Evaluating Voortman et al 2025 on Sea Level Rise

A paper was published recently that appears to be making a splash among contrarian influencers. Superficially, it appears to be a study that shows that 95% of tide gauges do not show any statistically significant acceleration, and the remaining 5% have non-climatic explanations for their observed acceleration:

Approximately 95% of the suitable locations show no statistically significant acceleration of the rate of sea level rise. The investigation suggests that local, non-climatic phenomena are a plausible cause of the accelerated sea level rise observed at the remaining 5% of the suitable locations. On average, the rate of rise projected by the IPCC is biased upward with approximately 2 mm per year in comparison with the observed rate.[1]

To evaluate this, it's important to look at what this paper actually does. The study evaluated a subset of two datasets, PSMSL and GLOSS. There are 1548 locations in the PSMSL network, and this study evaluated 204 of them, stating these were the only ones that met the "selection criteria." There are 294 locations in the GLOSS core network, and this study evaluated 39 of them, again stating these were the only ones that met the "selection criteria." There were 28 locations with data both in GLOSS and PSMSL, so this means they used only 215 locations, or less than 15% of the total number of available locations. Here is their selection criteria:

• "Latest year in the dataset not earlier than 2015...
• Data available over a period of at least 60 years
• At least 80% of the years in the range with data available."

They used these criteria to ignore all satellite data. But rather than calculating GMSL from this data and then calculating SLR rates and acceleration, this paper looked for acceleration in the relative sea level data from the selected tide gauges at these 215 locations, and they found that there was no statistically significant acceleration in 95% of these locations. They also concluded that the rate of SLR was smaller than that projected by the IPCC. "The mean rate of sea level rise is 1.4 mm/year, and the median is 1.5 mm/year."

There is, of course, a huge, gaping flaw in this analysis, because the authors did not account for vertical land movement (VLM) at these locations, meaning that this analysis is pretty much useless when used in comparison to observed SLR rates and acceleration globally. A proper analysis should use tide gauge data corrected for VLM and satellite measurements. In another post, I surveyed dozens of studies that detected statistically significant acceleration in both tide gauge and satellite data, since they were not limited by the self-imposed deficiencies in this analysis. Using all the available data, it turns out that tide gauges show more acceleration than satellites, and satellites show statistically significant acceleration. Veng et al 2021[2] used data from both the TOPEX/Poseidon/Jason missions (TPJ) and the European ERS/Envisat/CryoSat missions (ESA). The ESA missions expand the geographic coverage from ± 66° to ± 82° latitudes and to over 27 years of data (1991.7 - 2019.0) over earlier TPJ-based studies. The results of their acceleration calculations were:

ESA (1991-2019 and ± 82° latitude): 0.095 ± 0.009 mm/yr^2
TPJ (1993-2019 and ± 66° latitude): 0.080 ± 0.008 mm/yr^2

And the correctly measured SLR rate from NASA is 4.4 mm/yr. In other words, the self-imposed deficiencies of this study resulted in 1) a failure to detect statistically significant acceleration and 2) miscalculating current SLR rates, which are ~3x faster than this paper calculated. Nevertheless, this paper apparently fooled the Daily Caller and Michael Shellenberger into thinking that the IPCC has been getting the evidence here wrong. Shellenberger even writes, "A new, first-ever global study of real world data, not models, finds no evidence climate change accelerated sea level rise. This is a massive scientific scandal." I expect this kind of nonsense from the Daily Caller, but I wouldn't have expected Shellenberger to be so easily fooled. This is not even remotely the first "global study of real world data" (see my bibliography), and this paper was published in an mdpi journal that is widely regarded as being predatory. Anyone knowing what relative sea level means should be able to see through this paper. If you're interested in a more detailed rebuttal of the statistics in this paper, see Tamino's post.

Törnqvist et al 2025

Interestingly, another paper was recently published in a reputable journal that evaluated the IPCC's projections against empirical data for the last 30 years. This analysis showed that observed SLR since 1993 has slightly exceeded the middle projections of the IPCC. This is from the abstract;

With an acceleration of global sea-level rise during the satellite altimetry era (since 1993) firmly established, it is now appropriate to examine sea-level projections made around the onset of this time period. Here we show that the mid-range projection from the Second Assessment Report of the IPCC (1995/1996) was strikingly close to what transpired over the next 30 years, with the magnitude of sea-level rise underestimated by only ∼1 cm.

The analysis shows that while the overall projection was "strikingly" accurate, the IPCC underestimated the mass loss from ice sheets and overestimated the amount of SLR from thermal expansion: "thermal expansion was overestimated in IPCC-SAR, partly offsetting the low projections for ice-sheet contributions." This kind of analysis from credible scientific journals shows that the overall projections from the IPCC have been on target, but it also provides a helpful correction that could improve projections through the rest of the 21st century.

References:

[1] Voortman, H. G., & De Vos, R. (2025). A Global Perspective on Local Sea Level Changes. Journal of Marine Science and Engineering, 13(9), 1641. https://doi.org/10.3390/jmse13091641

[2] Tadea Veng, Ole B. Andersen. Consolidating sea level acceleration estimates from satellite altimetry. Advances in Space Research 68.2 (2021): 496-503. https://doi.org/10.1016/j.asr.2020.01.016.
https://www.sciencedirect.com/science/article/pii/S027311772030034X

[3] Törnqvist, T. E., Conrad, C. P., Dangendorf, S., & Hamlington, B. D. (2025). Evaluating IPCC projections of global sea-level change from the pre-satellite era. Earth's Future, 13, e2025EF006533. https://doi.org/10.1029/2025EF006533