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Sea level rises each year by a few millimetres. That’s less than the thickness of a line drawn by a marker pen. Why are ocean scientists worried about it?
What is Sea Level?
Sea level is the average level of the water surface. It’s taken as an average over time because the water level keeps changing thanks to the rise and fall of the tides, the presence of waves and currents, and processes that add or take away the volume of water.
Why is Sea Level Important?
Warming global temperatures cause the seawater to expand and the sea level rises as a result. Melting ice from glaciers and other land ice adds water to the sea and also contributes to sea level rise. A change in the sea level is therefore an indicator of climate change. This is so important that the European Space Agency says that monitoring sea level rise is essential for climate science, for policy-making and, ultimately, for protecting the lives of those in low-lying regions at risk.
How is Global Sea Level Measured?
The global sea level is observed using satellite altimetry. This is where radar altimeters are mounted on satellites that orbit the Earth at heights up to 1,340 km (about the distance between London and Rome) and circle the Earth in under 2 hours.
The satellite orbits are “near polar” so the satellite goes up one side of the Earth and down the other. Meanwhile, the Earth spins on its axis beneath, so the satellite passes over a different part of the Earth’s surface each time it flies overhead. This means that most of the world’s ocean can be observed in as little as 10 days.
The first truly high-quality oceanographic altimeter was TOPEX / Poseidon. It was launched in 1992 and its data was the first to allow accurate global sea level studies. Since then, more satellites have been launched which carry more accurate altimeters with improved data processing techniques. The latest addition is Copernicus Sentinel-6 which was launched last year on 21 November 2020 and is the 13th satellite altimeter to provide global sea level measurements.
The data from these altimeters are routinely compared with each other and also validated against sea level measurements made directly in the sea. Ocean scientists now have access to over 25 years of global sea level data with unprecedented accuracy.
What the Measurements Show
Most estimates of global mean sea level rise using satellite altimetry suggest a rate of around 3 mm every year – about the thickness of a line drawn by a marker pen. It’s significant because it covers all of the world’s oceans with a volume of water so huge that it would take 15 years to flow over the Niagara Falls.
On a volumetric basis, sea Level rise is 15 times faster than the Niagara Falls.
This isn’t all – sea level rise continues year after year. 3 mm a year adds up over the last 25 years to a rise of more than 7 cm – about the height of a tea cup – that’s enough to be responsible for the loss of about 8 m of beach along our coasts.
What do the Measurements Mean?
The WCRP Global Sea Level Budget Group says that most of the observed global mean sea level rise is due to thermal expansion and the melting of glaciers and the Greenland and Antarctica ice sheets. However, about 1 cm of the observed sea level rise in the last 25 years cannot be explained – that’s enough seawater to fill around 1.6 billion Olympic sized swimming pools. And it’s not clear where this water comes from.
There is widespread concern about the outlook for the future. Pierre Prandi and his team at CLS (Collectie Localisation Satellites) have studied local sea level trends. They expect sea level rise will continue over the next decades to centuries. And NASA reports that the rate of sea level rise is increasing.
Concluding Remarks
Sea level rise has far-reaching consequences for coastal societies, says Prandi. Global mean sea level is rising now and there are currently no signs of it slowing. The Committee on Earth Observation Satellites (CEOS) recognises the importance to monitor the current sea level, and indeed, a follow-on mission to Copernicus Sentinel-6 is planned for launch in 2025.
Whatever future the sea level holds for us, we can be assured that satellite altimeters will be monitoring it closely. But for now, we shouldn’t feel comfortable with what the satellite altimeters are showing us about the current rates of sea level rise. A rise of 3 mm every year is not a drop in the ocean.
References and Resources:
https://www.sciencedirect.com/science/article/pii/S0034425721001139 (Accessed: 2-8-2021)
https://www.esa.int/Applications/Observing_the_Earth/Copernicus/Sentinel-6/New_sea-level_monitoring_satellite_goes_live (Accessed: 2-8-2021)
https://www.nature.com/articles/s41597-020-00786-7 (Accessed: 2-8-2021)
https://ceos.org/observations/documents/Satellite_Altimetry_Report_2009-10.pdf (Accessed: 2-8-2021)
https://essd.copernicus.org/articles/10/1551/2018/essd-10-1551-2018.pdf (Accessed: 2-8-2021)
This article was first published in Brainstorm Magazine.
Paul Sterlini
