In this case, as each day is highly correlated to the previous one, it is safe to say that the distribution of daily sea ice variation is probably not Gaussian? (Though obviously, this is very bad...)
The comparison is between ice extent measurements at the same date, at differing years, so the day-to-day correlation is not the relevant metric here, but the year-to-year correlation, which should be very low.
There's still something funny about quoting tail probabilities converted to once in X years, though, isn't there? Maybe I'm thinking about this wrong...
