2-1-2-past-and-present-ice-cover

The Cryosphere

• The cryosphere is all the frozen regions on Earth and covers 13% of the planet’s surface

• The term comes from the Greek word ‘kryo’, meaning cold

• Ice can be found in

  • High latitudes – Arctic and Antarctic Circles of more than 65° north and south of the equator

  • High altitudes – found in mountain ranges, which can be at any latitude (Drakensberg Mountains, SA is over 3000m high at a latitude of 29° south of the equator)

• Features of the cryosphere include: 

  • Snow

  • Ice (69% of the world’s freshwater is stored as ice)

  • Permafrost and frozen ground – not all frozen ground is permafrost

  • Glaciers

  • Ice caps, sheets and shelves

  • Icebergs

  • Sea, river and lake ice

• Most of the cryosphere is found in Antarctica (85%) and the Arctic polar region (12%), as ice sheets, shelf ice, and permafrost

  • The largest, single ice mass on Earth is the Antarctic ice sheet, covering 8.3% of the global land surface 

  •  It took millions of years to form; is up to 4.8 km (3 mi) deep in parts; and covers approx. 14 million km² (5.4 million mi²) and contains 30 million km³ of ice

  • If it melted, it could raise sea levels by 58 meters (190 feet) 

• Permafrost areas are significant global carbon stores and help regulate levels of carbon in the atmosphere

• The cryosphere helps regulate Earth’s climate through its high surface albedo effect

• As the climate warms, the cryosphere also changes through feedback mechanisms, which further influences the climate:

  • Increased snow and ice melt, exposes more dark surfaces to insolation

  • Which increases surface absorption of solar radiation, causing further melting and release of stored carbon and methane into the atmosphere, which leads to further atmospheric warming

  • This is a positive feedback loop, which exacerbates the impacts of climate change

Classification of Ice Masses

• There are two groups of ice masses:

  • Constrained – these do not have a dome-like structure, so the flow and shape of the ice is influenced by its surroundings – valley, piedmont and cirque glaciers

  • Unconstrained – the flow and shape of this ice is not influenced by its surroundings – ice sheets, shelves and caps

    • These have the basic shape of a broad, slowly moving, central dome, with channels of faster-moving ice that flows to, and at, its margins

Unconstrained 

• Ice sheets

  • Continuous masses of ice, that cover areas greater than 50,000 km³ 

  • With no surrounding mountains or features to contain them, continental glaciers spread out and cover the surface

  • They spread out from the centre and can cover whole valleys, plains and mountain ranges with ice

  • Sometimes only the tips of mountain peaks show above the ice, called nunataks

  • In 2009, Antarctic scientists found a mountain range, as large as the European Alps, hidden under 2.5 miles (4km) of ice 

• Ice caps

  • Cover areas of less than 50,000km³

  • Usually centred on a mountain’s high point (called a massif), the ice flows flow in multiple directions to form a cap

  • This flow of ice feeds into a series of glaciers at its edges

  • Polar ice caps are not strictly ‘caps’ as they are greater than 50,000km³

• Ice shelves

  • These are thick, floating slabs of ice, permanently attached to a landmass

  • Found where ice flows down to the coast and out onto the ocean’s surface

  • Only found in Greenland, Northern Canada, Antarctica and the Russian Arctic

Constrained 

• Ice fields

  • Ice that covers a mountain plateau, but does not extend the high-altitude area

  • Not thick enough to bury the topography and covers 5 -1500km³ 

  • Examples include the Himalayas, Rockies, Andes, and the Southern Alps of New Zealand 

• Piedmont glaciers

  • Found at the foot of mountains, where a mass of ice has flowed downslope and fans out, forming lobes of continuous ice

• Valley glacier

  • Ice is surrounded by high mountains and fills the valley

  • They are usually ribbon-shaped and vary in length from a few kilometres to over 100km 

  • They can be a single feature or made up of multiple glacial tributaries from surrounding valleys

  • Most begin as mountain glaciers and spread/flow to gorges, basins and across the valley floor

  • Examples include the Andes, Himalayas and European Alps

• Cirque glaciers

  • Most common type of glacier and found in nearly all areas where snow and ice accumulate – e.g. alpine regions

  • Confined to either the upper parts of a glacial trough or within the hollowed, cirque basin itself

