Dynamic Planet/Glaciers

Glaciers are the topic of Dynamic Planet for the 2019 season. This was previously the event's topic during the 2013 and 2014 seasons.

What are glaciers?
Glaciers are large masses of snow and ice that have accumulated over years of snowfall and have flowed at some point in their lifespan. Glacier thickness can range from as little as 50 meters or less, to well over 2 kilometers. They form from years of unmelted snow being compressed into dense, glacial ice.

All glaciers originate on land, but can flow into the sea. Glaciers which extend into the ocean should not be confused with Sea Ice, which is formed from seawater freezing rather than snow compressing.

Glaciers are immense bodies of ice, often several Gigatons in mass. As such, they leave behind a very unique landscape, with dozens of types of landforms.



Distribution of Glaciers
Glaciers can form anywhere that the average annual temp is low enough for snow to last all year round. These locations are normally in high latitudes or at high elevations. The direction that a mountain faces can also affect their formation. In the Northern Hemisphere, glaciers will often form on the Northern face of mountains, as the sun's rays will always be coming from the South. Glaciers are found in/around all seven continents. The total area is about 15,000,000 sq. kilometers.

Worldwide Coverage (sq.km):



Antarctica:	  11,965,000 (without ice shelves and ice rises) Greenland:	  1,784,000 Canada: 	  200,000 Central Asia:	  109,000 Russia:	          82,000 United States:	  75,000 (including Alaska) China & Tibet:	  33,000 South America:    25,000 Iceland:	  11,260 Scandinavia:	  2,909 Alps:  	   2,900 New Zealand:	  1,159 Mexico: 	  11 Indonesia:	  7.5 Africa: 	  10 The Worldwide Coverage info was taken from the following site: http://nsidc.org/cryosphere/glaciers/questions/located.html

Formation
Glaciers form when snow and ice are able to remain throughout the year. Once snow begins to build up, it compresses the snow below it to form ice.

First year glacial snow is known as neve. Accumulated snow that has survived one melt season is known as firn. Eventually the ice will reach a critical mass that will allow it to flow, and a glacier is born.

There are a wide variety of conditions which affect the formation and maintenance of glaciers.

Movement
Glaciers flow because of gravity. While other factors affect the rate and specific ways in which glaciers flow, the driving force can always be attributed to gravity. Whether it's simply pulling downhill on the ice or other ice uphill being dragged down and pushing more along, the weight of a glacier is what makes it move. Unsurprisingly, the steeper the slope of the mountain that the glacier rests upon, the faster it will flow. Glaciers generally cannot flow on level ground or uphill until they are over 60m thick.

The advance and recession of a glacier should not be confused with its flow. A glacier never flows backwards or uphill, but it can have a net loss of ice at its terminus to make it appear to recede further up the mountain. Advance and recession only has to do with mass balance and gain/loss of snow & ice, not with how it moves.

The actual way in which a glacier flows is dependent on far more factors. There are three main ways a glacier can flow, but can often include two or even all three methods if the environment changes substantially between different parts of the glacier.

Basal Sliding
This process involves the movement of the base of the glacier across the bedrock upon which it lies, usually incorporating meltwater. When comparing glaciers which undergo basal sliding, thinner, steeper glaciers are most active. There are 3 ways in which basal sliding is accomplished.




 * Basal Slip is when a thin layer of water between the ice and rock lubricates the glacier, allowing for faster flow. This meltwater can come from a variety of sources, including pressure-melting, percolation, and moulins & conduits. This is generally more applicable to smoother bedrock surfaces, but still constitutes the majority of basal sliding. Additionally, if enough meltwater is present, Basal Slip can allow for a surge to occur, where the rate of flow is several orders of magnitude higher than normal.


 * Enhanced Basal Creep is when the ice encounters a large obstacle. The large increase in pressure causes the ice to deform plastically around the obstacle.


 * Regelation Flow is when ice encounters a small bedrock obstacle. Rather than deforming around it, it melts under the pressure and refreezes on the other side. This only happens if the object is small enough to allow the latent heat on the lee side (refreezing) to be quickly conducted to the stoss size (melting) and assist further melting.

