Comprehensive Study Guide on Sea Level Change, Paleoceanographic Proxies, and Future Projections L6

Recap of Sea Level Fundamentals

• Eustatic vs. Regional Sea Level:     * Eustatic (Global Mean) Sea Level: Represents the global average height of the ocean surface.     * Relative (Regional) Sea Level: Local sea level, which varies based on specific regional factors.

• Causes of Relative Sea Level Change:     * Mantle Dynamics: Movement within the mantle under the Earth’s crust can cause the crust to rise or fall.     * Isostatic Adjustment: Adding or removing ice sheets creates weird effects. When heavy ice is removed, the crust rises (rebound). Conversely, areas in front of ice sheets may collapse.     * Gravity Effects: The presence of massive ice sheets exerts gravitational pull on the ocean; removing them redistributes water globally.

• Sea Level Records:     * Satellite Data: Very accurate but only covers the last few decades.     * Tide Gauges: Date back to approximately $1880$.     * Reconstructions: Necessary to understand long-term changes beyond the limited instrumental record. Finding times in the past with dramatic shifts helps predict future trends.

• Biological Proxies:     * Foraminifera: Different species of these protists live at specific depths or in specific salt marshes.     * Corals: Excellent benchmarks because they grow near sea level. Specifically, microatolls grow exactly at the sea level interface and are easily datable using radiocarbon or uranium dating.

Stable Isotopes: Principles and Chemical Foundations

• Basic Atomic Structure:     * Elements are defined by their atomic number (number of protons in the nucleus).     * Atomic mass is roughly double the atomic number, accounting for protons plus neutrons.     * Chemistry is driven by electrons, which correlates with the proton count; neutrons do not significantly alter chemical reactivity.

• Isotopes:     * Different isotopes of the same element have the same number of protons but different numbers of neutrons (e.g., oxygen in water or limestone ($CaCO_3$)).     * Radioactive Isotopes: Spontaneously decay and are used for radiometric dating.     * Stable Isotopes: Do not decay; their total amount remains constant, though their ratios within molecules change.

• Oxygen Isotopes (${^{16}O, \^{17}O, \^{18}O}$):     * ${^{17}O}$ is rare and typically ignored in these studies.     * ${^{16}O}$ is much more abundant than ${^{18}O}$ (approx. $500 \times$ more).     * ${^{18}O}$ has two extra neutrons, making it slightly bigger and heavier, but it behaves chemically the same as ${^{16}O}$.

Oxygen Isotopes ($\delta^{18}O$) as a Sea Level Proxy

• Fractionation Processes:     * Changes in State: Evaporating water requires energy. It is slightly easier to evaporate water containing the lighter ${^{16}O}$ isotope. Consequently, atmospheric water vapor is depleted in ${^{18}O}$, while the ocean becomes enriched.     * Temperature: The temperature at which an organism (like coral) incorporates oxygen into its calcium carbonate ($CaCO_3$) skeleton affects the isotopic ratio (e.g., at $30^\circ \text{C}$ vs. $28^\circ \text{C}$).

• Measuring $\delta^{18}O$:     * Measured as a ratio compared to a standard, expressed in "per mille" (\text{\textperthousand} or parts per thousand).     * The standard is PDB (Pee Dee Belemnite), a fossil cephalopod used in early experiments.     * Formula concept: $\text{Sample Ratio} - \text{Standard Ratio} / \text{Standard Ratio} \times 1000$.

• The Glacial Cycles Mechanism:     * Evaporation: Water leaves the low-latitude ocean, enriched in ${^{16}O}$.     * Transport: Vapor moves toward the poles (Antarctica/Arctic).     * Precipitation: As it travels, ${^{18}O}$ rains out preferentially. By the time vapor reaches Antarctica, snow contains about $5\%$ less ${^{18}O}$ than where it started.     * Storage: Ice sheets trap ${^{16}O}$. During a "Glacial" period (max ice), the ocean is concentrated with ${^{18}O}$. During an "Interglacial" (like today), ice melts and returns ${^{16}O}$ to the ocean.     * Typical ocean values reach about $1$ during glacials relative to near $0$ today.

