External Controls on IS IC and Glaciers

Question 1

What are external controls?

Answer 1

Anything external to the ice mass which can affect its behaviour. E.g. Changes in precipitation, sea level change etc.

Question 2

What is the response time range from temperate glaciers to ice sheets?

Answer 2

5-10 years for temperate and up to thousands for Greenland and Antarctica.

Question 3

Johannesson et al., 1989

Answer 3

Thought ice sheets would not respond rapidly to changes in the oceans and atmosphere.
However we have since discovered that ice sheets can respond rapidly to external controls, which don’t result in a new steady state but change the mass balance and SLR contributions.

Question 4

Greenland Ice Sheet facts:

Answer 4

Extends 2500km N- S and 1000km W - E.
Volume of 2.6million km^3.
Complete melting would contribute 7.4m SLR (Bamber et al., 2011)
Has lots of fast flowing outlet glaciers, and floating ice shelves.

Question 5

Greenland Ice Sheet Mass Balance Trends

Answer 5

Until the 90’s the GrIS was thought to have been in a steady state (van den Broeke et al., 2016). Yet in 2012 a record mass was lost of 450 gigatons (1.2mm SLR).
Currently 60% of mass loss is from surface melting and 40% from solid ice discharge, yet this can shift.

Question 6

How much mass is GrIS losing?

Answer 6

Satellite gravimetry shows the GrIS is losing 400Gt/yr, (contributing 1mm/r SLR).

Question 7

Ice Caps and Glacier Mass Balance Trends

Answer 7

There is a similar picture of increased mass loss since the 1960s but they are contributing slightly less that the GrIS to sea level rise. However their smaller volume means they respond faster to external controls.
Current glacier extents are out of balance with current climate, indicating glaciers will continue to shrink into the future without further temp increase.

Question 8

2100 Climate Predictions

Answer 8

Albedo-Temperature feedback is likely to increase warming in high latitudes. The GrIS will be approximately 1.5 degrees warmer with 18% more precipitation.
Ice sheets and glaciers are variable but almost all warmer, some drier and some wetter.

Question 9

How would ice sheet geometry react to increased accumulation and ablation?

Answer 9

• Steepening of mass balance gradient.
• Steeper ice surface hence greater driving stress.
• Faster ice flow.

Question 10

How would ice sheet geometry react to reduced accumulation and ablation?

Answer 10

• Slackening of mass balance gradient.
• Shallower ice surface hence lower driving stress.
• Slower ice flow

Question 11

How does subglacial hydrology affect basal motion?

Answer 11

Basal friction is controlled by effective pressure from the ice mass and the roughness of the substrate. Water pressure reduces basal friction. Hence if subglacial water pressure increased, friction decreases and motion increases.

Question 12

Krabill (1999)

Answer 12

Used airborne laser altimetry surveys to show peripheral parts of the GrIS were thinning at up to 50cm/yr.
Most popular explanation was that surface melt could access the ice bed and lubricate basal ice motion, leading to ‘draw-down’ of inland ice.

Question 13

Zwally et al., 2002

Answer 13

First to discover rapid and large changes in GrIS ice flow rate which coincided with the timing and strength of surface melting, suggesting a direct link between surface and bed. now known informally as the Zwally Effect.

Question 14

Seasonal Hydro-Dynamic Coupling Summary

Answer 14

Early Melt Season: Increasing runoff overwhelms inefficient winter drainage system and then overwhelms and channels that form, leading to significant ice speed up.
Late Melt Season: Evolution in the efficiency of the drainage syste and broadly falling runoff leads to a reduction in water pressure and ice motion despite high surface melt rates.
Post melt season: Low pressure channels draw water from surrounding regions leading to a reduction in water pressure. Motion reaches a minimum following melt cessation, and recovers gradually over winter as basal water pressures increase once again.

Question 15

Inter-Annual Hydro-Dynamic Coupling

Answer 15

Observations show that within 80km of the margin there is no correlation between ice motion and surface melt on annual timescales (Sole et al., 2013). Larger and more extensive subglacial channels (result of extra summer melt) lead to a greater reduction in regional basal water pressure after melt ceases.

Question 16

What happens in years with more surface melt?

Answer 16

• Ice flows faster in summer, longer melt season with more variability.
• Ice flows slower the following winter due to larger decrease in basal water pressure.
  Further from margin: surface melt decreases, less input into bed. Thicker ice means more rapid ice creep closure of channels, and shallower surface slopes decrease hydraulic gradients, less turbulent flow melts less channel walls.