Reconstructing hourly coastal total sea levels and assessing current and future extreme sea levels threats to the Coast of China – Nature

Report on Extreme Sea Level Threats to the Coast of China in the Context of Sustainable Development Goals

1.0 Executive Summary

This report assesses the escalating threat of climate-driven extreme sea level (ESL) events along China’s coastline, directly addressing the urgent call for action outlined in the United Nations Sustainable Development Goals (SDGs). By reconstructing a comprehensive hourly total sea level (TSL) dataset, this analysis quantifies current and future coastal hazards. The findings reveal a significant increase in the intensity and frequency of ESL events, posing a direct threat to SDG 11 (Sustainable Cities and Communities) and SDG 13 (Climate Action). Under a high-emissions scenario (SSP5-8.5), the current 100-year ESL event is projected to occur every 10 years or less by 2100, undermining efforts to build resilient infrastructure (SDG 9) and protect coastal populations and economies (SDG 1, SDG 8). This report underscores the critical need for immediate, region-specific adaptation strategies to safeguard development gains and advance the 2030 Agenda for Sustainable Development.

2.0 Introduction: Coastal Risks and the Imperative for Climate Action

Global sea level rise (SLR), a direct consequence of climate change, presents a formidable challenge to the achievement of the Sustainable Development Goals. The intensification of ESL events threatens coastal infrastructure, ecosystems, and communities, jeopardizing progress on multiple fronts. This study provides a critical analysis of these risks along China’s coast, offering data-driven insights to inform policies aligned with key SDGs.

• SDG 13 (Climate Action): The accelerating rate of SLR necessitates a robust understanding of its localized impacts. This report provides an essential evidence base for developing adaptation strategies and strengthening resilience to climate-related hazards.
• SDG 11 (Sustainable Cities and Communities): China’s densely populated and economically vital coastal zones are highly vulnerable. Assessing ESL risk is fundamental to making these settlements inclusive, safe, resilient, and sustainable.
• SDG 14 (Life Below Water): Rising sea levels and extreme events degrade coastal ecosystems, which are vital for marine biodiversity and provide critical protection for coastal communities.

The primary objective of this assessment is to reconstruct and analyze historical and future ESL events to support the development of resilient coastal management frameworks that are integral to achieving sustainable development.

3.0 Methodology for Assessing Extreme Sea Level Events

A comprehensive methodological framework was employed to overcome the limitations of sparse tide gauge data and provide a robust assessment of total sea level dynamics. This approach ensures that planning for resilient infrastructure (SDG 9) is based on a complete understanding of coastal hazards.

3.1 Reconstruction of Total Sea Level (TSL)

A continuous hourly TSL time series from 1993 to 2023 was constructed by integrating four key components:

1. Sea Level Anomaly (SLA): Derived from multi-mission satellite altimetry data to capture long-term sea level rise trends.
2. Astronomical Tide (T): Predicted using the FES2014 tide model to account for gravitational effects.
3. Storm Surge (S): Incorporated using the AVISO+ Dynamic Atmospheric Correction (DAC) product to represent meteorological effects.
4. Wave Action (W): Estimated using an empirical formula with ERA5 data to include wave setup and swash, which are often overlooked but critical contributors to extreme events.

3.2 Extreme Value Analysis

The analysis of extreme events was conducted using a statistically rigorous process to ensure reliable risk assessment for coastal planning.

• Peak-Over-Threshold (POT) Method: This method was used to identify independent ESL events from the hourly TSL series. An optimal threshold between the 97.00th and 99.99th percentiles was selected for each location.
• Declustering: A 3-day interval was applied to ensure that clustered peaks from a single storm event were treated as a single occurrence, preventing statistical bias.
• Generalized Pareto Distribution (GPD): The identified extreme samples were fitted with a GPD model using Maximum Likelihood Estimation (MLE) to calculate the magnitude of events for various return periods (e.g., 1-year, 100-year).

3.3 Future Projections and SDG Alignment

Future ESLs were projected by combining the calculated present-day return levels with regional SLR projections from the IPCC AR6 under five Shared Socioeconomic Pathways (SSPs). This forward-looking analysis directly supports proactive climate adaptation planning (SDG 13) by illustrating the consequences of different global emissions trajectories on local coastal safety (SDG 11).

