From John Augustus Lloyd, “Account of Levellings Carried across the Isthmus of Panama, to Ascertain the Relative Height of the Pacific Ocean at Panama and of the Atlantic at the Mouth of the River Chagres Accompanied by Geographical and Topographical Notices of the Isthmus,” Philosophical Transactions of the Royal Society of London, 1830, 120:59–68. Lloyd’s profile of the Isthmus of Panama. As potential reference points they have a haptic tangibility that went missing with the introduction in the early nineteenth century of the mathematized mean sea level. 3 High and low tides can in fact be-almost physically-appreciated in the moment. As late as 1828, when the first interoceanic leveling in Panama was ordered by Simon Bolivar, the British surveyor John Augustus Lloyd-aware of the confusion around the exact definition of the mean level of the ocean-decided, rather than sticking to one specific point or average, to compare the levels of the Pacific and Atlantic oceans at various heights, registering how much they differed at both spring low and high tide and extrapolating from there the seas’ average difference (see Figure 1). 2 Extremely rare were instances in which it was defined explicitly with more precision.
While there has been a widespread tendency to refer to the average level of the sea, it was usually assumed-especially, but not exclusively, when measuring height by barometric means-that one would be able to determine it just by standing along the littoral, on a ship offshore, or on the frozen surface of a sound, without the need for any actual measurement. What sea level is, how it can be determined, how it can be compared diachronically, or which of its many incarnations it is best to refer to are all matters that for a long time have not been precisely defined or agreed on. In this essay I aim to tell how the early history of the idea of sea level as a plausible reference point is rooted in a scientific debate about sea-level variations that predates anthropogenic climate change. These histories can be understood only in conjunction with each other. As will be shown, at the local scale specific needs have determined which materially perceivable level of the sea ought to be used as a reference point, while on the global scale the inherent variability of the oceans has favored the development of the idea of an abstract and normalized mean sea level. Sea level and the scholarly debate about its long-term modification have a conceptual history, alongside a material history of changing landscapes, subsiding land, and rising water. 1 Its being so familiar means that sea level has become essentially naturalized-that is, we mention it, in daily life, without really thinking about what it truly means and tend to forget that mean sea level is a product of specific historically and culturally determined assumptions. It was first described as a crucial indicator of global environmental change in the early 1980s, but various locally and nationally defined mean sea levels served as the standard vertical datum long before that. Global mean sea level is the most widely adopted metric for assessing long-term changes in the relative positioning of land and sea.
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