Simon Engelhart
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sealevelchange.bsky.social
Simon Engelhart
@sealevelchange.bsky.social
390 followers 320 following 4 posts
Department of Geography, Durham University. Interested in sea-level and climate change across all spatial/temporal scales and geologic records of past subduction zone earthquakes and extreme events https://www.durham.ac.uk/staff/simon-e-engelhart
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Reposted by Simon Engelhart
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So I & 14 colleagues (incl. @richardtol.bsky.social @vsrikrish.bsky.social @goneri76.bsky.social) wrote an expression of concern, laying out the problems with the paper and calling for its retraction. We first provide scientific context, and the paper's failure to engage with related studies. 2/
We are writing to raise serious concerns regarding Voortman, H. G., & De Vos, R. (2025). A Global Perspective on Local Sea Level Changes. Journal of Marine Science and Engineering. Voortman & de Vos (2025) attempt to test for acceleration in relative sea-level (RSL) records from 243 tide gauges. They claim to have identified statistically significant acceleration in only 12 of the 243 gauges and conclude that “This pattern is inconsistent with sea level acceleration driven by global phenomena.” This result, which stands in contrast both to other studies conducting similar tide-gauge analyses (e.g., Wang et al., 2021, 2025) and the broader body of knowledge regarding global-mean sea level change and its drivers (e.g., Fox-Kemper et al., 2021), demands scrutiny. Yet the reviewers failed to recognise the context of this manuscript and the breadth of the existing literature, resulting in at best a superficial review. Upon close examination, we believe the paper contains fatal methodological flaws that compromise the validity of its conclusions and merit retraction. Scientific Context Comparison to other studies of RSL acceleration at tide-gauges and IPCC projections: The conclusions of this study are dramatically different from those of Wang et al. (2021, 2025). Wang et al. (2025) concluded significant acceleration was present at 68% of tide gauges, while this paper identifies it at only 4%. It is surprising that the reviewers did not insist that the authors provide a discussion to explain how they came to such different results, rather than ignoring the substance of Wang et al. (2021) and the existence of Wang et al. (2025). (We describe some likely statistical reasons below.) Notably, the authors state that “a comparison [of Intergovernmental Panel on Climate Change (IPCC) projections] to locally observed sea level appears to date to have been undertaken on a very limited scale”, and have made similar claims to the media, even though they cite without discussion Wang et al. (2021), which focuses precisely on a global-scale comparison of local observations and IPCC projections from 2013 and 2019 reports. They do not cite Wang et al. (2025), which extends this analysis using the same 2021 IPCC projections that are employed in this paper. Voortman and de Vos (2025) find that the 2021 IPCC median projected rates of relative sea level rise over 2010-2030 are higher than their preferred estimates of 2020 tide-gauge long-term rates by an average of about 2 mm/yr. Because the authors’ statistical tests reject the presence of acceleration at the large majority of tide gauges, they are effectively comparing IPCC projected rates — which account for the factors driving global-mean sea-level acceleration — with average rates over the length of the tide gauge record. By contrast, Wang et al. (2025) find that tide-gauge trends and accelerations extrapolated to 2050 agree with IPCC likely range projections at 96% of locations. Similarly, the Voortman and de Vos (2025) results are also in disagreement with regional studies where model-observation comparisons have been undertaken. For instance, Dangendorf et al (2023) report tide-gauge based rates of sterodynamic sea-level rise that are in 2020 at least twice as large as those from climate models along the U.S. Southeast and Gulf coasts, while Voortman and de Vos (2025) report IPCC rates that exceed tide gauge observations at the same locations (their Figure 9). The only exception in their model is Pensacola, which appears as an outlier in their assessment. Given the coherence of observed changes along this coast (Dangendorf et al., 2023), this also raises questions why their statistical model yields so different results for neighboring sites. Voortman and de Vos (2025) do not discuss any of these discrepancies. Comparison to other constraints on sea-level acceleration driven by global phenomena: In as much as the authors’ goal is to test for “sea le The authors also engage minimally with research on the global-mean sea level budget since 1900 (c.f. Fox-Kemper et al., 2021), providing a brief dismissal of one paper: “The current state of affairs is best illustrated by the paper by Frederikse et al. [(2020)] that claims to have “closed the sea level budget”. Unfortunately, a detailed inspection revealed a number of weaknesses and even errors in Frederikse’s work.” This statement is backed only by a reference to the first author’s own work (Voortman, 2023), which does not substantively engage with Frederikse et al. (2020). Regarding Frederikse et al. (2020), Voortman (2023) simply writes about his feelings: “Albeit Frederikse’s paper is an impressive scientific achievement, I feel that this conclusion is too rash. Estimating that the number of unknowns in this problem is larger than the number of equations available, I tentatively suggest that the reconstruction by Frederikse et al. is one of many plausible reconstructions that close the sea level budget.” No more “detailed inspection” of Frederikse et al. (2020) is provided in this paper or the cited source. The authors also highlight Munk (2002)’s enigma, which was an apparent inconsistency between estimates of 20th century global-mean sea-level rise compared with constraints on changes in Earth rotation and the sea level budget, but miss subsequent work resolving it. This inconsistency was subsequently resolved by newer estimates of 20th century global-mean sea level rise, improved modeling of glacial isostatic adjustment and its effect on Earth rotation, and improved interpretation of the constraints on Earth rotation from eclipse records (e.g., Dangendorf et al., 2017; Frederikse et al., 2020; Hay et al., 2015, 2016; Mitrovica et al., 2015). While the paper’s inadequate engagement with current scientific literature raises serious concerns, its statistical errors and biases render it internally invalid. Since these errors and biases fundamentally shape the paper, they cannot be corrected without complete rewriting and therefore necessitate the paper’s retraction. An examination of the reviewer comments reveal there has been no serious statistical evaluation of the methods or their appropriateness, or of the results. 1. Issues with model specification The paper hinges on the comparison of two models: one linear in time (no acceleration), and one with a changepoint after which quadratic and cubic terms (and thus acceleration) are present. The authors’ model with acceleration has 3 more fitted parameters than their no-acceleration model (a quadratic coefficient, a cubic coefficient, and a change point). We are not aware of any previous assessment of the third derivative of sea level (the cubic term), and a minimally larger model with acceleration requires only one fitted parameter (a quadratic coefficient; the change point could be fixed at, say, 1970 for consistency with other literature). Because hypothesis testing metrics penalize the loss of degrees of freedom, using three additional parameters raises the threshold of detection for acceleration more than is necessary and increases the risk of false negatives. It is also well known that interannual/decadal variability is one of the largest signals that makes detecting an acceleration in single time series challenging. However, there is no attempt to limit the impact of this variability on the detection of any acceleration (contrast Wang et al., 2021, 2025), and this critical issue is not even discussed. 2. Issues with model estimation 3. Invalid use of the F-test for hypothesis testing
September 9, 2025 at 2:13 AM
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