Update below February 22, 2016
Needless to say, “Ice Free” never happened. It is true that in the last ten years, August and September monthly extents declined slightly, but the other ten months have increased more than twice as much. So the over all trend has been slightly upward.
Here is the current image from NASA:
For comparison, here is the ice chart from MASIE:
The comparable MASIE image is showing about 500k km2 more ice than the NASA image. Through mid February, 2016 is following the average winter ice growth over the last ten years, and is greater than 2015 which had a maximum below average. The NSIDC Ice Index is running behind MASIE by about 600k km2.
Update February 22, 2016
Some additional information on the MASIE ice product:
MASIE: Human analysis of all available input imagery, including visible/infrared, SAR, scatterometer and passive microwave, yields a daily map of sea-ice extent at a 4 km gridded resolution, with a 40% concentration threshold for the presence of sea ice. In other words, if a gridcell is judged by an analyst to have >40% of its area covered with ice, it is classified as ice; if a cell has <40% ice, it is classified as open water.
The passive microwave sea-ice algorithms are capable of distinguishing three surface types (one water and two ice), and the standard algorithms are calibrated for thick first-year and multi-year ice (Cavalieri, 1994). When thin ice is present, the algorithms underestimate the concentration of new and thin ice, and when such ice is present in lower concentrations they may detect only open water. The underestimation of concentration and extent of thin-ice regions has been noted in several evaluation studies
Melt is another well-known cause of underestimation of sea ice by passive microwave sensors.
Meier et al. How do sea-ice concentrations from operational data compare with passive microwave estimates? Implications for improved model evaluations and forecasting