Internal Deformation
Also known as Creep, Internal Flow, Plastic Flow, and Plastic Deformation, this process involves ice crystals slowly sliding across each other from within the glacier. Ice can deform because it behaves plastically under standard glacier conditions, but can crack with large stresses. Internal deformation occurs in all types of glaciers, since it is not reliant on meltwater.

Bed Deformation
(Sometimes Subglacial Deformation) As its name suggests, Bed Deformation involves the shifting of softer sediments to allow the glacier to move downhill. Subglacial till is composed of unsorted sediments with a wide range of sizes, from boulders to clay. Finer sediments, such as clay and sand, deform readily when shear stress is applied and also have high power-water pressure (pressure of groundwater between particles). Much like basal sliding, bed deformation depends on meltwater at the base. Basal sliding is more efficient if water remains directly under the surface of the ice, whereas bed deformation is more prevalent where the sediment becomes saturated with water, reducing its strength.

Thermal Regime
Due to the importance of meltwater in two of the three methods of glacier flow, the thermal regime of a glacier cannot be ignored. Simply put, the temperature of a glacier, more specifically its base, determines its thermal regime. There are two sets of names for thermal regimes, which will be combined into one for the purposes of simplicity.

Cold-Based (Polar): These glaciers are frozen effectively year-round, excluding any seasonal melting near the surface. Most importantly, the base of the ice is frozen. These are generally found at higher latitudes and have lower seasonal variations in temperatures. There is very minimal to no meltwater. They move exclusively through internal deformation without any basal slip or bed deformation. The ice is generally frozen to the rock.

Warm-Based (Temperate): Also known as Wet-Based, glaciers of this thermal regime are characterized as being warm enough to have meltwater. They are generally at or very close to their melting point during the year throughout the entire thickness of the glacier. They are generally found at lower latitudes. The movement of these glaciers is largely through basal sliding (specifically basal slip). Meltwater plays a substantial role in the process, mainly coming from surface melt that is channeled to the bottom through moulins, tunnels, crevasses, and more. If the basal ice melts, either through temperature or pressure-melting, even more basal slip can occur. During the winter months, the glacier often refreezes to the bedrock, slowing the movement periodically. The meltwater of warm-based glaciers can also lead to an increase in plucking, leading to increased sediment transport.

Polythermal (Subpolar): Polythermal glaciers are those which have components of both warm- and cold-based glaciers, which vary depending on the location. Realistically, most valley glaciers are polythermal, containing both elements of warm- and cold-based glaciers, depending on the area being looked at. They can range from mostly Warm-based to mostly cold-based.

Factors which prevent movement
While various factors contribute to the movement of glaciers, others directly oppose it. First and foremost, friction between the ice and bedrock is (usually) the biggest contributor to stopping a glacier. Another factor which counteracts glacial movement is debris at the terminus, such as terminal moraines, which provide an extra "wall" that the glacier has to push along. Finally, there is Ice Shelf Buttressing. Here, an ice shelf prevents an outlet glacier from advancing any further into the sea, slowing or stopping its flow.

Mass Balance
A glacier's mass balance is defined as the difference between accumulation levels and ablation. Accumulation is the addition of snow or ice onto the glacier. Ablation is the depletion of ice from the glacier, through processes such as sublimation and evaporation. A glacier will advance when there is a net positive gain in ice, particularly at the terminus, making it grow further downslope. A glacier will retreat when the opposite occurs; it will melt away, leaving the terminus higher up the mountain. The visual appearance of advance and retreat should not be confused with the flow of a glacier, which is always downslope.

Glacier Parts
Glaciers themselves also have a few important parts. The top end of a glacier is known as its head, and the downhill end is known as the foot or terminus. Glaciers also have a zone of ablation, where snow melts in the summer, and a zone of accumulation, where is lasts all year. These two zones are separated by the snow line or equilibrium line, which moves during the summer.

