SEA levels are rising; it is one of the great worries of our age. But what are they rising relative to? Why, sea level of course. But how high is that?
It’s a momentarily confusing question that masks a real problem. In a flurry of cooperation we can only marvel at today, the world’s nations came together in the late 19th century to adopt international standards of time, longitude and metric measurement. But they never agreed a standard for the vertical.
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Even today, some 100 “points zero” are in use around the world, in some cases differing by metres. Technical difficulties and a lack of political will have hampered attempts to do away with this confusing mishmash. But now technology and the increasingly pressing need to measure rising seas look set to force a quiet revolution. “I expect we will see a global standard within five years,” says , a geodesist recently retired from the Technical University of Munich, Germany.
Thanks mainly to variations in water temperature and salinity, sea level differs around the world. That came as a surprise when European efforts to standardise latitude and longitude in the 1860s extended into the vertical. Several countries had already set up coastal tide gauges – essentially, a float attached to a pen that traced a line on a chart – and were calculating mean sea level, defined as the average of sea level measured at regular intervals between high and low tide. It turned out their levels weren’t the same.
In hindsight, says environmental historian of the Max Planck Institute for the History of Science in Berlin, the surprise is that they were surprised. The sea is only at mean sea level transiently several times a day. Over longer time frames, effects including climate change and the shifting of tectonic plates mean neither the sea nor the land stands still. Coastlines migrate, mountains appear and disappear. “The point is that there is no fixed point,” says von Hardenberg.
It was measurements of rising mean sea levels that first alerted us to the idea that we might be affecting the climate. And yet there was never one mean sea level to measure. “National pride always won, so instead countries compared levels and posted the differences at borders,” says von Hardenberg. This imperfect system remains in place today.
“Rising seas alerted us to climate change, but what are they rising relative to?”
Worse, most national benchmarks are now woefully out of date. The UK’s mapping agency, the Ordnance Survey, measures altitude with respect to in Cornwall – since when it has risen there by about 20 centimetres. “I got into trouble with the director of the Ordnance Survey once, for saying that the height of Mount Snowdon was really 20 centimetres lower than it was marked on the map,” says of the National Oceanography Centre in Liverpool, who studies sea level change.
Measuring mountain height is just one area in which the system’s drawbacks are apparent (see “Take your peak“). In the past, the national benchmark had to be carried through the country every half-century or so, for local benchmarks to be re-calibrated and all points expressed as new “heights above sea level”. These levelling operations were costly, error-prone and time-consuming. Of the three that have been carried out in the UK, the second got under way in 1912, reached Scotland in the 1930s and wasn’t completed until 1952.
And national benchmarks have caused the occasional expensive error – perhaps most notoriously at a bridge built over the Rhine between Switzerland and Germany at Laufenberg in the early 2000s. Germany uses a zero calibrated to a benchmark at Amsterdam in the Netherlands, while Switzerland refers to one at Marseille in France. The two differ by 27 centimetres, and somehow they got added rather than subtracted. For a while, a 54-centimetre vertical gap yawned between the two halves of the bridge.
Yet abandoning the status quo has long been seen as the more costly option – within nations, mean sea level is often used to determine property boundaries and insurance requirements, among other things. The global challenge of sea level rise has been a game changer. It turns out that , but it’s proving hard to get an overview. “Without a global standard, you are measuring apples and pears,” says Johannes Ihde of the German Federal Agency for Cartography and Geodesy in Frankfurt-am-Main, who has worked on setting on a common standard for Europe (see “On the level“).
If Earth were a perfect sphere, we might use GPS measurements: these calculate the user’s distance from the centre of the GPS satellites’ orbits. But Earth looks more like a rugby ball, with a radius 21 kilometres longer at the equator than at the poles. It’s a lumpy rugby ball too, with a depression of about 100 metres to the south of India, for example, and a peak of about 100 metres over Indonesia.
These lumps are in Earth’s geoid, or gravitational surface – a plane that you would move across if you did no work in the vertical dimension, like a marble rolling over a table. They occur because gravity is stronger where mass accumulates, as in a mountain or denser rocks. The geoid largely determines where the surface of the sea lies. If you were to swim from India to Indonesia, you would move 200 metres away from Earth’s centre.
Agreeing on a vertical standard, therefore, boils down to agreeing on a model of the geoid – and with the latest satellite measurements, we’re getting close to doing that. In 2002, NASA and the German Aerospace Center launched the , and seven years later, the European Space Agency launched its . GOCE orbited until 2013, while GRACE is still in orbit, and the two now have enough data to make a geoid model accurate to within a few centimetres. “The gravity field is smoothed because the satellite is far from the Earth’s masses,” says Rummel, who led the GOCE mission, “But it can be complemented by terrestrial gravity measurements.” Together, the two provide the millimetre accuracy required for, say, building bridges.
