3.1 Causes of sea level rise

Sea level rise is a complex phenomenon with multiple causes. Thermal expansion of oceans, melting land-based ice, and vertical land movements are key factors driving global and regional changes in sea levels.

Atmospheric and oceanic circulation patterns, gravitational effects, and human activities also play significant roles. Climate change amplifies these processes, creating challenges for coastal communities and necessitating adaptive engineering solutions.

Thermal expansion of oceans

• Thermal expansion of oceans plays a crucial role in sea level rise, impacting coastal resilience engineering strategies
• Understanding this process helps engineers develop adaptive measures for coastal communities and infrastructure

Heat absorption by oceans

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• Oceans absorb over 90% of excess heat in Earth's climate system
• Increased atmospheric greenhouse gases trap more heat, leading to warmer oceans
• Heat absorption varies with depth, with surface waters warming faster than deeper layers
• Thermal inertia of oceans causes delayed response to atmospheric temperature changes

Volume increase of water

• Water molecules expand as temperature increases, occupying more space
• Expansion coefficient of seawater varies with temperature, salinity, and pressure
• Volume increase contributes to approximately 30-50% of observed global sea level rise
• Thermal expansion affects entire water column, not just surface layers

Regional variations

• Thermal expansion impacts differ across ocean basins due to circulation patterns
• Western boundary currents (Gulf Stream) experience higher rates of thermal expansion
• Shallow coastal areas may see more rapid sea level rise from thermal expansion
• Deep ocean warming contributes to long-term sea level rise trends

Melting of land-based ice

• Land-based ice melt significantly contributes to global sea level rise, posing challenges for coastal resilience
• Engineers must consider the long-term impacts of ice melt when designing coastal protection measures

Glaciers and ice sheets

• Glaciers store approximately 70% of Earth's freshwater
• Ice sheets in Greenland and Antarctica contain vast amounts of frozen water
• Melting occurs through surface ablation and calving of ice into the ocean
• Rate of glacier retreat has accelerated in recent decades due to climate change

Greenland vs Antarctica

• Greenland Ice Sheet loses mass primarily through surface melting and runoff
• Antarctic Ice Sheet experiences mass loss mainly through ice shelf collapse and glacier acceleration
• Greenland contributes more to current sea level rise, but Antarctica poses greater long-term risk
• West Antarctic Ice Sheet considered potentially unstable due to marine ice sheet instability

Meltwater contribution

• Meltwater from land ice directly adds volume to the oceans
• Contribution varies annually based on seasonal melting patterns
• Accelerating melt rates increase the rate of sea level rise
• Meltwater alters local ocean salinity, potentially affecting circulation patterns

Vertical land movements

• Vertical land movements impact relative sea level changes, crucial for coastal resilience engineering
• Understanding these processes helps in developing localized adaptation strategies for coastal areas

Isostatic rebound

• Post-glacial rebound occurs as land rises after removal of ice sheet weight
• Affects regions previously covered by ice sheets during last glacial maximum
• Can cause relative sea level fall in some areas (Scandinavia) and rise in others (US East Coast)
• Rates of isostatic rebound vary from millimeters to centimeters per year

Tectonic plate shifts

• Plate tectonics cause vertical land movements through uplift and subsidence
• Subduction zones experience complex patterns of land level changes
• Earthquakes can cause sudden vertical displacements affecting local sea levels
• Long-term tectonic movements influence regional sea level trends

Subsidence due to extraction

• Groundwater and hydrocarbon extraction cause land subsidence
• Coastal cities (Jakarta, New Orleans) experience accelerated relative sea level rise due to subsidence
• Compaction of sediments in river deltas contributes to local land sinking
• Subsidence rates can exceed global sea level rise in some areas, exacerbating flood risks

Atmospheric and oceanic circulation

• Atmospheric and oceanic circulation patterns significantly influence regional sea level variations
• Coastal resilience engineers must account for these dynamic systems in their designs and projections

Wind patterns and currents

• Trade winds and westerlies affect ocean surface currents and sea level distribution
• Ekman transport causes water to pile up or be pushed away from coasts
• Western boundary currents (Gulf Stream, Kuroshio) influence regional sea levels
• Changes in circulation patterns can lead to local sea level rise or fall

El Niño and La Niña effects

• El Niño events cause higher sea levels in the eastern Pacific and lower in the western Pacific
• La Niña produces opposite effects, with higher sea levels in the western Pacific
• These oscillations can cause temporary sea level changes of up to 20-30 cm
• Frequency and intensity of ENSO events may change with climate change, affecting sea level patterns

Gulf Stream variations

• Gulf Stream transports warm water northward along the US East Coast
• Weakening of the Gulf Stream can lead to higher sea levels along the US Atlantic coast
• Climate change may affect the strength and position of the Gulf Stream
• Changes in this current system have implications for coastal flooding and erosion risks

Gravitational effects

• Gravitational effects play a crucial role in regional sea level variations, impacting coastal resilience strategies
• Engineers must consider these complex interactions when assessing long-term sea level rise projections

Ice mass loss impacts

• Large ice masses exert gravitational pull on surrounding ocean waters
• As ice sheets melt, their gravitational influence decreases
• Counterintuitively, sea levels may fall near melting ice sheets due to reduced gravitational pull
• Areas far from melting ice sheets experience higher than average sea level rise

Redistribution of water

• Melting ice sheets cause changes in Earth's rotation and gravitational field
• Water redistributes across the globe in response to these changes
• Equatorial regions may experience higher sea level rise due to redistribution
• Complex patterns of sea level change emerge from the interplay of gravitational effects

