, and not extent/area.
As this plot shows, it has been decreasing steadily for decades.
Measurement of sea ice volumes are much more difficult than measuring sea ice extent. The volume calculation here comes from modelling -- not Global Climate Modeling, but a simpler method of converting known data into volume data. (Prediction: skeptics/deniers will seize on this point in order to dismiss the data). But the model is pretty good, as its validation shows.