Ice sheets Flashcards
(81 cards)
1
Q
How much of the global land surface do the two ice sheets cover?
A
~10%
2
Q
What is the Sea Level Equivalent of the ice sheets ice
A
65m (30 x 10^6 km^3)
3
Q
Volume of the AIS
A
27 x 10^6 km^3 (57.9m potential SLR)
4
Q
Volume of the GrIS
A
3 x 10^6 km^3 (7.36m potential SLR)
5
Q
Why is there close to 0 surface mass accumulation on Antarctica
A
it is the dryest place on earth
6
Q
What are the two RS methods of measuring ice velocity
A
Image correlation and SAR interferometry
7
Q
What is insolation?
A
Energy per sq. metre
8
Q
What is the impact of every 1 cm rise in sea level?
A
6 million ppl globally exposed to coastal flooding
9
Q
What are the 3 methods of measuring ice sheet mass balance?
A
Input/ output
Geodetic
Gravimetric
10
Q
How is mass balance calculated in the input/ output method?
A
Mass balance = Surface Mass Balance - Ice Discharge
11
Q
How is Surface Mass Balance (SMB) calculated?
A
SMB = accumulation - runoff
12
Q
How is geodetic mass balance calculated
A
Mass balance = volume change x density
13
Q
How is mass balance calculated using gravimetry?
A
Mass Balance = Gravity - Gravitational Isostatic Adjustment (GIA)
14
Q
Would the AIS remain in contact with the ocean for longer than Greenland if they were to fully retreat
A
Yes
15
Q
What is the cycle of continent formation and what does it influence in the ocean
A
Continents form and deform every 200 - 300 million years.
This leads to large scale fluctuations in Global Sea Level
16
Q
How much has Sea Level Changed since the LGM
A
Sea level has risen about 120 m
17
Q
How does isostatic change impact sea level reading in Scandinavia
A
The isostatic rebound of scandinavia leads to a relative drop in sea level
18
Q
What is happening to Surface Melt area in greenland?
A
It is growing
19
Q
What is the albedo anomaly on greenland?
A
The albedo is decreasing
20
Q
What period do we find ourselves in at the moment?
A
An interglacial period - have been for 12ka
21
Q
How far back can you date with ice sheet cores?
A
100s ka
22
Q
What does the oxygen 18 fraction in ice tell you?
A
It informs you of local air temperature, as it tells you how much oxygen 16 has been taken out and into ice
23
Q
How long are the records provided by marine sediment cores?
A
Millions of years
24
Q
What are the three key factors influencing growth on large time scales?
A
- CO2 drawdown
- Major changes in ocean circulation
- Variations in earth orbit
25
What is the current trend of CO2 drawdown and timescale of effects
Since the Eocene there has been slow climate cooling due to an overall lowering trend of atmospheric CO2 (greenhouse effect)
time scale : effects over millions of years
26
What is the Atlantic Boundary Current
Also known as the gulf stream, this current warms the NH by transporting heat poleward
27
what is a key factor in the existence of the Atlantic Boundary Current
The presence of continental barriers preventing East -West flow
28
What is the theory put forward by Kennett in 1977 on the emergence of the AIS?
Until 35-45 Mya, the southern Atlantic had a similar barrier until the Drake passage opened, resulting in a cooling of the continent and the appearance of the Antarctic Ice Sheet 34 Mya.
This theory has been tested with ocean models that show that the opening of the Drake Passage and the Tasmanian gateway may have cooled the ocean by 10°C.
29
What is the effect on ice sheet growth when the earth is in the aphelion stage of its orbit?
It promotes ice sheet growth, as it is furthest from the sun
30
What is the Mid Pleistocene Transition (MPT)?
It's the point, 1 Mya, when the dominant period of glacial cycles was 40ka before becoming 100ka
31
How do you calculate the driving stress of the ice flow (theoretically)
If you look at a vertical cross section, the difference between the pressure from the two sides of the cross section provides driving stress for the ice flow.
32
What is a key difference between ice sheets and glaciers in terms of the influence of slope at the bed?