Moulins (French for Mill), also called Glacier Mills or Potholes, are narrow, near-vertical tubes within in a glacier which start at the surface. The are generally found in a flat section of within transverse crevasses. They vary greatly in depth, but are generally no more than 10 meters wide. They carry meltwater down to the base, which can lubricate the glacier as it slides along.



Crevasses are another important component of glaciers. They are deep cracks or fractures in the ice from brittle deformation (as opposed to the plastic deformation of flow). They should not be confused with crevices, which are a similar structure but in rocks. The presence of water in a crevasse can greatly affect the dynamics of a glacier, as, if it runs deep enough, it provides a direct connection between surface melt and the bed, providing lots of water to cause or increase basal slip. Crevasses can also tell us about how a glacier moved, as stresses and strains are preserved.

Crevasses form from stress, be it tension, compression, or shear stress. There are various types of crevasses which are categorized by how they form. Marginal crevasses form near the sides (margins) of a glacier. As a glacier grinds past the stationary valley walls, the ice in the center flows faster, creating a shear and tensional stress. This results in crevasses oriented roughly 45 degrees and pointed upslope. Longitudinal crevasses form parallel to the direction of flow. They form where the glacier expands in width, or the outside edge of a turn where the valley bends. When viewing from downslope, they also form a concave down shape, but are generally nearly parallel to the valley walls. Splashing or Splay crevasses (not to be confused with crevasse splay) typically form near the terminus, where the flow is compressional. The crevasses are approximately parallel to ice flow. They are similar in appearance and orientation to longitudinal crevasses, but form from compressional forces pushing ice out laterally. If a glacier spreads out wide enough at the terminus, such as in a piedmont glacier, splay crevasses will radiate outwards from a centerline. Transverse crevasses are the most common type of crevasse. They form in the zone of extending flow, where the stress is parallel to the direction of flow. The tension stretches the glacier until it fractures. They run side-to-side across the mountain, being nearly perpendicular to flow. They also form where the valley steepens, such as at an ice fall. An ice fall, much like a waterfall, is a region of the glacier's flow where there is a sudden change in altitude. This results in heavy amounts of crevassing as the surface layers of ice are stretched more than at the base.



There are two special types of transverse crevasses: Randklufts and Bergschrunds. A Randkluft (German for marginal crevasse) is the gap between the headwall of a glacier (particularly a cirque glacier) and the ice downslope of it. They are formed when the ice directly in contact with the rock is melted, and widen in the warmer months. They are generally found in lower-altitude glaciers. A Bergschrund, sometimes Schrund (German for mountain cleft), is a crevasse which forms between a stagnant block of ice above and a moving block of ice below. They generally are found at higher altitudes.

The last major component of glaciers are Ogives. Also known as Forbes bands, they are alternating crests and valleys in the glacier ice that appear as dark and light bands of ice. They are directly linked to the seasonal motion of glacier. The distance across one light and dark band is roughly equal to the annual movement of a glacier. They form downslope of icefalls, which contain large crevasses which can be filled with snow. This pattern of accumulation creates the wavy ridge pattern. Sometimes, ogives lack either the undulating surface or the distinct colored bands. The darker bands lack trapped air bubbles, forming from the melting and refreezing of ice. The lighter bands are filled with fresher snow and air pockets. They generally take on a crescent shape, being concave up when viewed from downslope. This is due to the increased rate of flow near the center of the glacier do to lower friction from valley walls. The variations in height of the different bands of the ogvies are caused by uneven melting due to the different colors, with darker bands absorbing more solar radiation.

Glacial Morphology
Glaciers are generally classified by their location and general features, and fall into two categories based upon constrainment by underlying topography.

Constrained
Glaciers which are constrained by underlying topography are generally also called mountain glaciers as they are all confined by mountains. The only exception from this list would be ice fields.

Valley: Valley glaciers are a general group of glaciers which flow through the valleys of mountains. Sometimes they originate from cirque glaciers which have spilled down into the valley.

