Coastal Zones and Sea Level Change

The Importance of Understanding Sea Level Change

• Coastal Management and Erosion: Higher sea levels lead to increased erosion in coastal environments. It is a core driver of risk and change.

• Economic Impact: Sea level rise is likely the most expensive climate-related factor in the future regarding infrastructure and erosion mitigation.

• Data Availability Strategic Goal: Historically, data from tidal gauges was inconsistent across different states/groups. The current strategy involves cleaning and making all data available.

• Salinity and Groundwater: Rising sea levels impact coastal groundwater systems, causing salinity changes in brackish or freshwater ecosystems.

• Traditional and Archaeological Significance:     * Coastal indigenous land use in the past was often $\text{several kilometres}$ offshore from current coastlines.     * Understanding past sea levels is vital for archaeology and managing traditional resources.

• Direct Observations:     * At the University of Queensland, the campus pool has filled with river water twice during the lecturer’s tenure due to rising water levels.

Defining and Measuring Sea Level

• Global Mean Sea Level (GMSL): The average sea level around the globe, also historically referred to as "eustatic sea level."

• Relative Sea Level (RSL): The local or regional sea level relative to a specific coastal area.

• Reference Theoretical Models:     * Reference Ellipsoid: A theoretical smooth surface of the Earth that accounts for the bulge at the equator caused by rotation (shrunken north-to-south).     * Geoid (Potsdam Gravity Potato): A surface of equal gravitational potential. It reflects mass and density variations on Earth. For example, the Indian Ocean has an area of lower gravity that affects water masses.

• Measurement Tools:     * Tidal Gauges: Range from simple "float in a pipe" systems to modern pressure and radar gauges. Modern versions provide high-resolution, precise time series to filter out noise from waves and currents.     * Satellite Altimetry: Uses LDA and interferometry to measure the ocean surface shape.     * SWAT (Surface Water and Ocean Topography): Launched recently to provide global coverage, though less effective at very high latitudes.     * GRACE (Gravity Recovery and Climate Experiment): Two refrigerator-sized satellites that measure gravity by tracking the microwave-linked distance between them. Changes in distance reflect gravitational pull variations. Subtracting gravity data from mean sea level allows scientists to map ocean currents and water densities.

Historical and Current Trends

• Global Trends: Compilation data from $1880$ to the present shows a general rising trend.

• Error Bars: Uncertainty increases as records go back toward $1880$ because tidal gauge data becomes less reliable.

• Current Rate of Rise: Satellite and modern data indicate an average global rise of approximately $3\,mm/year$.

• Acceleration: Recent data sits above the linear trend line, suggesting the rate of sea level rise is speeding up (potentially exponential).

• Regional Variability:     * San Francisco: Closely follows the global average.     * Stockholm: Sea level is falling due to local geological factors.     * Manila: Sea level rise is significantly faster than the global average.

• Brisbane Bar Case Study: Using data from $1982$ to the present collected by the Port Authority radar gauge, the rate of rise is approximately $3\,mm/year$. Predictions suggest an additional $7\,cm$ of rise by the year $2050$.

Drivers of Sea Level Change

• Ice Sheet Melting:     * Greenland Ice Sheet: Contains enough water for $6\,m$ to $10\,m$ of sea level rise. It is relatively stable in the center but susceptible to edge melting.     * West Antarctic Ice Sheet (WAIS): Contains approximately $20\,m$ of potential rise. It is "marine-based" (the ocean can get underneath), making it highly unstable as warming currents undercut the ice.     * East Antarctic Ice Sheet (EAIS): The largest, containing $50\,m$ to $60\,m$ of potential rise. Historically considered stable, but the Adele and Wilkesland basins (south of Australia) are now showing signs of instability.

• Mountain Glaciers: Important contributors but have smaller volumes compared to major ice sheets (contributing $centimetres$ to $metres$).

• Thermal Expansion (Steric/Thermospheric Rise): As ocean water heats, it becomes less dense and expands. This accounts for approximately $25\%$—or $\frac{1}{4}$—of current sea level rise.

• Tectonics and Subsidence:     * Plate Tectonics: Plates pulling apart cause subsidence (sinking); plates coming together cause uplift.     * Huon Peninsula, Papua New Guinea: Tectonic uplift is pushing the land out of the sea, creating terraces of fossilized coral reefs that allow scientists to track ecology over $400,000\,years$.     * Vanuatu: Currently experiencing tectonic uplift.     * Hawaii: Volcanic islands subside as the oceanic plate moves away from the plume (hotspot).

• Groundwater Extraction: Removing water from coastal sediments causes particles to compact, leading to land subsidence and relative sea level rise.

• Glacial Isostatic Adjustment (GIA):     * Near-field: Areas previously under ice sheets (e.g., Sweden) see the land rise and the sea level fall relatively.     * Forebulge: The area in front of a glacier that bulges up under pressure. When the ice melts, the forebulge collapses/sinks.     * Continental Levering: Melting ice adds weight to the continental shelf, pushing it down and causing the coastline to "seesaw" or rise up (e.g., Cleveland, Moreton Bay).     * Equatorial Siphoning: Changes in gravity fields near melting glaciers draw water away from the equator toward the poles.

Reconstructing Past Sea Levels

• Coastal Geomorphology: Notches and wave-cut platforms indicate past levels but are difficult to date.

• Speleothems (Stalagmites/Stalactites): Only form in air. Finding submerged speleothems in coastal limestone caves allows for precise depth and age dating (using carbonate dating).

• Fixed Biological Indicators:     * Oysters: Grow at or near sea level. Can be dated with radiocarbon ($\text{up to } 30,000\,years$).     * Foraminifera: Single-celled protists with calcium carbonate shells. Different species dominate at different depths. Transfer functions/algorithms convert the percentage of specific species in sediment cores into depth estimates.

• Coral Reefs:     * Uranium-Thorium Dating: Corals incorporate trace uranium from the ocean. Measuring the decay into thorium allows dating up to $500,000\,years$.     * Growth Forms: Massive corals grow near the surface; branching/foliaceous corals grow deeper.     * Micro-atolls (Porites): Corals that grow up to the mean low sea level and then grow sideways. They act as a "sea level ceiling."     * Cleveland, Moreton Bay: Dead coral reefs found above current low tide levels were dated to $5,000\,years$ old, indicating sea level (or the land surface) was higher then.     * Nicole Leonard's Research: Used micro-atolls across the Great Barrier Reef to reconstruct sea level changes over the last $7,000$ to $8,000\,years$.

Interannual Variability: ENSO

• El Niqo: Can drop regional sea level by $10\,cm$ to $15\,cm$ due to changes in wind and circulation patterns.

• La Niqa: Can raise regional sea level by $5\,cm$ to $10\,cm$. Higher sea levels during La Niqa years increase beach erosion and change wave orientation.

Questions & Discussion

• Question regarding PDB standard: What is the PDB (Peedee Belemnite) standard?

• Response: It is a standard used for measuring stable isotopes. It was based on a specific fossil (a belemnite, similar to a Nautilus) found in the Peedee Formation. Though researchers ran out of the original sample, all data is still reported relative to that standard.

• Discussion on Indigenous Archaeology: The lecturer noted that because the coastline was historically much further out, much of the evidence for early human habitation in Australia is now submerged.

• Workshop Interaction: Students used Excel to plot satellite data (mean rise $3\,mm/year$) and Brisbane Bar data, noting the acceleration in recent years.