Beneath the sea

A few miles along the coast from Aberystwyth, where I am writing, are the petrified remains of a forest. They are all that is left of Gwyddno Garanhir’s lost kingdom of Cantre’r Gwaelod, submerged beneath the sea in a single night’s cataclysm. That’s a myth, of course (though the petrified forest is real enough). Nevertheless, the story helps to highlight one of this highly topical book’s key themes: that many of the geographical features of the world around us that might seem immutable are not really so at all. They are features of our cultural, as much as of our natural, worlds.

In Sea Level: A history, Wilko Graf von Hardenberg sets out to historicize the concept, which is fundamental to the way we articulate the geography around us. How tall are mountains? How deep are the sea’s deepest trenches? It is by their relation to the inconstant constant of sea level that we answer such questions. Sea level is so ubiquitous in our perceptions of landscape that it is often, paradoxically, invisible. When we think of it at all, we see it as a permanent aspect of the natural world but, as the author makes clear, it is thoroughly cultural.

As this readable and thoughtful book explains, making sense of sea level was a matter of measurement. Von Hardenberg traces the ways in which philosophers, map-makers and bureaucrats from the eighteenth century onwards grappled with the problems of measuring elevation. Where, literally, might measurement begin? Even the perceived need for measurement is fairly recent, it turns out – the author notes that the first altitude to be recorded on a map was for the Steilerhorn in the Alps, approximated in 1712 by the mathematician Johann Jakob Scheuchzer. The relationship between land and sea is one of perpetual flux, so how are decisions reached about at what point in this perpetual motion lies the sea’s real level? Scheuchzer, for example, measured the mountain’s height barometrically, referring to an “unspecified location on the Mediterranean Sea” as his baseline. This is the story of how sea level was transformed from a local to a universal reference.

Sea level is a key metric for making tangible the effects of climate change, and in the mire of current climate politics a clear appreciation of how we understand and measure climate’s parameters is essential. We need to know what went into shaping them if we are successfully to take on the labour of overcoming current climate challenges. Von Hardenberg’s book is an object lesson in why activists and politicians need to understand their own histories.

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