GEOL 106 Lecture Notes - Lecture 24: Sea Level Rise, Sea Level, Anchorage, Alaska
25 May 2018
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11.3 Sea-Level Change
- change in sea level is caused by processes, some of which operate locally and others that affect
world’s ocean
- relative sea level: position of sea at shore, influenced by both movement of land movement of water
o movement can be local, regional, global in extent
- eustatic sea level: global sea level, controlled by processes that affect overall volume of water in
ocean and shape of ocean basins
o changes in eustatic sea level = one of many factors that cause change in relative sea level
EUSTATIC SEA LEVEL
- rises of falls he aout of ater i orld’s oceans increases or decreases of when there is a change
in overall shape of ocean basins
- climate exerts greatest control on amount of water in ocean today
- air temperature influences both average temperature of ocean and amount of water that is stored in
ice on land
o increase in average T of ocean → volume of water expands
o cools → volume of water contracts
o referred to as thermal expansion or contraction: global warming or cooling of atmosphere
is responsible
o e.g. eustatic rise in sea level in ocean is about 1.6 mm per year caused by warming of
atmosphere
- changes in air temperature also cause ice on land to melt or snowfall to increase → volume of water
frozen in glacial ice, ice caps, ice sheets, and permafrost is closely related to average air T over years
or decades
o recently global warming → melting in Greenland and Antarctica → increased amount of water
i orld’s oeas ad has otriuted to aout half of the eustati rise i sea leel
- plate tectonic processes → change in ocean basins → rate of seafloor spreading for example
o influence eustatic sea level over long periods of time, unlikely to contribute significantly to
changes in relative sea level
RELATIVE SEA LEVEL
- local or regional processes that influence movement of land and water
- lad a rise or fall sloly as Earth’s rust respods to eight of o-melted Pleistocene glaciers or
rapidly in reponse to tectonic movements during earthquake
o e.g. Good Friday 1964 earthquake in Montague Island southeast of Anchorage, Alaska →
shoreline rose close to 10m
- many coastlines that are tectonically active and experience frequent uplift in earthquakes may be less
strongly influenced by eustatic sea level rise caused by global warming
- movement of shoreline is also influenced by rates of deposition, erosion, subsidence along coast
o input and output factors in beach budget described in 11.5 determine whether coastline is
experiencing net erosion or deposition
- rates of coastal subsidence have major influence on relative sea level on some shorelines like
Louisiana coastline
- on daily basis: astronomical tides and weather conditions primarily control relative sea level
find more resources at oneclass.com
find more resources at oneclass.com
o astronomical tides: produced by gravitational pull of moon and to lesser extent the sun →
tides cause relative sea level to fluctuate daily and seasonally as position of coast relative to
moon and sun constantly shifts
- tidal fluctuations are entirely predictable but they can create hazardous currents and affect height of
storm surges
o storm surge occurring at high tide is more damaging than one occurring at low tide
- weather conditions also change relative to sea level over period of hours and days
o change sin wind speed, atmospheric pressure → influence level of sea
o wind speed has greater effect than atmospheric pressure on relative sea level
o in open ocean: high winds pile up water and increase wave height → swell
o swell increases both water level and wave heights when it reaches shore
o storm surge from tropical storms, hurricanes, extratropical cyclones crossing coast are
extreme cases of swell moving landward
o significant drop in atmospheric pressure in major hurricanes can add meter or more to height
of storm surge
o for weaker tropical and extratropical cyclones, there is smaller drop in atmospheric pressure
and less of an effect on storm surge
- over short time periods with El Nino conditions (atmospheric and ocean event characterized by warm
water and along US West Coast, stormy conditions)
- rapid changes in relative sea level contribute to coastal flooding and hazardous nearshore currents
whereas rising eustatic sea level over decades increases hazards from storm surge and coastal
erosion
o over long term, rising sea level threatens coastal cities and very existence of many islands
-
11.4 Geographic Regions at Risk from Coastal Hazards
- coastal hazards are present on both seacoasts and lakeshores
- large lakes like Great Lakes, Great Bear Lake, Great Slave Lake, Lake Winnipeg develop coastal
conditions similar to those of ocean
- in NA: hazardous along Atlantic, Pacific, Gulf, Arctic coasts as well as along Great Lakes
- strong nearshore currents, sea level rise, storm surge from cyclones, and tsunamis are not hazards on
all coastlines
o tsunamis and hurricanes are generally absent in lakes
o only Lake Okeechobee in NA and Lake Pontchartrain in Louisiana have experienced significant
storm surges from hurricanes before
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Plate tectonic processes change in ocean basins rate of seafloor spreading for example influence eustatic sea level over long periods of time, unlikely to contribute significantly to changes in relative sea level. Many coastlines that are tectonically active and experience frequent uplift in earthquakes may be less strongly influenced by eustatic sea level rise caused by global warming. Powerful currents form that carry large amounts of water between longshore bar and swash zone. Coastal erosion global rise in sea level and extensive development in coastal zones coastal erosion is more of a problem now coastal erosion is generally more continuous and predictable than other natural hazards. Many of fixes are temporary, if extensive development of coastal areas for vacation and recreational living continues, coastal erosion will become more of a serious problem. Sea cliff erosion littoral cell etc. sea cliff or lakeshore bluff along coastline may be subject to additional erosion problems.