Piedmont: (French: Foot of Mountain) Piedmont glaciers are valley glaciers which have flowed out beyond the edge of the mountains and into an open plain. They are characterized by a fan or mushroom shape at the foot of the mountain.

Cirque: Cirque glaciers are generally the smallest type of glacier and form in bowl-shaped depressions in mountains. They can expand beyond their original confines and become a valley glacier.

Ice Field: Ice fields are large expanses of glaciers which cover mountains up almost to their peaks, leaving nunataks. This means that ice fields are still partially confined by the mountains they reside in. They can form when a large number of valley glaciers or even smaller ice fields join together.

Outlet: Outlet glaciers are a special form of valley glacier which drain ice from ice caps, ice fields, and Ice Sheets through narrow mountain passages. These can terminate both on land, into an ice shelf, or simply into the ocean.

Unconstrained
Unconstrained glaciers are generally called continental glaciers, although this more often refers to ice caps and ice sheets, and less so constituent ice streams.

Ice Cap: An ice cap is a dome-shaped mass of glacier ice that spreads out in all directions. Ice caps are usually larger than ice fields but always under 50,000 sq. kilometers. The dome shape refers to the fact that accumulation, if it occurs, is generally near the center, leading to a raised area which will flatten out by Internal Deformation.

Ice Sheet: An Ice Sheet is the same as an ice cap, except it is greater than 50,000 sq. kilometers. Ice Caps and Ice Sheets are referred to as Continental Glaciers.

Ice Stream: Ice Streams are special areas of ice caps and ice sheets with substantially increased rates of flow, upwards of 500-1000 meters per year. They are important for the mass balance of the ice caps and ice sheets, and are often riddled with crevasses and shear margins from the tension and shear stresses the ice undergoes.

Other Glacier Types
There are other classifications for glaciers which do not specifically belong to the morphological categories, or are simply subtypes of the aforementioned examples.

General
These glacier classifications do not refer to one specific type.

Tidewater: Any glacier which terminates in water, but does not extend far beyond the coast, is considered a tidewater glacier. These are generally valley and outlet glaciers. They calve at very high rates, creating lots of icebergs which can be a hazard to oceangoing vessels. They generally have high flow rates due to the calving.

Ice Shelf: An ice shelf is a glacier or ice sheet which as flowed out into the ocean. These are very thick and composed of glacial ice, and should not be confused with sea ice, which is thinner and made from seawater. The area where an ice shelf is connected to land is known as the grounding line. They are large and relatively permanent, but have been known to break away and disappear, as was the case with Larsen B in 2002. Ice shelves are also responsible for ice shelf buttressing, where outlet glaciers are held back by the sheer mass of the ice shelf.

Valley
These glacier classifications usually apply to Valley glaciers and their derivatives.

Hanging: A valley glacier which terminates at a hanging valley is a hanging glacier.

Branched-Valley: Any valley glacier which has a tributary glacier is a branched-valley glacier.

Cirque
Niche: A niche glacier is very small glacier that occupies gullies & hollows on pole-facing slopes of a mountain which are covered by shadows. If the conditions become more favorable, it can develop into a cirque glacier.

Glacial Geology
There are two important parts to glacial geology: erosion and deposition. These two processes cause the many features associated with glacial landscapes.

Glacial Deposition


Drumlins: Elongated, streamlined hills formed from glaciers acting on till and/or ground moraine. The tapered end points in the direction of glacier flow. They are similar in form to a crag & tail. Drumlins almost always form in large groups, known as drumlin fields.

Erratics: Large, misplaced boulders or rocks transported far from their source by a glacier and left behind upon retreat. Their composition can be used to determine the direction of glacial flow by matching it with potential source rocks.

Eskers: Long, winding ridges of stratified deposit, left behind by glacial meltwater streams. They usually run parallel to the direction of flow of the glacier.

Kames: Irregularly shaped hill composed of sand, gravel & till that accumulates in a depression on a retreating glacier which is deposited on land upon further retreat.

