The record of formal sea level observations includes tide gauge measurements dating back to the 19th century and precise satellite altimetry measurements that began in the latter part of the 20th century.   (Images credits: NOAA, NASA)


One of humankind’s earliest attempts to keep long-term records of rising seas began in ancient Venice—appropriately enough, given its forced adaptation to slow inundation over many centuries [Carbognin et al. 2009]. The evolution of measurement technology soon accelerated. Marks cut into building walls gave way to tide gauges in the 19th and 20th centuries and, by the dawn of the 21st, satellite altimetry, with gravimetry soon to follow. Proxy records, meanwhile, had been kept over thousands of years by ocean organisms, sedimentary deposits and the action of waves against rocks and salt marshes, awaiting scientists who could learn to read them. Archaeological sites were interrogated for data on sea-level shifts, among them the Roman columns of the Phlagrean Fields. They revealed, via the borings of marine organisms, that this ancient marketplace had been submerged and uplifted over millennia [Morhange et al., 2006].

Critical realizations included the importance of vertical land motion in assessing changing sea levels, as well as the seasonal, year-to-year and decadal variations in regional climate, interwoven with the longer-term signal of global sea-level rise. Sea level observations today yield a rich tapestry of overlapping effects, though its threads must be unraveled to trace individual contributions to the larger mosaic.