Mean sea level
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Understanding Mean Sea Level: Trends, Variations, and Implications
Global Mean Sea Level Trends
Historical and Recent Trends in Global Mean Sea Level
Global mean sea level (GMSL) is a critical indicator of climate change, reflecting the cumulative effects of various natural and anthropogenic factors. Over the past century, GMSL has shown significant variability and acceleration. Before the advent of satellite altimetry, estimates of GMSL rise varied widely due to uneven tide gauge coverage and limited temporal sampling. Recent reassessments using advanced techniques have provided more accurate trends, showing a rise of 1.1 ± 0.3 mm/year before 1990 and 3.1 ± 1.4 mm/year from 1993 to 2012, indicating a larger overall acceleration than previously suggested2.
Recent Accelerations and Contributing Factors
Since 2010, the rate of GMSL rise has increased significantly, reaching 4.4 ± 0.5 mm/year, driven by accelerated land ice loss and steric changes (thermal expansion of seawater)9. This recent acceleration underscores the importance of continuous monitoring and reassessment of sea-level data to understand the underlying causes and future projections.
Components of Sea-Level Rise
Contributions from Ice Sheets and Thermal Expansion
The rise in GMSL is attributed to several key components: ocean thermal expansion, melting glaciers, and ice sheets from Greenland and Antarctica. From 1993 to the present, these components contributed 42%, 21%, 15%, and 8% to the GMSL rise, respectively1. The integration of various datasets, including space-based and in situ observations, has been crucial in quantifying these contributions and identifying uncertainties, particularly in land water storage.
Regional Variations and Influences
Regional departures from the global mean sea level can be significant, influenced by factors such as local geoid changes, vertical land motion, and oceanic internal variability. For instance, the Indian Ocean and Southern Pacific regions have shown a transition from lower-than-average rates before 1990 to unprecedented high rates in recent decades2. Understanding these regional variations is essential for accurate sea-level projections and risk assessments.
Implications of Sea-Level Rise
Vulnerability and Risk Assessment
Accurate estimates of GMSL rise are vital for assessing global and national population exposures to extreme coastal water levels. New digital elevation models (DEMs) utilizing neural networks have significantly improved these estimates, revealing that up to 630 million people could be living on land below projected annual flood levels by 2100 under high emissions scenarios7. This highlights the urgent need for effective mitigation and adaptation strategies to protect vulnerable coastal communities.
Historical Context and Future Projections
Reconstructing sea levels during past warmer epochs, such as the Pliocene, provides valuable insights into the response of ice sheets and sea levels to prolonged warming. During the mid-Piacenzian Warm Period, GMSL was about 16.2 meters higher than today, indicating the high sensitivity of ice sheets to temperature increases10. These historical data points serve as important calibration targets for future ice-sheet models and projections.
Conclusion
The study of mean sea level is crucial for understanding the impacts of climate change on our planet. Recent advancements in data collection and analysis have provided more accurate trends and insights into the contributing factors and regional variations of sea-level rise. As global temperatures continue to rise, ongoing research and monitoring are essential to inform effective mitigation and adaptation strategies, ensuring the resilience of coastal communities worldwide.
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Most relevant research papers on this topic
Global sea-level budget 1993–present
The global mean sea level can be closed to within 0.3 mm yr1 using ocean thermal expansion, glaciers, Greenland, and Antarctica, but land water storage remains uncertain.
Highly CitedReassessment of 20th century global mean sea level rise
Our new reconstruction of global mean sea level rise in the 20th century shows smaller trends before 1990 and a larger acceleration from 1993 to 2012, improving our confidence in climate change projections.
Highly CitedIdentifying the causes of sea-level change
Sea-level rise is likely to exceed one meter by the 21st century, with regional variations potentially reaching several decimetres.
Literature ReviewHighly CitedAn analysis of British monthly mean sea level
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Mean sea level is a complex concept, involving both period (e.g. monthly) and physical (physically constant) factors, and is often used for chart datums and other purposes.
New elevation data triple estimates of global vulnerability to sea-level rise and coastal flooding
The new CoastalDEM digital elevation model more than triples the NASA SRTM-based estimates of global population occupying land below projected sea levels in 2100, with more than 200 million people potentially affected by climate change.
Highly CitedAn increase in the rate of global mean sea level rise since 2010
The global mean sea level has been rising at a rate of 4.4 mm/yr since 2010, due to increased land ice loss and steric change, masking the true rise due to El Nio-Southern Oscillation fluctuations.
Highly CitedConstraints on global mean sea level during Pliocene warmth
During the mid-Piacenzian Warm Period, global mean sea level was about 16 meters higher than today due to global ice-volume changes and thermal expansion of the oceans.
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