Kettles: Often found as kettle lakes, formed by bits of glacial ice breaking off and forming depressions in the ground, which then melt. Kames and kettles are generally found around and about each other, creating kettle-kame topography.



Moraine: Any ridge or mound of glacial debris that is deposited in glaciated regions. Moraines can consist of boulders, gravel, sand and clay, among other sediments.


 * Terminal moraine: deposited at the terminus (end) of the glacier, marking its furthest advance. The term End moraine refers to both terminal and recessional moraines, as they are formed in the same way.
 * Recessional moraines: recessional moraines are ridges that are behind the terminal moraine- they mark other spots where the glacier had stopped in the past.
 * Lateral moraines: are material that has been pushed off to the side of glaciers.
 * Medial moraines: form between two glaciers which converge.
 * Ground moraine: the layer of till and other sediments underneath a glacier.
 * Supraglacial moraines: accumulations of debris on top of the glacial ice.

There are several other types of moraines, including Veiki, Interlobate, Washboard, De Geer, and Rogen moraines. These are comparatively rare in nature and are usually not found on tests, as either their formation mechanisms are unknown or they are specific to certain regions.

Glacial Erosion
There are many essential erosional landforms to know, many of them occurring as a result of alpine glaciation, rather than continental glaciation.



Arête: A sharp parallel ridge of rock that resists erosion, formed by two cirque glaciers coming together but not joining. The glaciers are usually flowing down opposite sides of a mountain.

Cirque: A large bowl shaped area carved out of a mountain by a moving glacier. They are bounded by a steep cliff know as a headwall. Cirques are also called corries or cwms.

Crag & Tail: A rocky hill, isolated from other peaks and hills, formed when a glacier passes over a resistant rock formation (often granite or a magmatic intrusion) surrounded by softer material. The softer material behind the block, relative to glacier flow, is sheltered, and creates a shallow, tapering tail on the leeward side of the crag. Post-glacial erosion can often remove the tail.

Hanging Valley: A valley cut across by a deeper valley. This is not exclusively glacial valleys, but can also result in hanging glaciers.

Horn: A pyramidal peak formed by three or more cirque glaciers meeting. They are also called pyramidal peaks.

Outwash Plain: Broad, low-slope angle alluvial plain made of glacially eroded, sorted sediment (outwash) that has been transported by meltwater.



Roche moutonnée: A knob of bedrock carved into an asymmetrical hill.

U-Shaped Valley: A standard glacially eroded valley. Contrasts with a V-shaped valley, which comes from water erosion. Also called a Glacial Trough. Fjords are U-Shaped valleys which have been opened up to the sea and filled partially with water.

Sediment Transport
Entrainment: The picking up of loose material by the glacier from along the bed and valley sides. Entrainment can happen by regelation or by the ice simply picking up the debris.

Basal ice freezing: It is thought to be made by glaciohydraulic supercooling, though some studies show that even where physical conditions allow it to occur, the process may not be responsible for observed sequences of basal ice.



Plucking: (Quarrying) The process involves the glacier freezing onto the valley sides and subsequent ice movement pulling away masses of rock. As the bedrock is greater in strength than the glacier, only previously loosened material can be removed. It can be loosened by local pressure and temperature, water and pressure release of the rock itself.

Supraglacial debris: Debris carried on the surface of the glacier as lateral and medial moraines.

Summer ablation: Surface melt water carries a small load and which often disappears down crevasses. Subglacial debris is moved along the floor of the valley either by the ice as ground moraine or by meltwater streams formed by pressure melting.

Englacial debris: Sediment carried within the body of the glacier.

Moraines: See Deposition section.

Till: Debris deposited directly by a glacier. It is unstratified and unsorted. Terminal and recessional moraines consist of till.

