How do we know sea levels are rising? VIDEOGRAPHIC
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00:00Up to the 1990s, sea level was calculated from the shore, using tide gauges, and taking
00:11the median between high and low tide.
00:14Since then, sea levels are also measured from space by satellites using altimeters.
00:19Their radar sends waves to the ground, where they rebound when they meet an obstacle like
00:24the ground or water.
00:26According to the time it takes the wave to leave and return, the distance between the
00:30satellite and the surface can be calculated to within a few centimetres.
00:35Based on the altitude of the satellite, and a reference model for the shape of the Earth,
00:40the sea level can be determined.
00:44Due to the melting of glaciers and the expansion of water due to heat, sea level has risen
00:48from 21 to 24 centimetres since 1880.
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