Are Near-Coastal Sea Levels Accelerating Faster Than Global during the Satellite Altimetry Era?

Round 1

Reviewer 1 Report

I distinguish two aspects in the text of the work. One of them refers to the processing of data obtained by satellites. It seems to me to be a very detailed and precise work, sometimes too precise to be considered as real. But that is scientifically acceptable, at least as a methodology for treating the acceleration of sea level variation. But I miss greater rigor in terms of justifying the causes why variations in sea level occur. Essentially, according to the authors, they are due to the melting of glacial ice due to Climate Change.Essentially the rises and falls in sea level do not depend only on the melting of ice. They also depend on the temperature of the water, on the deformation of the lithosphere due to the increase in weight of the water that gravitates on it (hydroisostasy), or on the decrease in the weight of the ice that gravitates on the continents (glacioisostasy). Or by the decrease or increase in weight produced by the accumulation of sediments that accumulate, or are eroded, on the lithosphere. But there are other factors that produce rises or falls in the lithosphere and that, consequently, are reflected in the sea level. In zones of plate collision they cause uplifts of the coastal edge. All these factors must be mentioned and collected, at least, in the background of the work. It is already quite risky to assume that the melting of glacial ice is due to Climate Change and ignore that we are in an interglacial, which means accelerated ice melting for thousands of years. Therefore, at least those factors that also affect sea level must be mentioned even if they are not taken into account in the calculations of acceleration in the rise of sea level.We emphasize this circumstance because it conflicts with the recommendations added in the conclusions "Sea level rise along the global coastline will increase the frequency of flooding and intensify the risks for the millions of people living in low-lying coastal areas. Policy makers and planners require a robust estimate of the rate of sea level rise and its acceleration for adequate adaptation options"We think that it is obvious to recommend to the affected countries what they should do if sea level rises or falls. In essence, the objective of the work is to find out, as precisely as possible, what is the speed of rise in sea level and the acceleration of that movement. It would also be convenient to show that there are some areas of the coast of the World where the sea level is falling rapidly due to glacioisostasy. Perhaps, if taken into account, the authors could explain some of the anomalies that they have detected in their interpretation of the data provided by the satellites.

Author Response

Please see attached a file (Reply to Reviewers' comments)

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Reviewer 2 Report

1.       Abstract: By incorporating these suggestions, the abstract can be further improved in terms of clarity, context, and the overall presentation of the research findings.

·Clarify the research objective: Begin by clearly stating the objective of the study in a concise manner. For example, "The objective of this study is to analyze regional sea level acceleration near coastal areas compared to global acceleration during the satellite altimetry era and identify potential drivers for regional sea level acceleration."

·Provide context and significance: Briefly explain why understanding regional sea level acceleration is important for impact and risk assessments in coastal areas. Emphasize the relevance and implications of the research.

·Clarify the methodology: Provide a concise overview of the methodology used to estimate regional sea level acceleration. Specify the data sources and analytical techniques employed.

·Present key findings: Clearly summarize the main findings of the study, highlighting the range of regional acceleration estimates and the comparison to global mean sea level acceleration. For instance, "The study reveals a wide range of regional acceleration estimates, varying from -1.2 to 1.2 mm·yr-2, which can be up to 20 times larger or smaller than the global mean sea level acceleration of 0.07 mm·yr-2. Notably, sea level acceleration near the global coastline is calculated at 0.10 ± 0.03 mm·yr-2, exceeding the global mean sea level acceleration by 40%."

·Discuss implications and contributions: Provide a brief discussion of the implications of the findings and their contribution to the existing body of knowledge.

2.       Introduction:

·Clearly state the global nature of climate change and sea level rise, emphasizing their impact on coastal areas.

·Highlight the significance of coastal areas due to high population densities, socio-economic activities, and vulnerable ecosystems.

·Emphasize the need for understanding regional sea level acceleration for developing projections and making informed decisions about adaptation options.