Subglacial Hydrology
Subglacial lakes are, as the name suggests, bodies of freshwater that are contained deep within the layers of ice sheets. The largest known subglacial lake is Lake Vostok, located beneath the East Antarctic Ice Sheet. It is beneath more than 3 kilometers of ice, is 230km in length, has an area of 14000 square kilometers, and a volume of about 2000 cubic kilometers. Subglacial lakes are very abundant. The water underneath the ice remains liquid due to geothermal heating and pressure-melting. Subglacial lakes do not necessarily conform to the underlying topography, being able to form on hills in some cases. The water can also affect or create ice streams, significantly increasing the movement speed of the overlying ice. \

Subglacial channels are water channels under glaciers which are roughly parallel to the flow direction. They are one of the ways in which eskers can form.

Glacial Periods
Glacial periods (also called stadials when referring to the Quaternary) are times in the Earth's history where average global temperatures were approximately 6 C lower and glaciers covered much of the planets surface. Glacial periods are usually found in groups, interrupted by interglacial periods. This collective cycle of several glacial and interglacial periods is known as an ice age. The last glacial period, known properly as the Last Glacial Period, ended approximately 11,700 years ago. There are 6 main factors that contribute to global climate and can cause glacial periods: solar variability, insulation, dust, atmospheric composition, ocean current circulation, sea ice, and atmospheric circulation. All of these are natural processes and the only one that is affected by humans is atmospheric composition.



Causative Factors
Ice ages and glacial periods are directly caused by lower average temperatures from lower amounts of solar radiation. While catastrophes and natural disasters can create the conditions to reduce insolation, glacial periods still happen on a regular cycle. This regularity is caused by changes in Earth's positioning and movement relative to the sun, which greatly impacts the amount of solar radiation and thus temperature. These are outlined mainly in the form of Milankovitch Cycles.

Milankovitch Cycles
In the 1920s, Serbian geophysicist and astronomer Milutin Milankovitch proposed that natural variations in three parameters of the earth's orbit caused fluctuations in the amount of incoming solar radiation, resulting in glacial periods:

1.	Eccentricity - the variation in the circularity of Earth's orbital path.

2.	Obliquity - (Axial Tilt) the variation in the degree of the tilt of Earth's rotational axis.

3.	Precession - (Axial Precession) the variation in the direction of the tilt of Earth's rotational axis.



Eccentricity

Eccentricity is one of the three major cycles, with a period of approximately 100,000 years. Eccentricity is a measure of how elliptical (or non-circular) the orbit is. The lowest eccentricity is 5.5 E-5, while the highest is mildly elliptical at 0.0679. There are various components to the eccentricity cycle, with some running at 413 Ka, 95 Ka, and 125 Ka, all coming together to form a cycle of approximately 100 Ka. The current eccentricity is 0.017 and declining. The cause of eccentric variations is the gravitational effects of Jupiter and Saturn. During a more eccentric orbit, the aphelion gets farther from the sun, while the aphelion gets closer. The semi-major axis does not change, and neither does the orbital period (Kepler's 3rd Law). Since the semi-major axis does not change size, the semi-minor axis shrinks during times of higher eccentricity. This means that the magnitude of seasonal changes is greater. The difference between the amount of solar radiation at the perihelion and aphelion is greater as well. While in the current eccentricity, Earth receives approximately 6% more radiation at the perihelion than at the aphelion, during the times of peak eccentricity, this can go up as high as 25%.

Eccentricity will also vary season length. Earth will move faster at its perihelion, spending less time there. The perihelion currently occurs around 3 January while the aphelion is usually on 4 July, meaning autumn and winter are shorter in the northern hemisphere and longer in the southern. A less eccentric orbit will even out season lengths more.



So, does high eccentricity or low eccentricity favor glaciation? Technically speaking, high eccentricity does cause a lower annual amount of insolation. However, this value is very small. At 0.167% less annual insolation, this comes out to give us a total change of about 0.12 degrees C. The effects of eccentricity are relatively small compared to those of obliquity and axial Precession and do not have as large of an impact of seasonal climate variations. However, when put into conjunction with axial precession, the effects of eccentricity can quickly get amplified.

