Our Very Strange Search for “Sea Level”

Parts of California’s Central Valley have fallen by nearly thirty feet as the aquifers beneath them have emptied, and areas of Louisiana, robbed of the Mississippi River sediment that once counteracted erosion, are sinking by as much as three-quarters of an inch per year.

A gravity anomaly in the Indian Ocean means that a large swath of its waters—an area nearly as large as India itself—has a top level that’s as much as a hundred and six metres lower than the global average.

We know that, because of the way the Earth bulges as it spins, the water levels of equatorial seas are some twenty-one kilometres higher than the sea ice at the North Pole. (This bulge also complicates how we think about mountain heights, since the peak of Mt. Chimborazo, rising close to the equator, is actually farther from the center of the plane... See more

The quest to find a mean sea level for the whole world was part and parcel of the quest to make the world governable. And thus, von Hardenberg writes, did our idea of sea level as a benchmark emerge from a very specific time and place, becoming intertwined with the colonial project and “a long-held perception of holocenic stability.”

Lyell turned out to be right: the Scandinavian peninsula was, and still is, rising relative to the sea, as a result of the phenomenon known as “postglacial uplift”—the slow bounce-back of land that was long compressed by the great weight of earlier glaciers.

People sometimes marked these extremes with rocks that could stand as warnings for a future that was in danger of forgetting the past. We know them now as tsunami stones and hunger stones. A famous one in the Elbe, commemorating a terrible drought, is carved with the words “If you see me, then weep.”

Between Nils’s time and Celsius’s sketch of the rock, in 1743, the water level had dropped nearly eight feet. Nils’s island was gradually becoming part of the mainland; today, it is a peninsula. In 2012, the scholar Martin Ekman located the rock that made Nils rich; by then, it was no longer in the sea at all. It stood in a young forest, surrounded... See more