第 29 题

1The peak of Mount Qomolangma, 8,849 meters above sea level. Chimborazo, 6,263 m; Mont Blanc, 4,806 m; Pradidali hut, at the foot of the Pala group in the Dolomites, 2,278 m. It is common practice to provide the elevation (高度) of a place. Simple readings are now readily available on our phones and wearables. But elevation is necessarily relative: only the choice of a reference point allows us to express numerically the height of an object or a location.

2The idea of sea level as a reference point for elevations has by now been around for so long as to go essentially unnoticed-we mention it without even considering what it means. That the concept has a history is easily ignored. We tend to forget that sea level is a product of technically and culturally determined assumptions. In this book, I tell a story of these assumptions.

3Appropriately, the book took shape during a summer spent between sea and mountains. A month after visiting the French Riviera and enjoying the sandy beaches of Nice and Menton, I went hiking with friends in the Pala group. Resting on a pass near the Pradidali hut, we began discussing the readings offered by our altimeters (测高仪). Why did they vary ever so slightly from one instrument to the next? This sense of uncertainty led me to think about the meaning of zero. This is how I began my search for the conceptual history of sea level.

4In this book I examine three crucial stages in the history of mean sea level. These stages mirror the development of human conceptions of the sea. Since I began thinking about this project in August 2011, the issue of sea-level rise and the need to place it historically have only become more urgent: between then and the end of 2022 the global sea rose by almost 5 centimeters, about half of the overall rise recorded by satellites since 1993.