Obliquity

Obliquity is also one of the three major Milankovitch cycles, with a period of 40-41 Ka. Obliquity is the measure of the axial tilt relative to the orbital plane, and varies from 22.1 to 24.5 degrees. The current axial tilt is 23.44 degrees and declining. An increased amount of axial tilt increases the seasonal variations in insolation, with more during the summer and less during the winter. This means that higher latitudes will receive more annual solar radiation, while the equator will receive less. The currently decreasing tilt will create milder seasons and an overall cooling trend. The reverse is also true, where less tilt creates more intense seasons and less insolation nearer the poles, leading to a higher likelihood of glaciation.



Axial Precession

Axial Precession, often just Precession, is the last of the three main Milankovitch cycles. It has a period of approximately 26,000 years, but this number can vary between 20 and 29 thousand years depending on the source. Axial precession means the movement of the direction that the axis of rotation points. This means that Polaris will no longer be the north star (or pole star). In other words, when the Earth is at its aphelion, it will no longer be summer for the northern hemisphere. Axial precession is caused by tidal forces from the sun and moon, in roughly equal amounts. If we take the current situation of the southern hemisphere's summer coinciding with the aphelion, the solar radiation from both the axial tilt and the proximity to the sun are both at their peaks during that time. The opposite is also true for their winters. This means to a more extreme variation of solar radiation in the southern hemisphere. In the northern hemisphere, axial tilt and distance from the sun have their effects working against each other, resulting in less extreme variations. In approximately 13 thousand years, the direction in which the Earth points will have flipped; the north pole will be pointed towards the sun at Earth's perihelion.



Apsidal Precession & Precession of the Equinoxes

Later in the 20th century, various other scientists proposed other orbital variations as other possible contributors to the Milankovitch cycle. One of these is Apsidal Precession, or the precession of the semi-major axis. If axial tilt, the cause of the seasons, stays fixed during this time, Apsidal precession will create a similar effect to that of axial precession. Halfway through one cycle, the location of the aphelion and perihelion will be switched, meaning the northern hemisphere will have summer coinciding with the perihelion. Of course, this is oversimplified, and there are too many factors at play to make a decisive statement about its effect, but it should not be ignored.

When apsidal and axial precession are combined, we see the trend of the solstices and equinoxes "rotating" around in the orbit. This combined cycle becomes known as the Precession of the Equinoxes.

Problems with the Milankovitch Theory
Milankovitch pacing seems to best explain glaciation events with periodicity of 100k, 40k, and 20k years. This pattern seems to fit the info on climate change found in oxygen isotope cores. However, there are some problems with the Milankovitch theories.

100,000-year Problem

In theory, obliquity should have the greatest effect on the climate by affecting insolation at high latitudes. This would suggest a ~40,000-year period for glacial periods. However, research has shown that the glacial periods in the past 1 million years are dictated primarily on a 100,000-year cycle, matching eccentricity. Eccentricity variations have a significantly smaller impact on insolation than precession or obliquity and is expected to produce the weakest effects.

There are several theories which attempt to explain the problem. Internal oscillations of the climate, such as atmospheric composition, can often override the power of astronomical alignment. There is also the argument that, since axial precession relies substantially on eccentricity, eccentricity can easily dominate the precession cycle.

Transition Problem

As an extension to the 100,000-year Problem, the Transition Problem deals with the fact that, between 1 and 3 million years ago, glacial cycles indeed lined up more with the 40-41-thousand year cycle of obliquity rather than eccentricity. This time 1 million years ago is known as the Mid-Pleistocene Transition. The problem has partially been addressed as changes in atmospheric composition, namely the decline of carbon dioxide.

400,000-year Problem (Marine Isotope Stage 11 Problem)

In addition to the basic 100 thousand year cycle, eccentricity variations also have a strong 400k year cycle. Every 400,000 years, rather than rebound up to nearly 0.05, eccentricity stays well below, around 0.02. Our present-day eccentricity is actually right after one of these stunted peaks in eccentricity, which matches the end of the Last Glacial Period & Pleistocene Glaciation. Since low eccentricity favors glaciation more, approximately 400,000 years ago should also be marked by a glaciation. However, Marine Isotope Stage 11 was an interglacial period. In fact, it was the longest and warmest interglacial in the last 500,000 years.

