One of the biggest concerns on everyone’s mind is the fact that global sea level has not only been rising steadily over the last 25 years but recently it is rising at a much faster rate.
And this disturbing trend is at the center of the Symposium on “25 years of Progress in Radar Altimetry,” being held on the 24th to 29th of September 2018, in Ponta Delgada, São Miguel Island, Azores Archipelago, Portugal.
This week’s symposium follows on the heels of the “15 Years of Progress in Radar Altimetry” Symposium in 2006, and the “20 Years of Progress in Radar Altimetry” Symposium in 2012, both deemed a very successful landmark by the participants and the readership of the Proceedings. This year will see the first presentation of ERS-1 and TOPEX/Poseidon data products to the Altimetric Community.
Progress in Radar Altimetry
In recent decades, radar altimetry has been the most successful technique for measuring oceanic surface elevations. Altimeters are profiling instruments that provide spot measurements of water-surface elevations relative to a reference ellipsoid, or satellite.
You could say that basically, radio altimetry records the surface topography along the satellite’s ground track. This is accomplished by precisely measuring the height of water, land, and ice by timing the interval between the transmission and reception of very short radar pulses.
The European Space Agency (ESA) explains that on average, as measured by satellites orbiting Earth – sea level has been rising by 3.1 millimeters (0.12 inches) per year over the past 25 years. Melting glaciers and ice sheets, and the thermal expansion of sea water as oceans warm are mainly responsible for the rise.
What is thermal expansion, you might ask? In physics, it is the tendency of matter to change in volume in response to a change in temperature. With the planet’s oceans, seas, rivers, and lakes, the same physics apply. As the oceans warm, the density decreases – meaning that even at constant mass, the volume of the ocean increases.
Water at a higher temperature or under greater pressure (meaning at greater depth) expands more for a given heat input, so the global average expansion is affected by the distribution of heat within the ocean.
What does this all mean?
You may be surprised to learn that sea level rise is not the same all over the planet. Just as the surface of the Earth is not flat, the surface of the ocean is also not flat—in other words, the sea surface is not changing at the same rate globally.
For example, sea level rise in San Francisco, California is not the same as it is in Wilmington, North Carolina. This is because there are local factors to take into consideration. We are talking about things such as land subsidence, upstream flood control, erosion, regional ocean currents, variations in land height, and whether the land is still rebounding from the compressive weight of Ice Age glaciers.
OK, this means that to get accurate measurements of sea levels, two methods are used – tide stations and satellite laser altimeters.
Tide stations, located all around the globe, tell us what is happening at the local level – like the height of the water measured along the coast relative to a specific point on land. Satellite altimetry – on the other hand – provide us with the average height of the entire ocean.
These two tools, when taken together, give us an accurate picture of our sea levels. Anny Cazenave from the Laboratoire d’Etudes en Géophysique et Océanographie Spatiales said, “Satellite altimeters are an essential tool for monitoring sea-level rise. We use reference missions such as the CNES–NASA series of Jason satellites along with other missions such as the Copernicus Sentinel-3 mission to gather a time series of data to understand how sea level is changing in the long term.