4.0 Findings: A Spatially Variable and Escalating Threat

4.1 Present-Day Extreme Sea Levels

The analysis reveals significant spatial variation in ESL risk along China’s coast, highlighting the need for tailored, rather than uniform, adaptation strategies to protect communities and infrastructure.

• High-Risk Zone (East China Sea): Stations such as Lusi and Kanmen exhibit the highest risk, with 100-year return levels exceeding 4.1 m. These regions face severe threats to urban sustainability (SDG 11) and require urgent investment in resilient infrastructure (SDG 9).
• Moderate-Risk Zones (Yellow, Bohai, and South China Seas): These areas show lower but still substantial 100-year return levels, generally ranging from 1.7 m to 3.7 m. While the immediate threat is less severe, proactive planning is essential to prevent future losses.
• Overall Baseline: Most coastal stations currently experience 1-year return levels exceeding 1.6 m, indicating that minor coastal flooding is already a regular occurrence.

4.2 Future Projections: The Amplifying Effect of Sea Level Rise

Future projections demonstrate a dramatic escalation of ESL hazards, directly challenging the resilience of coastal regions and the viability of long-term sustainable development.

1. Increased Magnitude: Under the high-emissions SSP5-8.5 scenario, the average 100-year return level is projected to rise by 0.83 m by 2100. This increase will push coastal defense systems beyond their design limits, threatening communities and economic assets.
2. Increased Frequency: The return period of historically rare events will shorten drastically. This shift transforms extreme events from generational concerns into decadal or even annual threats.
   • By 2050: The current 100-year event will have a return period of less than 50 years at most locations, regardless of the emissions scenario.
   • By 2100: Under the SSP5-8.5 scenario, the current 100-year event will become an event with a return period of less than 10 years for nearly all stations.

5.0 Conclusion and Recommendations for Sustainable Coastal Development

The findings of this report confirm that climate-driven sea level rise is set to severely increase the risk of extreme coastal flooding along China’s coast, posing a direct threat to the achievement of SDG 11 (Sustainable Cities and Communities), SDG 13 (Climate Action), and related development goals.

5.1 Key Conclusions

• ESL hazards are not uniform; the East China Sea is a particular hotspot requiring immediate attention.
• Climate change will dramatically increase both the height and frequency of extreme events, fundamentally altering the coastal risk profile.
• Without proactive adaptation, what is considered a “once-in-a-century” flood today will become a common event by the end of the century, leading to recurring damage to infrastructure, displacement of communities, and economic disruption.

5.2 Recommendations

To mitigate these risks and ensure a sustainable future for China’s coastal regions, the following actions are recommended:

1. Integrate ESL Projections into Policy (SDG 11 & 13): National and local governments must incorporate these localized, science-based ESL projections into all coastal development, land-use planning, and disaster risk reduction strategies.
2. Invest in Resilient Infrastructure (SDG 9): Prioritize investment in adaptive and resilient coastal infrastructure, including nature-based solutions and upgraded flood defenses, particularly in high-risk zones.
3. Strengthen Climate Action (SDG 13): The stark difference between outcomes under low- and high-emissions scenarios underscores the urgent need for ambitious global mitigation efforts to reduce greenhouse gas emissions.
4. Enhance Monitoring and Research: Continue to support and expand coastal monitoring systems to refine projections and improve early warning systems, thereby strengthening the foundation for adaptive management.

Analysis of Sustainable Development Goals (SDGs) in the Article

SDGs Addressed or Connected to the Issues

• SDG 13: Climate Action

  The article is fundamentally about the consequences of climate change. It opens by stating, “Climate-driven sea level rise (SLR) will intensify extreme sea level (ESL) events” and “Climate change is accelerating the global sea level rise (SLR)”. The entire study is framed as an effort to understand and project these climate-related hazards to “cope with the impacts of climate change” and develop “adaptation strategies.” This directly aligns with the goal of taking urgent action to combat climate change and its impacts.

• SDG 11: Sustainable Cities and Communities

  The research focuses on the threats to China’s coastline, an area characterized by “rapid economic development, especially in its coastal zones.” The article quantifies the human and economic toll of past disasters (“average annual economic loss caused by extreme sea level disasters was RMB 11.7 billion, 156 people died, and 13.4 million people were affected”), directly addressing the vulnerability of coastal communities. The study’s purpose is to inform “coastal defense strategies” and “future development planning,” which is central to making cities and human settlements inclusive, safe, resilient, and sustainable.

