Measuring Sea Level Change

Measuring Sea Level ChangeOverview

What is Sea Level?

Mean – Arithmetic average

Mean sea level – The average sea level over a large region

Mean high tide

Mean sea level

Mean low tide

Sea Level VariationAstronomical Tidal Forces

Earth’s water closest to the moon is acted on more strongly by the moon’s gravity

The solid Earth is pulled harder than the water on the side opposite the moon

The Sun produces similar tidal forces and accounts for about 1/3 of ocean tides

Sea Level Variation

Depth of Tampa Bay changes by 20 cm between winter and summer

Primary cause is thermal expansion (water expands when it warms)

Mangroves & coastal habitats must be able to handle the changing water line

Sea Level Variation

During El Niño years, sea level along West Coast is higher than in La Niña years

Higher seas caused by reduced trade winds, allows up to 8 inches of sea level rise

El Niño and Multi-Year ChangesReadings at Fort Point, San Francisco Depth Gauge

Measuring Sea Level RiseTraditional Stilling Well

Pulley andCounterweight

Float

Mechanical device “draws” sealevel on slowly rolling paper

Small openings near bottom of welldesigned to “filter out” wave motion

Sea bed

Tide Gauge Benchmark

Bedrock

Tides and changes in sea level are measured relative to a permanent local benchmark

Measuring Sea Level RiseModern Tide Gauge Station

Satellite Antenna

Acoustic Sensor

Calibration Tube

InstantaneousWater Level

PressureGauge

Data CollectionComputersTide Gauge Benchmark

Computers enable in-depth analysis Data available instantly Individual waves measured

Measuring Sea Level RiseModern Tide Gauge Station

Housing keepsequipment dry

Measures windspeed/direction

Solar panels

Satellite antennasends data toInternet

Computer to processand store data

Acoustic sensor

Measuring Global Sea Level

position dataJASON-2satellite GPS

Range

Altimeter

Microwave radiometermeasuring water vapor

Ellipsoid

Satellitealtitude Sea surface

height

• Deep ocean couldn’t be measured until about 25 years ago

• Satellites now measure elevation using altimetry

• Sea level determined by comparing measured height and GPS data

Causes of Sea Level Change

Terrestrial water storage,Extraction of groundwater,

Building reservoirs Subsidence in river delta,Land movements,

Tectonic displacements

Seepage into aquifersCirculation changes,

Storm surges,Water expands as it warms

Exchange of ocean waterwith

glaciers and ice sheets

Summary

• Mean Sea Level• Sea Level Variation• Coastal Tide Gauge Stations• Measuring Global Sea Level• Causes of Sea Level Change