These advances allow us to measure heights using GPS devices with a built-in geoid model. That represents a conceptual shift, because these devices measure land height relative not to sea level, but to Earth’s centre – a single point in space that, in theory at least, everyone should agree on. The geoid itself shifts over the long term with shifts in the structure of Earth’s crust, but these can be tracked by GRACE and successor satellites.
A map of the geoid can tell us new things about the oceans, too. The sea’s surface in general follows the geoid, but the correspondence is not perfect: temperature and salinity differences drive it into peaks and troughs that account for another metre’s worth of variability, on top of the hundred-fold greater variability in the geoid. If you were to swim across the English Channel from Dover to Calais, you would move uphill by about 9 centimetres with respect to the geoid. “You would do work in the physics sense,” says Woodworth.
GPS devices along coastlines and floating out at sea capture that extra layer of variability, allowing oceanographers to chart what they call the “physical surface” of the sea. Water circulates around those peaks and troughs like air around areas of high and low pressure. “If you know where that +/- 1 metre is, you know where the ocean currents are,” says Woodworth. He and his colleague Chris Hughes have been using this new data to build predictive models of phenomena such as El Niño.
The technical capability that underpins a geoid-based global standard of height is there – but is there the political will to agree on it? Perhaps. The US, Canada and Mexico have in 2022, and a meeting in Prague of the International Union of Geodesy and Geophysics in 2015 passed a resolution to . “We agreed,” says Ihde. “Now we have to put it into practice.”
“The technology for a global height standard is there – but is the political will?”
That may turn out to be the harder step, because the surveyors who lay out supermarkets and car parks still make measurements based on local mean sea level. That flurry of 19th-century cooperation hid a painful truth: it only came about long after the British clock-maker John Harrison invented the chronometer back in the mid-18th century, solving the problem of calculating longitude at sea and simultaneously laying the foundations of a unified system for measuring time. On that precedent, it could be a century or more before we’re all on the level.
On the level
Mean sea level means different things in different places (see map of Europe). In the UK, all heights are measured relative to the time-averaged height of the sea at Newlyn in Cornwall – except in Northern Ireland, where the reference point is Belfast. In the Republic of Ireland it’s Malin Head, while the French look to Marseille – unless they’re Corsican – with landlocked Switzerland following their lead. Since the 1990s, the Germans and Swedes have followed the Dutch in setting their point zero at Amsterdam.
Meanwhile, Italy refers to Genoa, except Trieste which refers to itself – as does Austria and most of the former Yugoslavia, which all belonged to the Austro-Hungarian Empire at the time the Trieste benchmark was fixed. Other parts of eastern Europe refer, along with Russia, to Kronstadt on an island in the Gulf of Finland, a legacy of Soviet influence. Cosmonaut Yuri Gagarin orbited at a height given relative to Kronstadt, reportedly inspiring him to call the town the hub of the universe.
Elsewhere in the world, both the US and Canada use benchmarks based on mean sea level as measured at Rimouski in Quebec, Canada. China worked with a tapestry of benchmarks until 1956, when under the new leader of the People’s Republic, Mao Zedong, these were swept aside in favour of a single reference at Qingdao on the Yellow Sea.
Take your peak
Devising an international standard for height (see main story) might conceivably rob Everest of its status as the world’s tallest mountain. The Himalayan peak was first awarded that title by the British during their 19th-century survey of the Indian subcontinent, the mammoth Great Trigonometrical Survey, in which surveyors measured its height relative to mean sea level as established at Karachi in modern-day Pakistan.
A widely accepted figure for Everest’s height is 8848 metres above sea level, although China and Nepal, whose frontier the mountain straddles, disagree over whether it should be measured by its rock height or its snow height, and the . It favours a figure of 8850 metres, as arrived at by its own team in 1999. Long-term tectonic movements are said to be pushing Everest up, while the Nepalese earthquake of 2015 is thought to have shaken it down slightly.
None of this matters much, though, when you start to look at Everest using other vertical benchmarks. The Hawaiian volcano Mauna Kea rises only 4207 metres above sea level, for example, but 10,203m above its base on the sea floor. Topping both when you start measuring heights from Earth’s centre, as a new standard might, is the Ecuadorean peak Chimborazo. It stands 6310 metres above locally defined sea level, but is almost on the equator, which thanks to Earth’s squished rugby ball shape is further from the centre than the Himalayas. On this basis, Chimborazo beats Everest by a whopping 2 kilometres.
This article appeared in print under the headline “Vertically challenged”
