This ancient oak tree on the outskirts of Stockholm is one of the world’s earliest – and most bizarre – measures of changing sea levels.
With a vast green canopy, this tree has thrived for centuries in the brief Nordic summers, resisting the bitter Swedish winters and storms that have left its trunk with the scars of lost branches.
No sign tells visitors of the almost forgotten history of this tree – it was examined almost two centuries ago by Charles Lyell, a Scottish scientist sometimes known as the father of geology.
Scientists had been puzzled for centuries because the level of the Baltic Sea was – and is – falling relative to the land, in some places by up to about a centimetre (0.3 inch) a year.
No one in the early 19th century knew if the land was rising, due to some unknown geological force within the Earth, or if the sea was falling, perhaps draining away into some mammoth underground cavern.
Lyell, a friend of Charles Darwin who uncovered the theory of evolution, travelled to the Nordic region in 1834, one of many leading scientists baffled by the Baltic.
He struck on this oak, on a slope about 40 metres from an inlet of the Baltic Sea, in a prime example of 19th-century data mining.
Since acorns and oak tree roots don’t grow in the brackish waters of the Baltic Sea, Lyell reasoned that the age of the tree, and the height of its base above the sea, would give a back-of-the-envelope guide to the pace of change.
Local Swedish experts told Lyell the tree was at least 400 years old, and that its base was about eight feet (2.4 metres) above the sea. So the maximum rate of land rising – or water sinking – was less than two feet (60 centimetres) per century.
Geologists now know that the land around much of the Baltic Sea is rising, in some places up to 1 centimetre a year, because the Earth is still reshaping after the end of the Ice Age 10,000 years ago melted a vast frozen weight of ice. That loss of billions of tonnes of ice pressing down the land means the planet’s crust is rebounding – just like a foam mattress takes a while to regain its shape after you get up.
That increase offsets modern sea level rise driven by global warming around parts of the Baltic. Scientists in Lyell’s day didn’t know about ice ages and didn’t have long, reliable records of sea levels, so they resorted to clues in old maps and in nature.
Back in London, he refined his estimates for a lecture in 1835: “It is clear that the rise (of the land) in each century must have been very slight, although it may undoubtedly have amounted to ten inches (25 centimetres) in a hundred years,” he said.
That’s not a bad estimate based on a plant.
Still, it’s about the twice the accepted rate of relative sea level fall recorded by a tidal gauge in Stockholm, of about 52 cms a century.
But when Lyell visited, that tidal gauge had only been in operation since 1774 – by contrast his arboreal calculation gave data back centuries and added to work by Swedish astronomer Anders Celsius in the 1730s when he examined maps and rocks used for hunting seals in the 16th century.
Lyell pieced together observations from along the Swedish coast – he found no change in sea levels in the south with bigger rates as he went north. He concluded that meant the land that was rising – if the sea had been falling the rate would have been the same everywhere along the coast.
His 1835 speech was entitled “On the proofs of a gradual rising of the land in certain parts of Sweden.” He provided drawings of the oak (centre-.right, below) and its place on a slope down to the water’s edge.
We measured (left) Lyell’s tree – it has a girth of 6.4 metres – according to the Woodland Trust a tree in Britain of that size would have been growing since the 16th century (trees grow more slowly in chillier Sweden, so Lyell’s oak is almost definitely older).
And it’s now about 60 metres from the water, against 40 in Lyell’s day.
Further north in Sweden, there are many signs of change as the land rises, transforming coasts and leaving ports high and dry. Northern ports need to be dredged to let ships dock.
Martin Eckman, an expert on Baltic Sea levels, reckons the earliest written record of changing sea levels is a Viking era inscription on a rock about 25 kms northwest of Stockholm, at Runby.
A woman named Ingrid, who lived around 1050, had an inscription carved on a rock (below, in runes) saying that she had a ”laðbro” on the site – literally a “loading bridge”, and taken to mean a quay or wharf, he said.
If you take the steps down below the tracks at the nearby railway station of Upplands Vassby – a stop between Stockholm and the city’s main airport at Arlanda – a mural shows the spot was about 5 metres below sea level 1,000 years ago – it asks you to imagine Viking ships and fish swimming above your head.
But global sea level rise could threaten coasts even here towards the end of this century if governments fail to restrict greenhouse gas emissions.
In the worst case outlined by a draft report by the Intergovernmental Panel on Climate Change, seas could rise by up to 1.33 metres this century.
That would outpace even the fastest uplift along the Swedish coast.