Regional sea level fingerprints

• Each ice sheet and glacier system produces a unique pattern of sea level change
• Greenland ice loss affects sea levels differently than Antarctic ice loss
• Fingerprints help identify the source of observed sea level changes
• Understanding fingerprints improves projections of future regional sea level rise

Human activities

• Human activities directly and indirectly influence sea level rise, presenting challenges for coastal resilience engineering
• Recognizing and mitigating these anthropogenic factors is crucial for sustainable coastal management

Groundwater depletion

• Excessive groundwater extraction transfers water from land to oceans
• Contributes to global sea level rise by adding water volume to oceans
• Causes land subsidence, exacerbating local relative sea level rise
• Particularly significant in arid regions and heavily populated coastal areas

Dam construction impacts

• Large-scale dam construction has temporarily slowed sea level rise
• Reservoirs store water on land, reducing the volume reaching oceans
• Effect is diminishing as new dam construction slows and reservoirs fill with sediment
• Future dam removal could release stored water, potentially accelerating sea level rise

Coastal development consequences

• Urbanization and land reclamation alter natural coastal processes
• Destruction of coastal ecosystems (mangroves, salt marshes) reduces natural buffers against sea level rise
• Coastal infrastructure can disrupt sediment transport, affecting local erosion and accretion patterns
• Hard engineering solutions may exacerbate erosion in adjacent areas, leading to increased vulnerability

Climate change factors

• Climate change significantly influences sea level rise, presenting complex challenges for coastal resilience engineering
• Understanding these factors is essential for developing adaptive strategies and long-term coastal management plans

Greenhouse gas emissions

• Increased atmospheric CO2 and other greenhouse gases trap heat in Earth's system
• Higher temperatures lead to thermal expansion of oceans and accelerated ice melt
• Different emission scenarios result in varying projections of future sea level rise
• Mitigation efforts to reduce emissions can slow the rate of sea level rise in the long term

Feedback loops

• Positive feedback mechanisms amplify the effects of initial warming
• Albedo feedback: melting ice exposes darker surfaces, absorbing more heat
• Permafrost thaw releases methane, a potent greenhouse gas
• Ocean acidification may affect marine ecosystems, indirectly impacting sea level through food web changes

Tipping points in climate systems

• Critical thresholds beyond which rapid and irreversible changes occur
• Potential collapse of West Antarctic Ice Sheet could lead to several meters of sea level rise
• Shutdown of Atlantic Meridional Overturning Circulation would affect regional sea levels
• Abrupt changes in climate systems pose significant challenges for coastal adaptation planning

Temporal variations

• Understanding temporal variations in sea level is crucial for coastal resilience engineering and risk assessment
• Engineers must consider both short-term fluctuations and long-term trends when designing coastal protection measures

Short-term vs long-term changes

• Short-term changes include tides, storm surges, and seasonal variations
• Long-term changes reflect climate-driven sea level rise and geological processes
• Combination of short-term and long-term changes determines flood risk and coastal erosion
• Coastal infrastructure must be designed to withstand both immediate and future sea level conditions

Seasonal and annual fluctuations

• Seasonal cycles caused by changes in temperature, wind patterns, and ocean currents
• Annual variations influenced by climate phenomena (El Niño, North Atlantic Oscillation)
• Steric effects due to thermal expansion and salinity changes contribute to annual cycles
• Understanding these fluctuations improves flood forecasting and coastal management

Historical sea level trends

• Geological evidence shows sea levels have varied by over 100 meters in Earth's history
• Rate of sea level rise has accelerated since the Industrial Revolution
• 20th century saw average sea level rise of 1.7 mm/year
• Current rate of global mean sea level rise exceeds 3 mm/year, with further acceleration expected

Measurement and monitoring

• Accurate measurement and monitoring of sea level changes are essential for coastal resilience engineering
• Multiple techniques provide comprehensive data for understanding past, present, and future sea level trends

Satellite altimetry techniques

• Satellites measure sea surface height with millimeter-level precision
• Global coverage allows for monitoring of open ocean and coastal areas
• Continuous measurements since 1992 provide crucial data on sea level trends
• Challenges include correcting for atmospheric effects and satellite orbit variations

Tide gauge networks

• Long-term records from tide gauges provide historical sea level data
• Coastal locations offer insights into local relative sea level changes
• Global network of tide gauges allows for regional and global trend analysis
• Limitations include uneven global distribution and land movement effects

Geological evidence interpretation

• Proxy records (coral reefs, sediment cores) provide insights into past sea levels
• Tree rings and ice cores offer information on past climate conditions
• Archaeological evidence of coastal settlements indicates historical sea levels
• Combining multiple lines of evidence improves understanding of long-term sea level trends

Regional differences

• Regional variations in sea level rise present unique challenges for coastal resilience engineering
• Understanding local factors is crucial for developing effective adaptation strategies

Hotspots of sea level rise

• Certain regions experience faster than average sea level rise
• Western Pacific shows higher rates due to wind-driven water redistribution
• US East Coast faces accelerated rise from land subsidence and ocean dynamics
• Identifying hotspots helps prioritize adaptation efforts and resource allocation

Coastal geometry influence

• Shape and bathymetry of coastlines affect local sea level changes
• Shallow continental shelves may experience amplified sea level rise
• Narrow bays and estuaries can amplify tidal ranges and storm surges
• Complex coastlines require detailed modeling to assess future flood risks

Local vs global sea level rise

• Global mean sea level rise differs from local relative sea level changes
• Vertical land movements cause divergence between local and global trends
• Ocean dynamics and gravitational effects create spatial variations in sea level rise
• Coastal communities must focus on local relative sea level rise for adaptation planning