The seas are rising, but how fast?

In November last year, residents of Italy’s gem Venice looked on helplessly as the second-highest tide in recorded history swept through the Unesco-protected city - considered by many to be one of the most beautiful in the world - flooding homes, historical squares, lobbies of expensive hotels, as well as splendid renaissance palaces and the precious San Marco basilica.

The governor of Veneto, the north-Italian region of which Venice is the capital, spoke of “apocalyptic devastation”. Venice, spread over 118 islands in a lagoon at the mouths of the Po and Piave rivers, is no stranger to flooding. Yet, the realisation that this was not the last time they would see their city “on its knees”, as the governor put it, must be a gloomy one for the desperate residents.

According to predictions compiled by Climate Central, a non-profit organisation that analyses climate reports, the entire north Adriatic coast of Italy, from Rimini to Trieste, will be below the annual flood level by 2050.

“If we follow a scenario of drastic reductions of greenhouse gas emissions as recommended by COP 21 [the UN climate change conference that took place in Paris in 2015], we will have an increase in sea levels of 43cm by 2100,” says Benoit Meyssignac, a climate scientist at France’s oceanography and geophysical science institute Laboratoire d’Etudes en Géophysique et Océanographie Spatiales (LEGOS). “If we don’t follow this scenario and there is no drastic reduction of greenhouse gas emissions, we will get to a sea level rise of at least 84cm by 2100 and multiple metres over the next centuries. That is hard to adapt to.”

Even though the worst problems will arise in underdeveloped countries of Africa and Asia, European regions are set to experience hardships too. The entire coast of Belgium and the Netherlands, as well as vast areas of the UK, including large swathes around Cambridgeshire, Peterborough and Hull, are on a trajectory for yearly flooding.

Over the last 25 years, scientists have gathered an enormous amount of data about global sea levels thanks to satellite altimetry.

It was data from Topex/Poseidon, a joint US-French mission, that enabled scientists to ascertain that sea levels were globally rising, a notion previously hinted at by measurements from coastal tide gauges. The network of tide gauges, however, is relatively sparse and has historically covered mostly coastal regions of developed countries.

“With Topex/Poseidon we were suddenly able to overfly and measure the same point on Earth every ten days,” says Francois Parisot, senior advisor for missions of the European Earth observation programme Copernicus at the European Organisation for the Exploitation of Meteorological Satellites (Eumetsat). “That was a huge step in terms of coverage and that enabled us soon to realise that the sea levels were indeed rising globally. It’s interesting because the mission wasn’t even designed with the objective of measuring sea-level rise but rather to measure sea circulation.”

Since Topex/Poseidon, which ceased operations in 2006, the international > < partners have launched a series of satellites called Jason, which have been continuously monitoring sea levels from the same 1,300km orbit. The newest addition to the US-European sea-level-monitoring fleet, the first of two Sentinel 6/Jason CS spacecraft, is readying for launch in 2020.

These satellites will carry a range of instruments, which together measure sea levels with the precision of several centimetres. By averaging a large amount of data, the scientists can determine sea levels with sub-millimetre accuracy.

“The satellites carry an altimeter, which is essentially a radar emitter that sends radar pulses to the surface of the ocean,” says Parisot. “The signal bounces off the ocean surface and travels back to the satellite. From the time it takes the signal to go there and back, we get a precise measurement of the distance between the satellite and the ocean surface.”

In addition, the satellites will carry three other instruments: a GPS receiver; Doris (Doppler Orbitography and Radiopositioning Integrated by Satellite), a positioning device based on the principle of the Doppler effect; and a laser retroreflector array, which enables the satellite to be tracked by laser measurements from the Earth.

The combination of measurements from these three instruments allows the scientists to determine the position of the satellite in the orbit with 1cm accuracy. By combining the positioning measurements with the data from the altimeter, the researchers determine the distance of the satellite from the sea surface and therefore the sea level.

In addition to that, the satellite is equipped with a radiometer, which assesses the disturbance of the radar signal caused by water vapour in the atmosphere.

“With Topex/Poseidon, we had the accuracy of the sea level measurements of about 8 to 10cm,” says LEGOS’ Meyssignac. “With Jason 1, 2 and 3, we gradually improved the accuracy to 2.5 and 2cm. With our new satellite – Sentinel 6/Jason CS – we expect to get down to the accuracy of 1.5cm.”

The improvements have been achieved by measuring the content of water vapour in the atmosphere with the radiometer at several frequencies for higher accuracy, and by further improvements in the determination of the orbital position.

The satellites cover 90 per cent of the world’s oceans every ten days. By averaging all data points from the ten-day period, the scientists estimate the global mean sea level, which has a much higher accuracy than the single measurement. By averaging large numbers of these measurements, the scientists eliminate the measuring errors further and eventually estimate the global sea level with an accuracy of 0.35mm, according to Meyssignac.

While Topex/Poseidon sampled the sea levels with a relatively low resolution of several kilometres, the frequency of measurements has since increased multiple times to a few hundred metres with today’s satellites, according to Parisot. As a result, the amount of data feeding the models has increased sharply over the years. Advances in computing technologies, improvements in models and algorithms, as well as the now nearly three-decades-long data record have provided a new level of insight into the sea level trends.

“The satellite data indicate a sea level rise of 3.4mm per year with the certainty of 0.34mm per year,” says Meyssignac. “But the longer data sets we have, the more these errors get evened out and the more accurate the estimate.”

In the ‘Special Report on the Ocean and Cryosphere in a Changing Climate’, released by the Intergovernmental Panel on Climate Change (IPCC) in September 2019, the scientists reported that the sea level rise is, in fact, accelerating. The data shows that in the decade between 2006 and 2015, sea levels were rising at 3.6mm a year, compared to the average 3.1mm per year of the previous decade.

The value for the period between 2013 and 2018 is a rather astounding 4.8mm per year, according to Meyssignac. The acceleration is fuelled by the increasing pace at which ice is melting in the polar regions.

On top of that, the data shows that sea levels do not rise at the same speed all over the globe. Improving the understanding of these regional differences is high on the agenda of scientific teams as they race to help at-risk communities to prepare for what is to come in the future.

“If we want to improve our predictions, we need to understand how concentrations of greenhouse gases in the atmosphere affect the sea-level rise,” says Meyssignac. “It is quite difficult to extract this link because there are other interferences such as the response to natural variations in the solar activity, volcanic forces and anthropogenic aerosol emissions. We know that, globally, the response is quite big – about 2.5mm per year – but it is much smaller on the regional level.”

According to Eumetsat’s Parisot, the next revolution in measuring sea levels will come with new technology, which is currently being developed by teams from CNES and Nasa (with a contribution from the UK Space Agency). The new type of a wide-swath altimeter that will be tested as part of the Surface Water and Ocean Topography (SWOT) mission, to be launched in 2021, will be able to take measurements of a wide band of water underneath the spacecraft in one sweep, unlike current altimeters, which only take measurements at the nadir, in one point directly below the spacecraft.

“With this kind of altimeter, which is a radar interferometer, we will be able to image the oceans instead of just taking measurements,” Parisot says. “This would open new applications through access to much higher resolutions.

“We hope this technology would help us improve our understanding of what is going on in the coastal areas, which is especially important since the majority of the global population lives within a few kilometres of the coast and is therefore at risk from the rising sea levels.”