Sea level is the average height of the ocean surface relative to
land. This has changed throughout Earth’s history in response
to climate change and the movement of tectonic plates. Eighty
million years ago, as a result of an expansion of mid-ocean
ridges (see pp.264–265) that caused displacement of water
upward, sea level was 820–1,150 ft (250–350 m) higher than today.
At the peak of the last Ice Age (about 25,000 years ago), when a
huge volume of water was stored on land in polar ice caps, sea
level was 400 ft (120 m) lower than it is today. As global warming
melts the ice caps and warmer seawater expands, sea level is
once again on the rise—possibly by as much as 20 in (50 cm) by
the end of the 21st century.
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