Fake ice

The ice is real ice, but the satellites are being fooled.  Actually, not even that.  The satellites are correctly reporting what they're seeing, but the humans have been making an assumption that no longer holds true due to change in the Arctic.  A friend pointed to an interview (audio only) of David Barber, on Quirks and Quarks on CBC.  Early on, he mentioned how the satellites were being fooled.  That isn't exactly what's up, but was the right answer for the circumstance.  I'll take a bit more time to discuss the details of what is happening on this part of the story. 

Our standard method for observing sea ice from space is to use satellites to measure the microwave energy emitted from the earth's surface.  Sea water is a very bad emitter, so has a very low brightness temperature.  Sea ice is a pretty good emitter, so has a much warmer brightness temperature.  You go through a couple of elaborations on this, and out pops the fraction of the surface that is sea ice instead of sea water.

You can also be a little more demanding than that.  Particularly in the Arctic, this makes sense.  The elaboration comes from the fact that not all ice emits microwaves equally well.  Salty ice is a better emitter than fresher ice.  In the summer time, when ice floes do some melting (but not enough to get rid of the whole floe for the ones we're interested in), it is the saltier parts of the floe that melt away first.  This is the same thing happening when you salt the sidewalk -- the salt lowers the melting point, and the salty parts melt first.  When you get to this time of year, and the melting stops, what is left is a relatively fresh ice floe.  It is also called 'multiyear' ice, since it's now in its second winter.  With a more detailed analysis of the microwaves, you can try to distinguish between the first year ice (saltier and a better emitter) from the multiyear ice (less salty, but still a better emitter than sea water) from the sea water (very poor emitter).

Analogy for the remote sensing: The satellite is listening to the surface.  Sea ice is much louder than sea water, so you can start by just checking how loud things are.  Louder = more sea ice.  You can then listen a little more carefully.  Multiyear ice is a little quieter than first year ice, and a little different pitch.  So there's a choir of sea ice, and the multiyear ice is, say, the sopranos singing a little quiter than the basses (first year ice), but both are much louder than the baritones (sea water).  If you've listened to a choir, a band, or just a room of people talking (and deciding how many people were in it, and how many of them were men vs. women), you've done the same sort of discrimination that we're doing with the satellite observations.

Now for the spot where we were fooled. 

Up until basically this year, it was a standard assumption that multiyear ice was thick ice.  It made sense.  Ice that was thick enough to survive the summer can't be terribly thin (we figured).  And in the next winter it has a chance to pile up even more thickness either by freezing more ice on, or by getting piled up as the circulation jammed ice floes on top of each other.  This had been confirmed many times in field expeditions starting long before the satellites were flying.

What Barber found instead was that the ice breaker was cruising along at 13 knots (25 kph) through what the satellite said was multiyear ice.  He was amazed.  That is about the speed the breaker would go through open ocean!  What he realized had happened, after going up to the bridge, was that the satellite was seeing just a thin scum of the fresh multiyear ice, that was on top of also a thin layer of first year ice.  There was only just enough multiyear ice for the satellite to see that.  It is no longer the case that we can assume that the sound of multiyear ice means thick ice.  There's still ice, so methods for deciding total ice coverage are ok.  But we can't use the multiyear ice signature to mean 'thick' any more.  Probably (my opinion) this has become a progressively less accurate thing to do over the last few years, and it's only now that the difference has become as drastic as David found.

This is only a small part of the interview, and the whole thing is well worth listening to.