Causes and consequences of sea level rise Flashcards
(25 cards)
What is mean sea level rise?
Mean sea level (MSL) is the average height of the sea over longer periods of time (usually a month or year), filtering out short term variations (e.g., waves, tides, storm surges etc.).
These longer periods range from months - thousands of years
Briefly outline the historical changes in mean sea level rise. Link this to the temperatures experienced in these times.
Last interglacial (125,000 ya) = 5m above today, only 2 dC warmer than now
Last ice age = >100m below today (water trapped in ice sheets)
Little change from 2,000 ya to 19th century, but a gradual increase
How has the rate of mean sea level rise changed?
During late 19th and early 20th centuries MSLR = ~ 1.7mm yr-1
1993 – 2021 MSLR = ~ 3.3 mm yr-1 – increase in rate
Which regions are most vulnerable to mean sea level rise?
Coastal zones with:
- Dense populations (e.g., Asia and US coast)
- Low elevations
- Appreciable rates of subsidence
- and/or inadequate adaptive capacity (parts of Africa and Asia)
Is the rate of sea level rise uniform? Why?
No - varies regionally with ENSO, ocean currents, ocean circulation (driving temperature and density changes) etc.
What are three major contributors to mean sea level rise (source of water)?
- Thermal expansion (water expands as it gets warmer)
- Glaciers
- Ice sheets (Greenland and Antarctica)
What is the usual seasonal equilibrium of glaciers and polar caps?
Summer - naturally melt back
Winter - snow balances out melting (mainly formed from evaporated seawater)
Essentially - melting = freezing
How has rising temperatures disrupted the natural equilibrium of glaciers and polar caps?
- Greater than average summer melting
- Reduced snowfall (late winters and earlier springs)
This has caused the equilibrium to shift towards increasing melting.
What is the actual mechanism of ice sheet melting?
Seawater from below seeps into the bottom of the ice -> this increases melting from below as the seawater is warmer -> parts of the ice sheet break off and melt.
How is mean sea level rise projected to change under different melting scenarios? Hint: different projections based on the melting of different ice sheets.
Melting of small glaciers & polar ice caps = projected MSLR = ~ 0.5m
Melting of Greenland ice sheet = projected MSLR = ~ 7.2m
Melting of Antarctic ice sheet = projected MSLR = ~ 61m
What are five impacts of mean sea level rise?
- Inundation, flooding and storm damage
- Wetland migration and coastal “squeeze” - increases vulnerability of marshes
- Erosion
- Saltwater intrusions
- Rising water tables
What are the effects of inundation, flooding and storm damage as a consequence of MSLR?
Modifies propagation of tides
Storm surges raise risk of extreme events.
What is an example of a group of islands that have been/will be impacted by inundantion, flooding and storm damage as a consequence of MSLR? What is expected to happen?
Kwajalein Atoll - more than 100,000 islands in this atoll are likely to be affected by SLR.
Likely to experience:
- Greater wave heights
- Greater wave driven run up
- Raised ground water levels
- Salinisation of freshwater lens (island water supply)
- Complete island flooding
How do wetlands, such as saltmarshes, typically respond to rising SL?
Typically migrate inland
How do seawalls/physical barriers (e.g., cliffs) impact the natural response of wetlands (e.g., saltmarshes) to rising SL?
Seawalls protecting the land or physical barriers prevent inward migration of wetlands as sea levels rise. This causes the habitat to become “squeezed”, as the wetlands are pushed inland with no where to go.
Aka - “Coastal Squeeze”
Where is a region where SLR is expected to cause coastal squeezing as a result of urban barriers?
The Gulf of Mexico -> there is potential for wetland inland migration here, however these areas coincide with regions of high urban density.
Where in the UK is the threat of marsh loss especially high?
Southeastern UK - this is also associated with crucial economic regions and ecosystems.
What is the estimated rate of erosion in the past vs now along the southern coast of the UK?
Past: 1-6 cm per year
22-32 cm per year
What is the main issue associated with saltwater intrusions? What sediment feature makes this more likely to occur?
The salinisation of freshwater lenses (crucial water source).
This is more likely to occur with low permeability sediments.
How are freshwater lenses/aquifers typically recharged? What controls the size of them?
Precipitation recharges them.
Size controlled by:
- Rainfall rates
- Island size
- Water use
- Infiltration rates
- Saltwater intrusion frequency and intensity
How can saltwater intrusions impact freshwater systems via disruption of zonation?
Normally: Clear mangrove zonation along seaward edge, sawgrass marshes behind (much less resistant to salinity)
Upon saltwater intrusion: Mangroves expand landward, reducing land available for sawgrass marshes.
Sawgrass marshes are typically built on top of peat soil covering freshwater - the loss of this habitat causes the loss of freshwater systems.
How does RSL affect the water table?
Causes it to rise.
What is an example of flooding as a result of water table changes?
Honolulu.
Where groundwater is inundated on top of marine inundation, there is much more significant flooding.
What is the difference between groundwater and marine inundation?
Marine Inundation: Flooding caused by seawater overtopping the land surface, usually from storm surges, high tides, or sea level rise.
Groundwater Inundation: Flooding when the water table beneath the surface rises to the point where it emerges at the land surface (so floods regions that aren’t directly connected to the shoreline, extending the reach of SLR inland).
