Going, going … when will it be gone?
Guardian– In winter when the sea surface is frozen up here, scientists can be pretty sure how much ice there is. But in the summer months when the ice is melting and there’s much more water around, the satellite can become confused.
It can think that melt water sitting on the ice floes is open water; it may not be able to tell the size of the floes or the distance between them; it can have problems “seeing” the ice because of clouds and fog.
In short, the melting effect makes it much harder to quantify the amount of ice there is and the satellite tends to see more ice than there actually is. That’s why monitoring groups such as NSIDC or the university of Bremen try to compensate with weather filters or by calculating the ice extent over a number of days rather than on individual ones.