·Provide a brief overview of the existing knowledge on global sea level rise and acceleration, including recent trends and contributions from ice mass loss and ocean heat uptake.

·Address the uncertainties and limitations in estimating regional sea level accelerations.

·Highlight the need for a better understanding of the drivers behind regional variations and the challenges in interpreting and discussing coastal sea level acceleration.

·Clearly state the objectives of the study, including the use of satellite altimetry data to analyze regional and near coastal sea level accelerations.

·Mention the intention to identify areas with acceleration, discuss potential drivers, and address challenges in estimating near coastal sea level accelerations.

·Acknowledge the importance of effectively communicating the findings to end users for adaptation purposes.

3.       Materials and methods: By incorporating the following suggestions, the Materials and Methods section will provide a clearer and more comprehensive understanding of the data sources, analysis techniques, and statistical models employed in the study.

·Explain the purpose and significance of calculating regional sea level acceleration using satellite altimetry observations, providing context and motivation for the data selection and analysis.

·Specify the sources of steric sea level data, highlighting its advantages and relevance to the study. Describe how the IAP steric sea level data is obtained and any data processing techniques or corrections applied.

·Include information on the resolution and coverage of the IAP steric sea level data, specifying the depth range (upper 2000m) covered and mentioning any data gaps or limitations.

·Briefly explain the ENSO and PDO climate indexes and their relevance to sea level acceleration. Describe how these indexes are calculated and provide the source references for obtaining the data.

·Clarify the reasons for not applying corrections for Pinatubo in the gridded data sets of altimetry and steric sea level time series, explaining the impact of volcanic eruptions on sea level and the rationale for excluding the corrections due to lack of regional pattern information.

·Provide a concise overview of the regression model used to estimate regional and global mean sea level acceleration, emphasizing the removal of annual and semi-annual cycles prior to analysis.

·Clarify the meaning and interpretation of coefficients ?1 to ?3 in Equation (1) and their relationship to sea level acceleration calculation. Specify the statistical method used for estimating these coefficients (least-squares fit) and mention the software or programming language (MATLAB) utilized.

·Explain the calculation of acceleration as twice the quadratic coefficient ?2 and justify this approach. Briefly describe the estimation of confidence intervals at the 95% level using the polyparci function in MATLAB.

·Provide a brief summary of the linear regression model used to determine the contribution of internal variability to regional and global mean sea level acceleration. Explain the coefficients ?0 to ?4 in Equation (2) and their relationship to the climate indexes (MEI and PDO). Specify the reference (Moreira et al., 2021) for the application of this model.

·Mention any assumptions made in the analysis, such as assuming the residuals are pure noise following Veng and Andersen. Explain the rationale behind these assumptions and their potential impact on the results.

4.       Results:

·Provide more interpretation of results rather than just describing outputs. For example, discuss implications of regional variations compared to global.

·Include statistics/figures where possible to support findings rather than just stating results.

·Improve flow between paragraphs by linking results back to objectives/questions stated in intro/methods.

·Highlight key conclusions and takeaways at the end to summarize what was learned.

·Use past tense consistently when describing results/findings.

·Check language for clarity, conciseness and passive to active voice.

·Ensure consistency in formatting/labeling of figures, tables across text and caption.

·Consider dividing content into regional subsections for better organization (Pacific, Atlantic etc).

·When describing outputs, provide context on what each figure/table shows (e.g. "Figure 1 maps regional acceleration patterns across the study area calculated from satellite altimetry data.").

·Highlight any unexpected/interesting regional patterns observed rather than just stating results globally.

·Discuss similarities/differences between altitude, steric and their difference results to aid interpretation.

·Explain physical explanations for observed acceleration patterns based on known ocean dynamics/climate drivers.

·Analyze impact of removing variability - does it create more uniform patterns? Changes acceleration magnitude/significance where?

·Statistical analysis could help quantify differences e.g. percentage increase in coastal vs global acceleration.

