CLIMATE Scientists determine the global water level
by means of satellites equipped with altimeters. Every
second the altimeter emits some 1700 pulses of
microwave energy towards Earth. By measuring the
time passing before the reflected energy from the ocean
surface returns to the satellite, it is possible to determine
the level of the surface with impressive precision.
Before the first water-level satellite was launched in
1992 (the American-French TOPEX/Poseidon), harbour
water-level markers were scientists’ most importanttools, but they provide only local data. The markers
caused many sources of error, which satellites eliminate.
Satellites also have the advantage that they can cover
the entire planet in about 10 days, providing an overall
picture of water levels for all seas and oceans. By
collecting all data from multiple overflights, scientists
can calculate the average global water level over a
calendar year, screening out the effects of wind, waves,
ocean currents, tides and so on, all of which can
influence individual data points. They can also dismissSatellites watch ocean levels rise
Measurements of global water levels have become increasingly important as the effects
of global warming are assessed. When harbours and cities are to be protected against
flooding caused by water level rises, ocean-level data from satellites becomes crucial.Since 1992, a water-level satellite
has been orbiting Earth constantly.
Currently it is Jason-3, which will be
joined by Sentinel-6 in 2020.A few other satellites can also
measure ocean water levels. Their
data contribute to supporting measure-
ments of the real water-level satellites.The data show that ocean-level
rises are accelerating. Not only
have water levels risen in the past 25
years, the rise is taking place ever faster.How are global water
levels measured?
How are general changes in the world’s ocean levels measured?
It must be difficult given the need to take local factors such as
tides and temperature differences into consideration.90
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101993 1995 1997 1999 2001 2003 2005 2007 2009 2011 2013 2015 2017 20190Ocean-level riseYEAROCEAN-LEVEL
RISE (MM) TOPEX/Poseidon 1992-2006 Jason-
2001-2013 2008-2019 Jason-2 2016-2021 Jason-S
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