Glacier motion is determined by the bed topography. This is not the case for Ice Sheets, where the opposite, surface slope determines driving stress at the bed
33
What are conditions for less ice sheet flow
Thinner ice and Gentle surface slope
34
What does a higher sliding constant mean for ice thickness and slope
Higher sliding constant means thinner ice and/or gentler slope
35
What is the explanation put forward by Pollard and Clarke 1988 for 100 000 yr cycle skipping
There is evidence that a regolith (sediment from weathering) covered North America before the NH glacial cycles began - it is now gone
There is also evidence that earlier N American ice sheets extended farther south than later ones
Given that sediment allows faster sliding in the presence of water, past past glaciations, with the same input would have formed into thinner and flatter, more extensive ice sheets
Once the regolith was cleared away, the decrease in sliding speed lead to ice sheets becoming thicker and less extensive
SO, thicker ice sheets --> longer melting time --> cycle skipping
Thinner ice sheets are more likely to melt away when climate warms
Maintenance of the Laurentide ice sheet would have cooled globally, leading to skipped cycles
This model allowed Pollard and Clarke to capture the Mid Pleistocene Transition (MPT) with true orbital forcing
36
When observing ice sheet termination in timelines, it appears to occur very fast, why?
Numerous feedback loops:
* including loss of albedo from melt --> accelerates warming
* Altitude - melt feedback --> Ice sheet surface warms as it thins due to elevation dependence of temperature
37
What is Melt Water Pulse 1a (MWP1a)
a rapid rise in sea level ~14,200 years ago
38
In recent years steric and eustatic have been similar
In recent years, ocean expansion (steric) and mass input (eustatic) have been similar
39
Is SLR distributed evenly across earth?
No
40
What is the long term (10 - 20 ka) effect of ice loss from ice sheet?
GIA - which affects perceived SLR
41
What are the 3 short term (yrs - decades) effects of ice loss form ice sheet
crustal rebound,
gravitational effects
rotational effects
42
What is crustal rebound?
Following ice loss from an ice sheet, the crust rebounds, bringing up the ocean floor, displacing water
43
What are the gravitational effects of ice loss form ice sheets?
As ice attracts the ocean by gravitation, following shrinking of ice, local ocean level drops, so water has to go elsewhere
44
What are the rotational effects of ice loss form ice sheets?
SLR changes the shape of the geoid, which can then lead to change in the direction of the earth's rotational axis
45
What is True Polar Wonder?
When the when the pole axis moves toward area of loss
- Observed in response to Greenland ice loss
46
MWP1a origins?
Option 1:
- Sea level records in Barbados and Tahiti which are consistent with sea level response to loss of ice from West Antarctica and not Laurentide
- However, this could be wrong as the record does not come from a continuous core
- MWP1a represents a large amount of fresh water, meaning West Antarctica must have been a lot thicker
- However, there is mounting evidence that it was not thick enough 15 ka ago to source this much fresh water
Option 2
- Using GIA-gravity-rotation modelling, more data constraints and more careful attention to statistics
- Lin et al 2021 provide an alternative solution to MWP1a:
§ 12m N. America
§ 4.6m from Scandinavia
§ 1.3m from Antarctica
47
What do ice streams do?
They are responsible for transporting a large art of the interior
When there is rapid ice loss, it's due to ice streams speeding up (particularly in Antarctica)
48
What are the two types of basal sliding?
Hard bed sliding and Deformable bed sliding
(both require water at the bed)
49
What is regelation and when does it occur?
phenomena when upstream pressure lowers the melting point of ice. During hard bed sliding, upstream ice when reaching an obstacle melts and freezes downstream once the obstacle has been passed
50
What is enhanced creep?
During regelation, localised stress softens the ice and allows it to deform more easily
51
WHen does caviation occur?
occurs when there is less 'pushing' on the lee side (downstream) as a result of water pressure being too high and the driving stress remaining the same
§ In scenarios like this, the bed offers less resistance but the driving stress remains the same and the flow becomes faster
§ When speed increases, high water pressure opens cavities further
52
How are basal channels closed?
By the weight of the ice above
53
What is the motion of ice when water pressure is low during deformable bed sliding?
Slower, the sediment does not respond to the shearing force (basal stress) as the sediment particles remain tightly packed
However, when the water pressure is high, the sediment particles become loosely packed and can move around
54
What are the sources of basal water in Antarctica?
Geothermal heat and Frictional drag
55
What are the sources of basal water in greenland?
surface melt, Geothermal heat, Frictional melting, Heat carried to bed by surface meltwater
56
How much summer surface meltwater is produced in greenland
400 Gt/yr
Majority drains to bed
57
How much melting occurs at the ice sheet bed in greenland
~20Gt/yr basal melt water
58
How much melting occurs at the ice sheet bed in Antarctica
~70 Gt/yr
59
What disproves the infinite velocity assumption?
