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Some sea ice is permanent, persisting from year to year,and some is seasonal,melting and refreezing from season to season.Because the extent of the sea ice is important both for the Arctic marine ecology and for the role it plays in the Earth's climate, understanding the variation of this extent during the year and from year-to-year is vital.Each year,the minimum sea ice extent in the northern hemisphere occurs at the end of summer,in September.By comparing the extent of the sea ice in September over many successive years,long term trends in the polar climate can be assessed.
This means that the thawing of ice in the Arctic will take less time each year as instead of having to thaw thick ice it only has to thaw relatively new ice. If the multiyear ice disappears completey then it is reasonable to expect that the Arctic will become ice free in some summers.