Sea Level

Relative sea level (RSL)

The height of the sea relative to a specific piece of land or coast.

Eustatic sea level

Global average sea level; it changes when the volume of ocean water or the size of ocean basins changes.

How do tectonics affect relative sea level?

Tectonics can raise or lower land and the seafloor, changing sea level relative to a coast.

How do water and ice affect sea level?

Their volume changes with temperature and with the amount of water stored as land ice.

Oceanic crust production

The creation of new oceanic crust, mainly at seafloor-spreading centers.

How can seafloor-spreading rates affect sea level over geologic time?

Faster spreading produces more elevated, young oceanic crust, which can reduce ocean-basin capacity and raise sea level.

How does seawater composition change with seafloor spreading?

Faster spreading is associated with more calcium, while slower spreading is associated with more magnesium.

Why can ancient sea-level changes matter today?

They can affect the locations of landscapes, cultural environments, and potentially human settlement or voting patterns.

Sedimentation

The process by which eroded material is transported and deposited somewhere else.

Erosion of mountains

The wearing away and removal of mountain material.

What happens to sediment eroded from mountains?

Gravity moves it downhill toward lower elevations, often eventually toward sea level.

Sedimentary basin

A low area where sediment accumulates.

How does sediment filling a basin affect seawater?

The sediment displaces seawater, contributing to sea-level rise.

Why can sedimentation contribute to rising sea level?

Sediment occupies space that would otherwise hold seawater.

Sequence stratigraphy

The study of sedimentary layers and depositional patterns caused by changes in relative sea level.

Accommodation

The space available for sediment to accumulate.

Transgression

A landward shift of the shoreline caused by relative sea-level rise.

Retrogradation

A landward movement of depositional environments, commonly during transgression.

Aggradation

The vertical buildup of sediment deposits.

Progradation

A seaward advance of sediment deposits and the shoreline.

What happens during a combined eustatic fall and uplift?

Relative sea level falls because global sea level falls while the land rises.

What happens if uplift is faster than eustatic sea-level rise?

Relative sea level falls.

What happens if eustatic sea level rises faster than land uplift?

Relative sea level rises.

What happens if eustatic sea-level fall is faster than subsidence?

Relative sea level falls.

What happens if subsidence is faster than eustatic sea-level fall?

Relative sea level rises.

Uplift

Upward movement of Earth's land surface.

Subsidence

Downward movement or sinking of Earth's land surface.

Why is the rate of change important for relative sea level?

Whether sea level appears to rise or fall at a coast depends on the relative rates of eustatic change and land movement.

Sea ice

Sea water that has frozen and floats in the ocean.

What is the major sea-level effect of melting sea ice?

Very little direct eustatic change, because floating ice already displaces nearly its own mass of seawater.

Land ice

Ice resting on land, such as glaciers and ice sheets.

How does melting land ice affect eustatic sea level?

It raises eustatic sea level because water previously stored on land enters the ocean.

Glacial rebound

The adjustment and rebound of land and mantle after the weight of a large ice sheet is removed.

Why can glacial rebound affect relative sea level?

Land rises after ice melts, which can make local relative sea level fall.

Thermal expansion

The increase in water volume as water warms.

How does warmer ocean water affect sea level?

Warmer water expands, causing eustatic sea-level rise.

How does colder ocean water affect sea level?

Colder water contracts, contributing to eustatic sea-level fall.

Mercury thermometer analogy

Like mercury expands and rises when warmed, seawater expands and takes up more space when warmed.

Groundwater and lakes

Water stored on land that can affect sea level if it is moved into or out of the ocean.

External source of Earth's heat

Solar radiation from the Sun.

Internal source of Earth's heat

Heat from radioactive decay within Earth, especially in the mantle.

Greenhouse gases (GHGs)

Gases that trap heat in Earth's atmosphere.

How do greenhouse gases affect sea level?

They trap heat, warming water and melting land ice, both of which can raise sea level.

Milankovitch cycles

Long-term changes in Earth's orbit and orientation that change how solar energy is distributed.

Eccentricity

A Milankovitch cycle involving changes in the shape of Earth's orbit around the Sun.