Ocean Drilling and Paleoreconstruction: The JOIDES Resolution

• Foraminifera as Recorders:     * Globigerinoides ruber: A common species that grows in surface waters, then dies and accumulates in deep-sea sediments.     * By drilling sediment cores, scientists analyze the shells of these protists to reconstruct ocean chemistry over hundreds of thousands of years.

• The JOIDES Resolution Ship:     * History: Originally an oil exploration vessel; converted in the $1980s$ for scientific research.     * Specs: Approximately $125\,m$ long, carrying $100+$ crew and $30$ scientists.     * Moon Pool: A large hole in the center of the ship through which the drill string is lowered.     * Drilling Capacity: Can reach ocean depths of $4\,km$ and drill a further $2\,km$ into the substrate.     * Drill String Weight: A $6\,km$ drill string weighs approximately the same as $1.5$ fully fueled Airbus A380s.

Data Analysis: The Ong Tong Java Plateau Study

• Site Context: Located in the tropical Pacific, north of New Guinea. It is a large, raised volcanic seafloor area far from land-based influence.

• The Exercise: Using isotopic data from benthic and planktonic foraminifera to reconstruct sea level over the last $750,000$ years.

• Choosing the Correct Indicator:     * Benthic (Bottom-dwelling): Preferred for chemistry reconstructions because deep-ocean temperatures are more stable. This isolates the change in ice volume (sea level) from temperature fluctuations.     * Planktonic (Surface-dwelling): Subject to high temperature variability, which complicates the isotopic signal.

• Sea Level Curve Interpretation:     * A high ratio of ${^{18}O}$ in benthic forams indicates massive ice sheets and low sea level.     * Last Glacial Cycle: Sea level fell slowly over $100,000$ years as ice sheets grew, then rose rapidly over $20,000$ years during the melt.     * The Last Interglacial ($125,000$ years ago): Sea level was roughly similar to or slightly higher than today.     * Calibration: Oxygen isotope records (which provide a continuous relative curve) must be calibrated against absolute markers like the New Guinean Terraces, which contain datable corals.

The Astronomical Theory of Climate: Milankovitch Cycles

• Concept: Variation in the heat received from the sun at specific latitudes (specifically $65^\circ \text{N}$) drives ice sheet growth and decay.

• Eccentricity:     * The Earth’s orbit changes from nearly circular to more elliptical.     * Cycle Duration: Approx. $100,000$ years.     * Driver: Gravitational pulls from Jupiter and Saturn.

• Obliquity (Tilt):     * The angle of the Earth\'s axis relative to its orbital plane varies between $22\,degrees$ and $24\,degrees$.     * Current tilt is $23.5\,degrees$.     * Cycle Duration: Approx. $40,000$ years.

• Precession:     * The "wobble" of the Earth's axis affecting whether a hemisphere points toward the sun during its closest approach.     * Cycle Duration: Approx. $20,000$ years.

Ice Sheet Dynamics and Climate Feedbacks

• Mass Balance:     * Accumulation: Snowfall turning into ice.     * Ablation: Mass loss through melting, calving (into the ocean), or sublimation.     * Ice always flows downslope. If accumulation exceeds ablation, the sheet grows; if ablation is higher, it shrinks.

• Nonlinear Response:     * The response of ice sheets to orbital forcing is disproportionate. A small "push" from summer heat can cause a massive collapse.     * During melt periods, sea level rise can reach rates of $30\,mm/year$ to $40\,mm/year$.

• Climate Feedbacks:     * Albedo Effect: Ice sheets reflect light. Losing ice reduces planetary albedo, leading to further warming and more ice loss (positive feedback).     * Blue Carbon Release: Rising seas inundate wetlands/peat bogs, releasing stored carbon and methane into the atmosphere.     * Ocean Off-gassing: Warmer oceans release more $CO_2$.     * Marine-Based Stability: Marine-based ice sheets (like the West Antarctic Ice Sheet or Thwaites Glacier) are especially unstable because seawater can undercut them, accelerating collapse.

