DISCUSSION ON: Robust validation of trends and cycles in sea level and tidal amplitude in the Dutch North Sea

The paper “Robust validation of trends and cycles in sea level and tidal amplitude in the Dutch North Sea” by Hessel G. Voortman published in the Journal of Coastal and Hydraulic Structures reconstructs mean sea level time series from tide gauge data and analyzes these data. It concludes that "Higher-order terms in time, expressing acceleration of the rate of rise, were found to be insignificant". This conclusion appears to contradict a paper that we published recently (Keizer et al. 2023). We show that this discrepancy arises from multiple issues in Voortman (2023): errors in the reconstruction of mean sea level from tide gauge data (section 2.1), an inappropriate framework for testing (section 2.2), errors and suboptimal choices in designing the statistical model (section 2.3). These issues invalidate the conclusions regarding the detection of an acceleration in sea level and exaggerate the mismatch between observations and Intergovernmental Panel on Climate Change's Sixth Assessment Report (IPCC AR6) sea level projections over the period 2015-2022 (section 2.4). Additionally, the author makes a number of false claims about Keizer et al. (2022), the preprint version of Keizer et al. (2023), which we point out and correct here (section 2.5).