Let me get this straight.
They measure the height of Arctic ice relative to sea level - right?
And they measure sea level relative to the ice - right?
So how do they know which one changed?
Flip a coin. It works every time.
And they measure sea level relative to the ice - right? Catastrophically incorrect. Do some research.
No. Arctic Sea Ice extents (and Antarctic sea ice extents too) are measured continuously fron near-polar orbiting radar satellites. “Local Ocean Level” and the rougher, more irregular ice surface heights are measured, compared to an algorithm for assumed and measured thickness to estimate height of the ice surface. Sea Ice Area is assumed present if the difference is 90% consistently greater than 1.0 meter above local ocean surface. Sea Ice Extents is reported if the ice height is greater than the local ocean surface by 1 meter over at least 15% of the area. The results are totalled up and reported each day for 12 different regions - but not for fresh water like Lake Superior or Russia’s Lake Baiyal (-1 sp) .
(Sea Ice extents are always greater than Sea Ice area each day.)