·Correlate results with other studies to further validate findings.

·Consider adding validation with in-situ tide gauge records where available.

·Qualify conclusions by discussing limitations/uncertainty in analysis methods or data.

·Weave conclusions back to research objectives/questions at each subsection.

·Ensure consistency in units used when comparing results.

·Provide context on time periods analyzed to interpret evolution of patterns over time.

5.       Discussion: By incorporating these suggestions, the discussion section becomes more comprehensive, clearer, and provides a better understanding of the research findings and their implications.

·Provide context and emphasize the implications: Start the discussion section by providing context for the study and highlighting the implications of the findings. Emphasize the threat posed by the higher rate of sea level rise in coastal areas due to global warming and the need for a better understanding of coastal processes for adaptation purposes.

·Explain the reasons for faster acceleration near coastal locations: Discuss the various near coastal processes and ocean adjustment mechanisms that contribute to the faster acceleration observed in these areas. Mention factors such as local atmospheric effects, coastal trapped waves, baroclinic instabilities, shelf currents, and fresh water transport in estuaries.

·Acknowledge limitations of satellite altimetry data: Address the limitations of satellite altimetry data near the coastline, including land contamination and the lack of optimized geophysical corrections. However, also highlight the successful applications of satellite altimetry data in various coastal studies.

·Discuss the need for longer and improved data records: Emphasize the need for longer records of high-resolution satellite altimetry data and improved corrections to enhance the understanding of coastal, regional, and global sea level acceleration. Highlight that the relatively short period of satellite altimetry data limits the present understanding.

·Explore the impact of natural variability: Discuss the impact of natural climate variability, such as ENSO and PDO, on sea level acceleration. Explain how the removal of natural variability reveals changes in acceleration patterns and highlight the consistency with significant climate shifts, such as the PDO in the Pacific Ocean.

·Provide explanations for acceleration patterns in specific regions: Explain the observed acceleration patterns in specific regions, such as the weakening of spatial acceleration patterns in the Indian Ocean and the strengthening of regional acceleration patterns in the North Atlantic Subtropical Gyre region. Discuss possible mechanisms driving these patterns, such as the decadal Indian Ocean basin mode (DIOB) and surface wind and heat flux associated with climate modes like the NAO and AMO.

·Address uncertainties and limitations: Acknowledge the large uncertainties in observations and the variability in relatively short time series, which contribute to the lack of a robust estimate for acceleration. Mention the limitations of the study and the challenges in interpreting observed changes due to the complex interactions of climate signals.

·Highlight long-term trends: Discuss the long-term trends observed in steric sea level data prior to 1993, showing a replacement of areas of positive acceleration with negative acceleration over the past 50 years.

6.       Conclusions:

·Reduce the overuse of citation numbers in the middle of sentences, which interrupts the flow. Move some of the citations to the end of sentences instead.

·Clarify what "it" refers to in "However, it remains an open question...". Consider rephrasing to directly state what remains an open question.

·Explain acronyms before using them - e.g. "Pacific Decadal Oscillation (PDO)" and "El Niño Southern Oscillation (ENSO)".

·Simplify wordy phrases like "acts to amplify" and "considering the ocean dynamic variability that in regional scale acts to amplify" for clarity.

·Make sure connections between ideas flow logically. For example, explain how the regional acceleration relates to the open question in the last sentence.

Conclude by summarizing the key takeaways from the study results rather than introducing new discussion points. What are the main conclusions and implications from this research?. The key takeaways and implications from this research that could be included in the conclusion may be the following: 1. Satellite observations show the acceleration of sea level rise along the global coastline exceeds the global mean acceleration by 40%, highlighting greater risks for coastal regions. 2. Regional ocean dynamics, rather than ice melt, appear to be the key driver amplifying sea level acceleration along coastlines. 3. It remains uncertain whether the high coastal accelerations reflect a persistent trend or natural variability. Constraining this uncertainty is critical for coastal adaptation planning. 4. Ongoing monitoring of regional sea level change is needed to differentiate the contributions of long-term climate-driven trends versus ocean variability. 5. Improved understanding of the processes contributing to spatial differences in sea level acceleration will allow refinement of local projections and mitigation strategies.

