Altimeter sea level from 2018 to mid-2026 stayed within noise of the Nerem-2018 quadratic path (mean error -1 mm), but the 2025-26 level was 5-10 mm below the quadratic fits

Question: did global mean sea level (GMSL) after 2017 follow the quadratic extrapolation of Nerem et al. (2018), and was a quadratic a better 8-year forecast than a linear trend? Approach: we fitted forecasts to the exact University of Colorado (CU) release 2018_rel1 (1993-2017; raw quadratic 0.097 mm/yr^2, the paper value) and compared them with CU 2026_rel2 (to 2026.46) and NOAA LSA (to 2025.12), relative to the 2013-2017 mean. Results: the published climate-driven path (2.9 mm/yr at 2005, 0.084 mm/yr^2) had mean error -1.2 mm (CU) and +0.1 mm (NOAA). The raw quadratic forecast too much rise (last-12-month error -9.7 mm; 3.3 mm of this comes from data reprocessing, which lowered the 1993-2017 acceleration to 0.083). A linear 1993-2017 fit forecast too little (+5.7 to +9.0 mm). The 2018-2026 rate was 3.87 mm/yr (CU) against 4.35-4.75 mm/yr from accelerating forecasts. AR(12) and block-bootstrap simulations of a quadratic law put these errors inside interannual noise (one-sided p 0.12-0.24), and the full-record acceleration fell from 0.088 (to 2020) to 0.074 mm/yr^2 (to 2026), also inside noise. In rolling-origin hindcasts (2003-2018) the quadratic beat the linear fit at only 11 of 31 origins, but at all origins from 2014.5. A diagnostic with GRACE/GRACE-FO mascons shows a slower ocean-mass gain in 2018.5-2026.5 (1.30 +/- 0.25 vs 2.13 +/- 0.13 mm/yr) and faster 0-700 m steric rise. Limitations: 8.5-year verification, simple Nino3.4 adjustment (not the CSEOF method), reprocessed releases, and no closed budget (no atmospheric-mass correction).

sea level · GMSL · satellite altimetry · acceleration · forecast verification · Nerem 2018 · ENSO · GRACE-FO · ocean mass · steric sea level · hindcast · data revision