|Review of “Sea-level rise in Venice: historic and future trends”|
This is the second round of review of the paper, and the authors have substantially improved the manuscript, and I only have some minor comments left. The exception to this is the section on sea-level projections (section 6.2), which still contains a substantial number of confusing statements with regards to how sea-level projections are made.
When section 6.2 is properly revised and the other minor issues have been taken care of, I think the paper can be published.
Remarks on section 6.2
L710: The SROCC report also provides regional sea-level projections based on CMIP5 model output. Now the text suggests that some sort of down-scaling is needed from these projections, while they are already available in gridded format. These projections form the basis of Kopp et al. (2014) and follow the methodology outlined in Slangen et al. (2012).
The big question, which is touched upon, but not answered in this section, is whether the CMIP5 models and other projections capture the relevant processes that cause sea-level changes in Venice. Can anything be said about this question?
L712: “Here “likely” corresponds to the IPCC uncertainty language, meaning that the probability of future sea-level change within this range is estimated from ≥66% to 100%, and therefore does not exclude values outside this range” What does this mean?
L716: “Deep uncertainty” is a very specific term with a specific definition. Suggestion to remove it here.
L717: “Therefore”: how does this sentence follow from the previous? And where does the “up to 2m” come from?
L720: “Slangen et al. (2017) suggest”: That paper doesn’t “suggest” that number, it shows model results, and this number comes from model results and is not a ‘suggestion’.
L722: The coupled climate models used for CMIP3/AR4, CMIP5/AR5 do simulate the Mediterranean Sea directly. See for example Landerer et al. (2007, doi: 10.1175/JPO3013.1). Sterodynamic effects are computed directly by these models (the ‘zos’ and ‘zostoga’ variables in CMIP5/6 models) and do not need to be computed offline. The same holds for the associated water mass redistribution: this effect is also included in these models and is stored as variable ‘pbo’.
L723: “Pioneering work in this regard is Slangen et al. (2012)” the pioneering work in this paper is not about the ocean models and statistical downscaling: it’s about combining ocean sterodynamics from CMIP models with GRD effects and GIA to make regional sea-level projections that include all relevant processes.
L742: As noted in the previous round: there’s no reason to assume that GMSL is equal to sea level in Venice, so there’s no ‘consistency’ when both numbers are close. Suggest to replace ‘consistent with’ by ‘on the same order as’ or ‘similar to’.
L747: “which is ignored in the computation of the pure steric effect”: This might be a bit of a strawman argument: to my knowledge, no projections just use the steric effect to approximate the total sterodynamic (steric + bottom pressure) effects. See also my comments for L722
L764: “Their projections build on the decomposition of the recorded historical sea level into several processes, including the “background non-climatic local sea-level change” corresponding to GIA, tectonics, and other non‐climatic local effects.” This is not an accurate description of the Kopp et al. (2014) framework. They use the AR5 projections for most terms (or switch to an alternative projection for the ice sheets in the Kopp et al. (2017) update) and use a statistical framework to estimate the non-climatic component at each tide-gauge location.
L779: “The Mediterranean sterodynamic sea-level projections are estimated by relying on those of the Atlantic area near Gibraltar.” This is an interesting remark, as it’s a much ‘coarser’ approximation than the coarse CMIP models. Is it a better approach and do the CMIP models suggest something different? From Figure 7 in Slangen et al. (2017) that doesn’t seem to be the case.
L787: I think I know what is meant by “added linearly” but it might be clearer to write out the equation for the combination of the uncertainties.
L788: “The projections do not include expert elicitation and rely only on IPCC-like assessments, so the RCP2.6 is rather symmetric and RCP8.5 slightly asymmetric.” The IPCC process is pretty close to an expert elicitation, and the link to the symmetry/asymmetry of the uncertainties is not related to that per se.
The definitions on L69ff:
“The acronym RSL is therefore used for tide-gauge data”: I’d say: ‘Tide gauges typically measure RSL’ instead. Same for altimetry: they measure GSL.
“VLM-corrected RSL” That is the same thing as GSL: GSL = RSL + VLM. Like above: tide gauges corrected for VLM measure GSL.
GMSL is spatially-averaged RSL.
L443: ‘consistency’. I think the authors mean ‘similarity’ here. GMSL and sea level in Venice may have a similar trend, but that has nothing to do with ‘consistency’.
L445: “to put local changes in the context of global mean changes”. Vague and a circular reasoning. It now reads like it instructive to compare both, in order to see if they’re comparable.
L451: ‘connection’ as said above, there’ no connection. Only similarity. Check also the sentences after L450 for similar suggestions of connections/discrepancies etc.
L570-L575: “The two-way water exchange regime…” This and the following sentence are vague and I don’t understand what’s being said here. What message should I get from these sentences?
L582: “strengthened by steric changes since the late 1950s”: what does that mean? From Frederikse et al. (2020): “Before the end of the 1950s, in situ observations are too sparse to derive unbiased steric changes”. How do we know they did not play a role before the 1950s?
L885: “would be reliable only in the basin mean tendencies”: where does this conclusion come from? Same from the next sentences. A citation or an experiment to prove these statements is needed.
L888: “Improved assessment and progress is hoped in this direction as well”. Hope is the mother of disappointment. Suggest to remove this.
L889: Good et al. (2013) and Ishii et al. 2017 are not about new observations, but they’re about optimal interpolations of in-situ temperature and salinity profiles.
L896ff: Circling back to the first round of review. What would be the outcome of ‘further research’ on estimating a trend in a sea level record? I’d say that ‘The shape of the local sea level trend’ is just the first derivative of the time series of local sea level. You can decompose that time series into contributions from various processes (wind, subsidence, ice melt etc.), but I can’t see what approaches like SSH or EMD can add. while these methods often create more confusion than that they solve, see for example https://npg.copernicus.org/articles/22/157/2015/.