Lecture 5: Ice sheets in Earth's system Flashcards
What are potential theories for the causes of glaciation?
Volcanoes, Sun spots, Milankovitch cycles. (Effects on seasonality most important as need cool summers).
What are the three possible amplifiers of glaciation?
- Increased land surface albedo
- Increased GHGs
- Ocean circulation
Describe the feedback between the atmosphere and ice sheets on the earth?
- Creation of equator-pole temperature gradient.
- Precipitation changes caused by interruption of regional climates
- Strength of zonal (E-W) and meridional (N-S) circulation changes with glacial extent.
Describe the feedbacks that occur through albedo.
Albedo = reflected / incident.
Ice sheets have high albedo, snow reduces absorption on glacier surfaces (by up to 80%), more ablation on ice surfaces as no snow and dirty.
How much of the radiation that hits earth is reflected by the Antarctic ice sheet and sea ice?
80-85%
The ‘snowball earth’ hypothesises that 650Ma B.P. the earths continents were all near the equator, and ice covered the globe. Through what processes is this hypothesised to have occurred?
- Runaway ice albedo feedback
- Balance between CO2 outgassing and consumption (via chemical weathering)
- orbital forcing
Evidence against a ‘snowball earth’ includes the impossibility of both life and having a totally frozen ocean. What other arguments act against it?
- Global deposits not synchronous
- Continents have to be in a specific position to allow total glaciation / extreme orbital obliquity (60 degrees)
- Huge quantities of CO2 would have been required to melt ice
Where might refuge for life have been found in a ‘snowball earth’?
Sea Ice cracks and cryconite holes.
The ocean has feedback interactions with glaciation - what 4 types of sea level change occur from glaciation?
- Glacioeustacy (glaciers lock up water from the water cycle)
- Glacioisostacy (earths crust depressed by weight of ice sheets)
- Geoidal isostacy (ice sheet mass changes the Earth’s gravitational field)
- Steric sea level change (water becomes less dense and expands above 4 degrees celsius)
How do polar regions interact with thermohaline circulation?
Evaporative cooling in polar regions causes cold, salty water. This is denser so sinks, driving circulation.
We can use iceberg debris patterns to tell us about former ice sheets, but how may they have interacted with glaciation in the past?
Increased input of icebergs from ice sheets (to ocean) may alter temperature, salinity and ocean currents, as well as iceberg debris deposited on ocean floor,
What physical evidence is there of heinrich events?
- Coarse grained layers in North Atlantic ocean floor sediment cores correlate with ice core records and short term cooling trends.
- Iceberg rafted debris
What causes heinrich events?
Binge-purge cycles of laurentide ice sheet.
Orbital insolation changes.
Describe the ‘binge’ phase of heinrich events?
Ice sheet grows until reaching a critical thickness.
Describe the purge phase of heinrich events.
Thinning and basal melting lead to Iceberg rafted debris fall out. Once initial debris melted off there is a mid-purge IRD hiatus. Basal debris then freeze-on during the late-purge IRD output.
What is the effect of heinrich events?
Increased freshwater output to the oceans, so altered thermohaline circulation.
Which of the two Antarctic ice sheets is land-based?
EAIS
What two properties make the WAIS particularly vulnerable to collapse?
WAIS is grounded below sea level, and is buffered by a floating ice shelf (risk of disintegration).
Describe a simplified 3 stage process of the collapse of larson B.
Warming caused build up of meltwater ponds on surface.
Water seeped through crevasses.
Shelf broken into blocks which fell away.
Arctic sea ice saw its lowest extent in 2012, which opened the NW passage. What combination of mechanisms is thought to have made a similar event in 2007 possible?
- Wind patterns brought warm air to the region
- Ice already weakend
- Clear skies due to unusual atmospheric pattern
- Lengthened melt season
In what year did Greenland have record melt?
2012
What percentage of the oceans does sea ice cover each year?
15% ( around 25million km^2).