Horizontal stresses from change in bed resulting in extensions and compression. (train analogy)
Extension as ice speeds up after entering weak bed from strong bed leads to tension in the ice behind and compression ahead, as the strong bed following the weak bed pushes back on the ice
Longitudinal stresses transmit the driving stress from weak bed areas to to parts of the bed than can support it
60
What takes up the imbalance between driving stress and basal stress?
longitudinal stress and lateral shear
61
What is the velocity and the driving stress of the ice streams on the Siple Coast
High velocities (800 m/yr) and low driving stress (10 kPa) - suggesting weak sediment at the bed
62
How high can driving stresses be in fast moving ice streams
100 - 150 kPa
63
What controls the speed of an ice stream, not including forces
the width of a stream controls how fast a stream goes
64
Why are there ice streams on the Sipe coast, where troughs are minimal?
Numerical simulation by Kyrke-Smith et al. 2014 shows spontaneous formation and organisation of ice streams
Formation: heating leads to water then sliding then higher speeds which leads to more heating (positive feedback)
65
What is the story of the Kamb ice stream?
An ice stream that spontaneously shut down, ~150 years ago
4 options:
1 - cooling by dynamic thinning
- where ice spread as a result of the fast flow. The continuity can lead to thinning
- The thinning leads to the bed cooling down through cold ice being pushed down.
- The bed is also less insulated, more heat gets 'sucked into' the ice interior, leading to the basal heat balance becoming negative and the bed freezing
This is important in the case of terminating thermal surges
2 - ice stream narrowing
3- water diverted to another stream - water piracy
4 - Nonlocal effect of a change in the driving stress on the basal stress
66
Does climate change influence variability in ice stream base temperature in Antarctica?
No
67
How do you locate subglacial lakes
Look at surface roughness in DEM - Areas of smooth topography represent subglacial lakes, a smooth subglacial environment
Ice penetrating radar
Look at radar reflection
- Rough surface - a lot of energy reflected back
- Smooth surface, specular reflection
Drilling - rare
68
What is used as an indicator of subglacial lakes draining?
Surface elevation change over time
69
Does ice velocity increase coincide with onset of surface melting in Greenland?
Yes
70
2013 lake drainage in Antarctica
- The flux of water drainage form subglacial lakes in antarctica is equivalent to that of Greenland in the summer
Hard to tell how much basal melt is occurring here
However, you can use events like the lake draining to test your model as the rate of refilling is the rate of melt water production
It has been found in malczyk et al. (2023) that observed recharge rates generally agree with predicted values from the model
There is little evidence that the ice speed changed as a result of the drainage
71
How does ice flow influence mass balance?
it regulates the proportion of ice in accumulation/ ablation dominated area, it modulates ice discharge
72
What is the current trend of surface melt on GrIS
GrIS surface melting has increased dramatically, starting form the mid-1990s
73
What is the paradox for Greenland's decadal slow down in ice velocity?
Although there has been a dramatic rise in surface meltwater production, there was a reduction in basal sliding, contributing to 90% of the change.
Conclusions for this slowdown propose that the increase in subglacial drainage efficiency are responsible for the slowdown
74
What does subglacial water discharge do when it reaches the ocean?
Fresh water exits from the base when it reaches the ocean, as the water is fresh and less dense, it is buoyant and rises towards the surface in a plume (they don't always reach the surface)
It brings up warmer water from deep under the water surface, warming the upper layers and increases turbulence (a factor for melting) leading to submarine melting along the ice - water interface
75
What is the contribution of atmosphere to submarine melt in Greenland
Atmosphere contributes to 1/3 of submarine melt and Ocean Terminal Forcing contributes to 2/3
76
what do hydropotential maps tell you?
they tell you where water goes under ice sheets, taken into account bed elevation and surface topography, telling you where water will collect
77
what is a latent heat polynya
where wind opens areas of the sea ice
78
What are sensible heat polynyas
created by upwelling of warm water which melts the sea ice
79
What is used as an indicator of ocean temperature?
the depth of the 0.8°C thermocline
80
what happpened to the ice shelf at thwaites after the drainage of the subglacial lakes?
there was a grounding line retreat and a record amount of thinning on the ice shelf
The increase in basal melt and change in basal friction near the subglacial outflow is predicted to have had the largest impact on Thwaites' volume change
the drainage of the subglacial lakes impacted a part of the ice shelf which was highly sensitive, leading to further impacts upstream (this was shown in the sensitivity of grounding line to changes in ice shelf document)
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