A Century of Mean Sea-Level Change in Ireland (1925–2024)

Abstract. Understanding mean sea level (MSL) change is crucial for assessing coastal vulnerability and guiding adaptation planning, particularly by identifying regions most at risk. Central to this understanding is the availability of long-term sea-level data. In Ireland, digitized records longer than 40 years are rare and mostly confined to the northeast; however, data archaeology—including the digitization of historical marigrams—can fill spatial and temporal gaps where undigitized records exist. Previous studies have focused primarily on individual sites in the north and east. Here we show that integrating previously undocumented records from the southwest with well-documented datasets provides a comprehensive assessment of MSL change across the country. We find clear regional variability, with the highest mean rates in the south and west. Long-term mean instantaneous rates—representing the modelled rate of sea-level change at each site—vary systematically between regions, reflecting coherent spatial patterns. Rates range from ~1.07 mm yr⁻¹ in the north to 2.48–2.74 mm yr⁻¹ in the south and west, with the highest observed at Cork (2.74 mm yr⁻¹). The regional mean is 1.96 ± 0.1 mm yr⁻¹ for 1925–2024, decreasing to 1.88 ± 0.1 mm yr⁻¹ after accounting for Glacial Isostatic Adjustment (GIA). Annual rates increased from below 1 mm yr⁻¹ during 1925–2000 to above 4 mm yr⁻¹ during 2000–2024, peaking at ~6 mm yr⁻¹ in 2024, highlighting pronounced 21st-century acceleration. After applying atmospheric and datum adjustments and accounting for GIA within a Bayesian framework, these historical records provide a robust basis for reconstructing regional sea-level rise and contextualizing future coastal risk, with implications for coastal planning and adaptation strategies globally.