The Shifting Tides
An authoritative exploration of Earth's dynamic oceanic reference point, from measurement intricacies to global implications.
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Defining Mean Sea Level
The Baseline Reference
Mean Sea Level (MSL) represents the average surface level of Earth's coastal bodies of water. It serves as a critical reference point for measuring elevations and altitudes. Globally, MSL functions as a standardized geodetic datum, essential for cartography, marine navigation (as chart datum), and aviation (for calibrating atmospheric pressure and flight levels).
Global vs. Local
While a common method involves averaging long-term tide gauge readings at a specific location, the concept of global MSL is more complex. It's an average across the entire ocean surface, influenced by Earth's geoid—a surface approximating mean sea level influenced by gravity anomalies. Local MSL can differ significantly from the global average due to these gravitational variations and other factors.
Relative Terminology
The term "above sea level" generally refers to height above mean sea level (AMSL). Conversely, "APSL" (Above Present Sea Level) compares current sea levels with historical levels. Understanding the distinction between local and global sea level changes is crucial, as land elevation can also change, affecting relative sea level measurements.
The Nuances of Measurement
Complex Influences
Precisely determining MSL is challenging due to numerous dynamic factors. Instantaneous sea level fluctuates due to tides, tsunamis, wind, atmospheric pressure, local gravitational variations, temperature, and salinity. To establish a stable MSL, measurements are averaged over extended periods, often a full Metonic 19-year lunar cycle, using data from tide gauges.
Modern Techniques
Satellite altimetry has revolutionized sea level measurement since the 1990s. Missions like TOPEX/Poseidon, Jason-1, and Jason-2 provide highly precise data on sea surface topography. These advanced methods complement traditional tide gauge records, offering a more comprehensive global perspective.
Reference Points
Various locations serve as reference points for national or regional MSL datums. Examples include Newlyn in Cornwall (UK), Marseille (France), Alicante (Spain), and Kronstadt (Russia). These historical benchmarks provide continuity for long-term sea level monitoring.
Height Above Mean Sea Level (AMSL)
Elevation Reference
Height Above Mean Sea Level (AMSL) is the elevation of an object relative to the MSL datum. This is fundamental in land surveying and cartography, where contour lines on topographic maps indicate elevation relative to MSL. Locations below sea level, such as Death Valley, have negative AMSL elevations.
Aviation Context
In aviation, AMSL is a critical reference. Altimeters are typically set to the local barometric pressure expected at MSL (QNH). This allows pilots to estimate their height above ground by referencing terrain altitudes marked on charts, which are also based on MSL.
Geoid vs. Ellipsoid
While AMSL is geoid-based, systems like GPS use a reference ellipsoid (e.g., WGS84). The difference between ellipsoid height and local MSL can be significant, up to 100 meters. Geoid models (like EGM96) or specific datums (like NAVD88) are used to reconcile these differences for accurate vertical referencing.
Understanding Sea Level Change
Eustatic and Isostatic Factors
Changes in sea level can be categorized: Eustatic changes refer to global shifts in ocean volume (due to melting ice or changes in ocean basin capacity) or water density (thermal expansion, salinity). Isostatic changes involve the vertical movement of landmasses relative to a fixed point, often due to post-glacial rebound or crustal loading/unloading.
Recent Trends and Projections
Global sea level has risen significantly since the end of the last ice age. In the 20th century, the average rise was approximately 1.5-2.5 mm/year, accelerating in recent decades to over 4.6 mm/year. This acceleration is primarily attributed to anthropogenic climate change, driven by thermal expansion and melting ice.
Societal Impacts
Rising sea levels pose substantial risks, including increased coastal flooding, amplified storm surges, saltwater intrusion into freshwater sources, and damage to critical infrastructure like ports. Low-lying regions and small island nations are particularly vulnerable. Adaptation strategies, ranging from hard defenses like seawalls to soft approaches like dune restoration and managed retreat, are essential but vary in feasibility based on economic capacity.
Sea Level in Aviation
Altimeter Settings
Aircraft altimeters are calibrated using barometric pressure settings. Pilots typically set their altimeter to the local atmospheric pressure expected at Mean Sea Level (MSL), known as QNH. This allows for altitude estimation relative to the ground, using terrain elevations provided on aviation charts, which are themselves referenced to MSL.
Standard Atmosphere
Above a certain altitude (the transition altitude), altimeters are set to the International Standard Atmosphere (ISA) pressure value of 1013.25 hPa (or 29.92 inHg). This ensures consistent altitude reference for flight levels in the upper airspace, independent of local surface pressure variations.
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References
References
- US National Research Council, Bulletin of the National Research Council 1932 page 270
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Academic Disclaimer
Important Notice for Learners
This educational resource has been meticulously crafted by an AI, drawing exclusively from the provided source material. It is intended for academic enrichment and informational purposes at a postgraduate level.
This content does not constitute professional advice. The information presented is a synthesis of data and should not replace consultation with subject matter experts in oceanography, geography, climate science, or related fields. Always verify critical information with primary sources and qualified professionals.
The creators assume no liability for any inaccuracies, omissions, or actions taken based on the information herein. Users are encouraged to exercise critical judgment and consult official documentation for definitive guidance.