sea level change (2B.7)

Question 1

what is long term sea level change?

Answer 1

long term sea level changes occur over thousands of years and is due to eustatic and isostatic factors, and tectonics.

Question 2

eustatic change

Answer 2

= volume of water in oceans, occurs on a global scale and relatively quickly

1. negative eustatic change (= sea level decreases)
   - during glacial periods as ice builds up, precipitation does run down into sea but snow does not so over time sea levels decrease partly due to lack of excess rainfall (and rivers freeze!)
2. positive eustatic change (= sea level increases)
   - at end of glacial periods ice melts and runs from land into the oceans increasing global sea level
   - thermal expansion (caused by increasing temperatures) also leads to sea level rise (& vice versa!)

Question 3

isostatic change

Answer 3

= movement of the land relative to the sea, occurs locally and changes take much longer

1. eustatic positive= isostatic negative (land sinks, this is called isostatic subsidence)
   - during glacial periods enormous weight of ice (can be several km thick) makes land sink by pushing crust into mantle
2. eustatic negative= isostatic positive (land rises, isostatic recovery)
   - end of glacial periods, temps rise and ice melts; weight is lifted causing accretion

! subsidence in one area causes accretion in the other !
- in UK during last ice age, Northern part sunk due to weight of ice whereas South (where ice sheets did not cover) accreted- magma in mantle is forced to move elsewhere. only took a few 100 years for ice to melt but isostatic rebound is still occurring in the north 11000 years later: sea levels are falling as isostatic rebound exceeds eustatic rise in global sea levels…..

and southern England is still subsiding as magma returns back north putting South England at greater risk of flooding: subsidence is accelerating a rise in sea level produced by global warming (eustatic). Land’s End in Cornwall is sinking isostatically by 1.1 mm p.a., and there is a 2.8 mm eustatic rise in sea level due to climate change, producing a sea level rise of 3.9 mm per annum.

Question 4

tectonic activity

Answer 4

in general tectonic activity= sea level rise

eustatic:
- if shape of ocean basin is altered capacity to hold water will change, if basin reduces (due to rise in magma/ lift in crust) there will be a positive change and vice versa

isostatic:
- can also change land level which in turn alters the sea level. when continents rise periodically= fall in relative sea level
EG during 2004 Indian Ocean Tsunami, extension of crustal plate caused isostaic fall in land on island of Sumatra by 20cm

Question 5

landforms created

Answer 5

1. emergent castlines: exposed from negative sea level change/ isostatic recovery (raised beaches with fossil cliffs)
2. submergrnt coastlines: flooded from positive sea level change (rias, fjords, dalmatian)

Question 6

emergent coastlines

Answer 6

emergent coastlines are being produced by post- glacial readjustment. they have landforms reflecting previous sea levels.

1. raised beach: relict beach now above high tide level
   - flat surface covered by sand/ rounded pebbles
   - usually vegetated by plant succession
2. fossil cliff: steep slope found at back of raised beach
   - this is evidence of former marine erosion as they often have erosional features eg caves and arches

! these 2 features are common on west coast of Scotland where often remains of eroded cliff lines (fossil cliffs) can be found behind the beach with wave cut notches and waves as evidence of past marine erosion ! EG Isle of Arran has raised beach 5m above sea level and has 3 levels of raised beaches produced at different stages of postglacial adjustment.

Question 7

submergent coastlines

Answer 7

sections of the littoral zone where sea level rise has flooded often due to post-glacial adjustment

1. rias: drowned river valley, section that is flooded by the sea making it much wider
   - most common landform
2. fjord: drowned glacial valley, section of a glacially eroded valley flooded by the sea eg Western Norway/ Scottish Sea Lochs
   - relatively straight profile
   - glacier erosion is often deep meaning fjords are often deeper than the adjacent sea.
   EG Sognefjord in Western Norway is 205km long, 1.3 deep and 4.5 wide
3. Dalmatian coast: composed of long, narrow islands running parallel to the coastine
   - produced by rising sea levels drowning valleys and leaving the peaks above sea level
   EG Dalmatian coast of Croatia: 520km long with 1,240 islands parallel

Question 8

contemporary sea level change

Answer 8

sea level change from global warming or tectonic activity is a risk to some coastlines.
- climatic warming leads to eustatic sea level rise, warming leads to the melting of mountain glaciers (Alps, Himalaya) and polar ice sheets increasing the amount of water in the ocean store.
! KEY NOTE ! melting of sea ice has no effect on sea level as the ice was already displacing the equivalent water volume to that produced by melting.
!!! statistic to support !!! IPCC attributes 50% of sea level rise 1990-2010 to ice sheets melting (Greenland ice sheet 15%, Antarctic ice sheet 10%)

• warming also leads to the thermal expansion of existing ocean water as its temperature rises: 94% of increased heat energy in the climate system is transferred to oceans.
• > IPCC attributes 40% of sea level rise 1990-2010 to this

in the future…

• complete melting of Greenland ice sheet would raise global sea levels by 7 m
• complete melting of Antarctic ice sheet would raise sea levels by 50 m