CoAuthors

Benjamin Hamlington (Old Dominion University, USA); Felix Landerer (NASA JPL, USA); Surendra Adhikari (NASA JPL, USA)

Long tide gauge (TG) records provide invaluable information about 20th century sea level rise. It is not straightforward, however, to infer the rate of global ocean volume change from these records, because TGs also capture local sea level change due to ocean dynamics, land motion, and changes in Earth's gravitational field. Simply averaging 20th century trends from a small number of long, high-quality TG records typically produces `global' rates of 1.5-1.8 mm/yr. In contrast, a probabilistic estimate of global mean sea level (GMSL) rise that combines TG records of various lengths with information about the spatial structure in sea level change produces a lower rate of 1.2 ± 0.2 mm/yr (Hay et al., 2015). Taken together, these findings suggest that the best TG records are poorly located for capturing the global mean rate and tend to sample the ocean at locations where sea level rose faster than average during the 20th century. Here, we use observations and simulations of spatial structure in sea level change to estimate the likelihood that ocean dynamics and/or geodetic processes result in historical trends from the longest and highest-quality TG records that are systematically greater than the true rate of GMSL rise. On the contrary, we find these gauges are more likely to underestimate – not overestimate – the 20th century global mean rate.