Overall, the quality of English language usage in the article is high, effectively conveying scientific concepts and findings.

Author Response

Please see attached file (Reply to Reviewer 2)

Author Response File: Author Response.docx

Reviewer 3 Report

In this study, the authors analyzed the near coastal sea level acceleration compared with global 13 during satellite altimetry, and discussed the potential drivers for regional sea level acceleration. This is a good work to evaluate the near coastal sea level acceleration combined with multiple data sources.

My main comments are as follows.

1. Several datasets from different sources are used in this analysis. Here, the spatial resolution and observation time of the original datasets should be provided.

2. There are many expression errors in the paper, such as“…make it comparable of sea level acceleration 83 between near coastal and global as they have the same corrections. 84”.

3. What is the difference between “Regional and near coastal sea level acceleration”?

4. “Using satellite altimetry, we have calculated a global mean sea level acceleration of 128 0.07 ± 0.01 mm·yr-2.” “acceleration of 0.08 ± 0.01 mm·yr-2 from Veng and Ander-130 …”. The satellite altimetry is the key data source in this work, however, what is the accuracy of the data source? Is it possible to obtain the acceleration value by using the satellite altimetry?

5. How to obtain the conclusion “Regional acceleration in most locations is 10-20 times larger/smaller 134 than the global mean number (Figure 1a),”?

6. How to obtain the conclusion “There is an agreement between the patterns for steric sea level acceleration calculated 141 using IAP steric sea level data sets from Cheng et al, 2017 [26] and for sea level acceleration 142 from satellite altimetry (Figure 1 a, b, c).”? 

7. I can not understand the contents in “…be divided by subheadings. It should provide a concise and precise 158 description of the experimental results, their interpretation, as well as the experimental 159 conclusions that can be drawn.”.

8. “Global mean sea level trend, calculated from satellite altimetry along the coastline, is 182 3.46 ± 0.3 mm·yr-1”, while “global near coastal sea level acceleration decreases from 0.10 ± 0.03 mm·yr-2 to 0.09 ± 187 0.03 mm·yr-2,”. That is, the sea level change in one year is only 0.10 ± 0.03 mm·yr-1. Is it possible to obtain such value?

9. “the satellite altimetry patterns of acceleration for moving 20-239 year time window by one year since 1993” are different in different time spans. Why? This probably means that the acceleration is not regular on a global scale. Then, what is the destination of the work?

10.   The contents in Discussion part should be further improved.

Extensive editing of English language required

Author Response

Please see attached a file (Reply to Reviewers' comments)

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

I am pleased to see that the authors have addressed many of the concerns raised in my initial review and made a concerted effort to improve the manuscript. Specifically:

• The abstract has been reworked to more clearly convey the objectives, methods, and key findings. This enhances engagement and clarity.

• The introduction now provides better context on the threat to coastal areas from sea level rise, gaps in understanding regional acceleration, and the significance of this work.

• Explanations of the data sources, analytical techniques, and statistical models are expanded in the methods section, imparting more methodological detail and rigor.

• The results section includes more insightful interpretation of the findings, improved flow, and linkage back to the research aims. Additional statistics, figures, and comparisons strengthen the presentation of results.

• The discussion explores the implications more thoroughly, including examining the impact of natural variability and limitations. Regional acceleration patterns are also explained in greater depth.

• The conclusions focus on clear takeaways from the research.

These revisions have strengthened the manuscript considerably. The scientific analysis appears sound and the study represents a valuable contribution to the field.

Reviewer 3 Report

The authors have addressed all my concerns, and I do not have further comments or suggestions. I recommend that the manuscript be published in its present form.