Obliquity

A Milankovitch cycle involving changes in Earth's axial tilt.

Axial precession

A Milankovitch cycle involving the slow wobble in the direction of Earth's axis.

Milankovitch-cycle order from longest to shortest period

Eccentricity, obliquity, axial precession.

Oxygen-16 (O-16)

A lighter oxygen isotope that evaporates more readily.

Oxygen-18 (O-18)

A heavier oxygen isotope that is left relatively enriched in seawater when lighter O-16 becomes locked in ice.

What happens to O-16 during glacial periods?

More O-16 evaporates and becomes locked in land ice.

What happens to the O-18 content of seawater during glacial periods?

It becomes relatively higher because more light O-16 is stored in ice.

Marine isotope stages

Alternating warm and cold periods reconstructed from oxygen-isotope patterns in marine sediments.

How do oxygen isotopes help reconstruct ancient sea level?

The O-16/O-18 pattern records changes in global ice volume, which are linked to eustatic sea level.

Internal climate forcings

Forcings that originate within Earth's climate system, including greenhouse gases.

Examples of greenhouse gases

H2O (water vapor), N2O, CH4, and CO2.

Why is CO2 especially important as a greenhouse gas?

It has a long residence time in the atmosphere.

Carbon cycle

The movement of carbon among the atmosphere, oceans, living things, rocks, and other Earth systems.

Convection currents

Heat transfer through the physical movement of a fluid or other mobile material.

What happens to hot material during convection?

It becomes less dense and rises.

What happens to cold material during convection?

It becomes denser and sinks.

How does convection affect beach winds?

Different heating of land and water causes winds to change between day and night.

Why is the equator warmer than the poles?

The Sun's rays strike the equator more directly, while sunlight reaches the poles at a lower angle.

Coriolis effect

The apparent deflection of moving air or water caused by Earth's rotation.

Why is the Coriolis effect important for winds and currents?

It changes their direction as they move across Earth's rotating surface.

Why does Earth rotate faster at the equator than near the poles?

Points near the equator travel a greater distance during each rotation.

Why does a single global convection cell break into multiple cells?

Earth's rotation and instability in a single cell cause atmospheric circulation to divide into several cells.

How many major atmospheric circulation cells are in each hemisphere?

Three.

Hadley cell

The warm tropical atmospheric circulation cell.

Ferrel cell

The middle-latitude atmospheric circulation cell.

Polar cell

The cold, high-latitude atmospheric circulation cell.

Global atmospheric circulation

The system of wind cells that moves heat around Earth through three cells in each hemisphere.

Ocean currents

Moving seawater driven by winds, the Coriolis effect, and density differences.

What density factors help drive ocean currents?

Temperature and salinity differences.

Thermohaline circulation

Global ocean circulation driven by temperature and salinity differences; often described as a conveyor belt.

Ekman transport

The net movement of surface water caused by the combination of wind and the Coriolis effect.

What happens to wind influence as depth increases in Ekman transport?

Frictional wind influence decreases with depth, while Coriolis influence becomes relatively more important.

What is the approximate direction of net surface transport in Ekman transport?

Roughly 45 degrees from the wind direction, according to the notes.

Ekman spiral

The pattern in which water movement changes direction with increasing depth because of wind and the Coriolis effect.

Atlantic Meridional Overturning Circulation (AMOC)

A major Atlantic current system that helps move heat through the ocean.

What could happen to sea level if the AMOC weakens?

Warm, thermally expanded water can build up like a traffic jam and raise sea level.

Why are the Carolinas important in ocean circulation?

They are near a critical crossroads of important ocean currents.

Rip current

A narrow, fast-moving current flowing away from the shore, driven mainly by incoming waves.

Longshore current

A current moving parallel to the shoreline, driven by waves arriving at an angle.

How do waves create a longshore current?

Waves wash up the beach at an angle, and gravity pulls water back down, creating a net current along the shore.

Longshore drift

The net movement of sediment along the coastline caused by longshore currents.

What is the difference between a longshore current and longshore drift?

The current is moving water; longshore drift is the sediment moved along the coast by that water.