You can double check the data here

northSeaIndex = 
  Import["ftp://sidads.colorado.edu/DATASETS/NOAA/G02135/north/daily/\
data/N_seaice_extent_daily_v3.0.csv", "Data"];
title = First@northSeaIndex;
northSeaIndex = northSeaIndex[[3 ;;]];
northDates = DateObject[{#[[1]], #[[2]], #[[3]]}] & /@ northSeaIndex;
tsNorthSeaIndex = 
  TimeSeries[Transpose[{northDates, northSeaIndex[[All, 4]]}]];
DateListPlot[MovingAverage[tsNorthSeaIndex, 365], 
 PlotLabel -> "Artic Sea Level Index", 
 FrameLabel -> {None, "Sea Ice Extent (10^6 sq. km)"}, 
 PlotTheme -> "Detailed", ImageSize -> Large]