Sea LevelTraces a commonplace averagesea levelfrom its origins in charting land to its emergence as a symbol of global warming. News reports warn of rising sea levels spurred by climate change. Waters inch ever higher, disrupting delicate ecosystems and threatening island and coastal communities. The baseline for these measurementssea levelmay seem unremarkable, a long familiar zero point for altitude. But as Wilko Graf von Hardenberg reveals, the history of