• SDG 14: Life Below Water

  The article explicitly mentions that understanding ESL events is essential for “safeguarding vulnerable coastal ecosystems.” This establishes a clear link between the physical threat of sea-level rise and the need to protect marine and coastal environments, which is the core objective of SDG 14.

Specific SDG Targets Identified

SDG 13: Climate Action

1. Target 13.1: Strengthen resilience and adaptive capacity to climate-related hazards and natural disasters in all countries.

   The article directly supports this target by providing detailed analysis and projections of increased ESL intensity and frequency. It concludes by highlighting the “urgent need for spatially differentiated adaptation strategies” and provides a methodology that can be used globally to assess risks and build resilience against coastal flooding.

2. Target 13.3: Improve education, awareness-raising and human and institutional capacity on climate change mitigation, adaptation, impact reduction and early warning.

   The study itself serves as a tool for building institutional capacity. By providing “a better understanding of current and future extreme events,” it equips policymakers and planners with the scientific knowledge needed to develop “appropriate policies and planning” for climate adaptation and disaster risk reduction.

SDG 11: Sustainable Cities and Communities

1. Target 11.5: By 2030, significantly reduce the number of deaths and the number of people affected and substantially decrease the direct economic losses relative to global gross domestic product caused by disasters, including water-related disasters, with a focus on protecting the poor and people in vulnerable situations.

   The article provides a baseline for this target by citing historical data on deaths, affected people, and economic losses in China due to extreme sea-level events. Its projections of future hazards, such as a “100-year event” becoming a “less than 10 years” event by 2100, underscore the increasing risk and the importance of mitigation actions to achieve this target.

2. Target 11.b: By 2020, substantially increase the number of cities and human settlements adopting and implementing integrated policies and plans towards inclusion, resource efficiency, mitigation and adaptation to climate change, resilience to disasters…

   The research directly advocates for the implementation of such plans. The conclusion that findings “highlight the urgent need for spatially differentiated adaptation strategies that are tailored to specific emission pathways” is a direct call to action for coastal cities to develop and implement the integrated policies mentioned in this target.

SDG 14: Life Below Water

1. Target 14.2: By 2020, sustainably manage and protect marine and coastal ecosystems to avoid significant adverse impacts, including by strengthening their resilience, and take action for their restoration in order to achieve healthy and productive oceans.

   The article connects its findings to this target by stating that advancing the understanding of ESL events is “essential for… safeguarding vulnerable coastal ecosystems.” This implies that the data and projections from the study should be used to inform management plans that protect these ecosystems from the adverse impacts of increased coastal flooding and sea-level rise.

Indicators for Measuring Progress

SDG 13: Climate Action

• Rate of Sea Level Rise: The article mentions the observed increase in the global mean sea level (GMSL) rise trend from “1.3 ± 0.7 mm/year from 1901 to 1971 to 3.7 ± 0.5 mm/year between 2006 and 2018.” This serves as a direct indicator of an accelerating climate hazard.
• Frequency and Intensity of Extreme Sea Level (ESL) Events: The study’s core analysis is on ESL return levels. It projects that under the SSP5-8.5 scenario, “centennial return levels rise by 0.83 m by 2100,” and the return period of a 100-year event shortens to “less than 10 years by 2100.” These metrics are clear indicators of increasing climate-related risks.

SDG 11: Sustainable Cities and Communities

• Economic and Human Losses from Disasters: The article provides specific historical data that can be used as indicators: “average annual economic loss caused by extreme sea level disasters was RMB 11.7 billion, 156 people died, and 13.4 million people were affected.” These figures directly align with the official indicators for Target 11.5.
• Implementation of Disaster Risk Reduction Strategies: The article’s repeated call for “regional adaptation strategies” and “coastal defense strategies” implies that the adoption and implementation of such plans by coastal authorities would be a key indicator of progress.

SDG 14: Life Below Water

• Threats to Coastal Ecosystems: While the article does not provide a quantitative measure of ecosystem health, its identification of intensified ESL events as a primary threat implies that an indicator would be the area of vulnerable coastal ecosystems (e.g., mangroves, wetlands) exposed to projected future flood levels.

Summary Table of Findings

Source: nature.com