Monitoring rising sea levels – from space
Sea level varies as the ocean warms or cools, as water is transferred between the ocean and continents, between the ocean and ice sheets, and as water is redistributed within the ocean due to the tides and changes in the oceanic and atmospheric circulation.
However, since the 1850s, the rate of sea level rise has been larger than the mean, or average, rate during the previous two thousand years.
And between 1901 and 2010, the global mean sea level has risen by 19 cm due to increased ocean warming – when seawater warms it expands – and melting glaciers and ice sheets.
Following sea level rise with Jason-2
One of the key tools that is being used to help monitor global sea levels is the Jason-2 ocean altimetry satellite , which was launched on 20 June 2008.
The satellite orbits the Earth at an altitude of 1336 km and its accurate observations of variations in sea surface height provide scientists with information about the speed and direction of ocean currents and heat stored in the ocean. This information, in turn, reveals global climate variations.
How does it work?
The principle of satellite altimetry is that the onboard altimeter bounces radar waves off the ocean surface and measures the time it takes to receive the echo.
This measurement allows scientists to calculate the distance from the satellite to the surface very accurately – to within cm. With precise data on the altitude and location of the satellite in relation to a reference surface, it is then possible to accurately calculate sea surface height. Find out more
These measurements are continuously calibrated against a network of tide gauges. When seasonal and other variations are subtracted, they allow estimation of the global mean sea level rate.
As well as measuring sea surface height, the Jason-2 satellite also provides data on wave height and wind speed.
Twenty year time series
Since 1992, a series of highly accurate altimetry satellites have been in continuous operation. The first satellite, Topex/Poseidon was followed by Jason 1 and now Jason-2. To continue the continuous line of ocean measurements, the third in the line of Jason satellites, Jason-3, will be launched in 2015.
Because the Jason satellites have all been on the same orbital path around the globe they are a reference against which data from other altimetry satellites can be checked. Together they demonstrate that over the last 22 years sea level has been rising by an average of 3.1 mm per year.