  • It is the basin that dictates the size, shape and flow of the glacier

  • Niche glaciers are smaller versions of cirque glaciers

Thermal regime of ice masses

• This refers to an ice mass’s basal temperature and indicates whether water or ice will be present

Pressure melting point (pmp)

• The temperature at which ice melts at a given pressure is the pressure melting point (pmp)

• The melting point of water depends on air pressure above the ice

• As air pressure increases, the temperature at which ice melts lowers

• At 1 atmosphere pressure, the melting point of ice is 0°C

• At 200 atmospheres, the melting point decreases to -1.85°C

Warm-based glaciers

• Occur in temperate regions such as southern Iceland and western Norway

• They are relatively small and range in width from hundreds of meters to a few kilometres

• Melting occurs during the summer months

• It is this meltwater that ‘lubricates‘ the base and sides of the glacier, which assists movement (called basal sliding) and increases rates of erosion, transportation and deposition

• As such, all ice in these glaciers is at, or close to, the melting point of ice

• Temperatures at the base are, therefore, at or just above the pressure melting point

Cold-based glaciers

• Occur in polar regions such as central Greenland and Antarctica

• They are large, vast sheets and caps of ice covering hundreds of km²

• Temperatures remain below melting point, with low rates of precipitation, resulting in low levels of accumulation 

• Basal temperatures remain below the pmp, therefore, basal sliding does not happen

• This results in little erosion, transportation and deposition

• Any movement is by internal deformation

  • The ice stays frozen to the bedrock and moves slowly at 1-2cm a day

  • Orientation of the ice crystals in the glacier, to the direction of movement, allows the crystals to slide over each other

Polythermal glaciers

• These are glaciers with both warm and cold bases but at different altitudes

• They usually show a cold base in their upper reaches (high altitudes)

• At the lower altitudes, their bases are warm with meltwater

Present Day Distribution of Ice Sheets

Past glaciation

• The last glacial maximum was 21,000 years BP, where over 30% of the Earth’s surface was glaciated

• The polar ice sheets covered much of the UK and major parts of southern Europe were periglacial

• Sea levels dropped, and shorelines extended farther out, creating more land (water was trapped in ice sheets)

• The climate was drier, because most of the water on Earth’s surface was ice, resulting in less precipitation

• Earth’s average temperature was 6°C (average now is 14-15°C)

• The present-day distribution of cold environments can be divided into polar, glacial, alpine and periglacial areas

• Polar – considered areas of permanent ice within the northern and southern extremes of the Antarctic and Arctic regions

• They are found in areas of high latitude, with long winters and short summers, with high levels of storms and cold winds

  • The Arctic polar environment can be defined either by the Arctic circle at 66° N or by the July isotherm of 10° C

    • Isotherms are areas of the same temperature

    • July is the hottest month and areas north of this line have an average of 10°C or below

    • Winter sea ice is shrinking

  • The Antarctic is much colder than the Arctic, with strong westerly winds, cold oceans and a large landmass

  • Winter sea ice is increasing 

  • Defined by the 10°C January isotherm (January is the hottest month in the southern hemisphere)

• Other examples include Greenland and northern Canada

Upland Glaciated Landscapes Today

• Glaciated landscapes vary, dependent on location – polar, glacial, periglacial and alpine

• Glaciated landscapes can be divided into active (current) or relict (past) landscapes

• Geology influences the nature of a glaciated landscape

  • Igneous rock is harder to erode and often makes up high mountains with steep sides and hollows

    • Large amounts of poorly sorted sand, gravel, and boulders are plucked and pried from the surface and mountains

    • As the glacier flows over bedrock, the sediments trapped in the ice, are ground into a fine powder called rock flour

    • Rock flour acts as sandpaper, that polishes the surface of exposed rock to a smooth finish called glacial polish

    • Larger rock pieces scrape over the surface creating grooves called glacial striations

    • The Highlands of Scotland, the Lake District and Snowdonia (Eryri), North Wales show many relict landscapes from the Pleistocene epoch, including arêtes, erratics, cirques/corries, and corrie lakes

  • Sedimentary and metamorphic rocks are found mainly in low-lying areas (already eroded from the uplands) and are easier to erode

    • During the last Ice Age, the advancing ice sheet moved chalk, boulder clay etc. into the south and east of England