<b>Trends in Extent <a href="https://en.wikipedia.org/wiki/Northern_Hemisphere" class="extiw" title="en:Northern Hemisphere">Northern Hemisphere</a> (Arctic) Sea Ice</b>: Ice extent anomalies are plotted as a time series of percent differences between the total extent for the month in question, and the mean for that month, where the mean is based on the January 1979 - December 2000 portion of the data set. The trend, in percent change per decade, is obtained using least squares regression, and a 95% confidence interval for the resulting slope is given.
National Snow and Ice Data Center
<a rel="nofollow" class="external free" href="http://nsidc.org/data/seaice_index/n_plot.html">http://nsidc.org/data/seaice_index/n_plot.html</a>