Modeling Future Sea Level: IPCC Projections and SSPs

• Assessment Report 6 (AR6 - 2023): Uses Shared Socioeconomic Pathways (SSPs) to project outcomes based on global choices.

• SSP Scenarios:     * SSP1-1.9: Best case; aggressive green technology and renewable adoption. Temperature rise remains low (approx. $1.19^\circ \text{C}$).     * SSP2-4.5: "Middle of the road"/steady progress.     * SSP3: Regional rivalry.     * SSP4: Inequality.     * SSP5-8.5: Fossil fuel development "goes nuts"; temperature/sea level rise drastically.

• Modeling Components and Uncertainties:     * Thermal Expansion: Accounts for roughly $25\%$ of sea level rise; relatively easy to model but depends on temperature.     * Greenland/Antarctica Dynamics: Surface mass balance is understood, but "internal dynamics" (cracking, flowing, melting underneath) are a "black box" with high uncertainty.     * Land Surface: Local projections depend heavily on knowing whether the land itself is rising or sinking.

• NASA Projections for Brisbane (2050):     * Relative to a $1992-2014$ average, estimates suggest an increase of approx. $19\,cm$ ($0.19\,m$) under middle-of-the-road scenarios.     * Current observed rise is roughly $3\,mm/year$.

Coastal Hazards and Inundation Impacts

• Direct Impacts:     * Submergence: Permanent land loss.     * Coastal Erosion: Increased destruction of beaches and infrastructure along cliffs.     * Salination: Saltwater intrusion into coastal aquifers, soils, and groundwater.     * Impeded Drainage: Higher sea levels make it harder for rivers and land-floodwaters to drain, worsening inland flooding (e.g., the Brisbane River bursting banks during heavy rain).

• Amplification Factors:     * Raising the mean sea level makes Storm Surges and high-tide flooding more severe.     * A "$1$ in $100$ year extreme sea level event" could occur annually in some scenarios by the end of the century (amplification factor of $100$).

Adaptation Strategies: Protection, Retreat, and Ecosystem-Based Solutions

• Protection: Hard structures like concrete seawalls. Problem: Prevents natural habitat migration.

• Accommodation: Raising buildings on stilts or modifying existing infrastructure.

• Advance: Reclaiming land from the sea.

• Retreat: Moving communities and infrastructure inland.

• Ecosystem-Based Adaptation:     * Coral Reefs: Slow down waves and break up incoming surges. They must be able to "keep up" with the pace of rising water to remain effective.     * Mangroves: Act as baffles for waves and traps for sediment. As water slows, sediment deposits, potentially allowing the shoreline to build itself up.

Case Studies: Mangroves and Coral Reef Resilience

• Mangrove Thresholds:     * Global data shows mangroves consistently grew when sea level rise was below $7\,mm/year$.     * Above $7\,mm/year$, mangroves often fail to establish or drown.     * Moreton Bay Analysis (UQ Study): Surface elevation in some parts of the bay (e.g., Adams Beach) is rising at $5-6\,mm/year$, nearly keeping pace with localized projections.     * Coastal Squeeze: If infrastructure (roads, sea walls) exists inland of a mangrove forest, the forest has no space to migrate as seas rise and is eventually squeezed out.

• Coral Growth Models:     * Keep up reefs: Growth matches the rate of sea level rise.     * Catch up reefs: Sea level initially rises faster, but corals eventually grow back to the surface after a delay.     * Give up reefs: Water rises so fast that light diminishes, and the reef drowns.     * Current challenges: Ocean acidification and thermal bleaching make it much harder for modern corals to survive rapid sea level shifts compared to pre-human history.

Questions & Discussion

• Question (Audience): Is modeling complicated by local data?     * Answer: Yes, projections are only well-constrained when there is good local data for the land surface.

• Question (Audience): What is the rate of sea level rise in 2050?     * Answer: Using the NASA tool, students found rates ranging from $5.5\,mm/year$ to $6.6\,mm/year$ in Moreton Bay depending on the scenario chosen.

• Question (Audience): Why do ice sheets melt so much faster than they grow?     * Answer: Due to positive feedback loops like Albedo and greenhouse gas releases from warming oceans and wetlands.