Marine Isotope Stage 5 Problem

The MIS 5 Problem refers to the timing of the interglacial dating from 130 to 80 thousand years ago that appears to have begun 10 thousand years in advance of the orbital alignments hypothesized to have caused it. MIS 5 was also the last major interglacial prior to the end of the Pleistocene glaciation and entrance into the Holocene Epoch, the current epoch.

Effect exceeds cause

Very often, climate behavior is much more intense than calculations show they should be. Various internal characteristics of climate systems are believed to be sensitive to the insolation changes, causing amplification(positive feedback) and damping reponses(negative feedback), leading to skewed data compared to what is expected based solely on orbital parameters.

Oxygen Isotope Analysis
There are 3 stable isotopes of oxygen, 16O, 17O, and 18O. There is approximately 1 atom of 18O for every 500 atoms of the most abundant isotope, 16O.

Water molecules containing the light isotope, 16O, are more active, evaporating slightly more readily than molecules containing the heavy 18O. Thus, the 18O/16O ratio in water vapor is smaller than in ocean water; oxygen in water vapor is "lighter". Similarly, when the vapor condenses, 18O does so more readily, leaving the vapor depleted in 18O. This leaves snows precipitated onto glaciers "lighter" than the ocean water. The depletion is even more noticeable at colder temperatures, kaing winter snow isotopically lighter than summer snows.

In oxygen isotope analysis, variations in the ratio of 18O to 16O by mass present in the calcite of oceanic core samples are used to find ancient ocean temperature change, and therefore climate change. Cold oceans are richer in 18O (which is included in the tests of the microorganisms, giving us calcite).

Temperature and climate change are cyclical when plotted on a graph of temperature vs. time. Temperature coordinates are measured by deviation from today's annual mean temperature, taken as zero. Ratios are converted to a percent difference from the ratio found in the Standard Mean Ocean Water (SMOW). Either form of the graph appears as a waveform with overtones. Half of a period is a Marine Isotopic Stage (MIS). It indicates a glacial (below 0) or interglacial (above 0). Earth has experienced 102 MIS's; early Pleistocene stages were shallow and frequent while the latest were the most intense and widespread.

Interglacial Periods
Glacial periods are characterized with large ice sheets and are normally known as ice ages. The periods between these are known as interglacial periods and currently we are in the Holocene interglacial period. The last glacial period was between 120,000 to 11,500 years ago and was during the Pleistocene Epoch.

Interglacial periods are caused by the same Milankovitch Cycles which cause glacial periods. For the Quaternary period, this has been on a roughly 100,000-year cycle.

Antarctica is a great indicator if Earth is in a glacial or interglacial period because the amount of ice and snow on it indicates the amount of solar radiation that is hitting the Earth as well as the average temperature of the Earth.

CO2 is also an indicator of the changing from a glacial to interglacial period or vice versa. As the CO2 levels increase the Earth's average temperature will increase and it will move into an interglacial period whereas if the CO2 levels were to fall, the average temperature would fall and the Earth would change to a glacial period.

Marine Isotope Stages
Marine Isotope Stages (Marine Oxygen-Isotope Stages, Oxygen Isotope Stages (OIS)), abbreviated as MIS, are alternating glacial and interglacial periods. They are calculated directly from Oxygen isotope data from various core drillings. MIS's are counted in reverse, with MIS 1 being the present interglacial period, MIS 2 being the Last Glacial Period, and so-on. Interglacials are noted with odd numbers while Glacials are noted with even numbers. Over 100 MIS's have been properly identified, going back for over 6 million years, but could reach as far back as 15 million years. Some stages are divided into substages, most notably MIS 5, where each sub stage references a peak within the main MIS. More recent MIS's have more precise dating than those with higher numbers.

The Quaternary/Pleistocene Glaciation
The Quaternary Period lasted from approximately 2.588 million years ago to the present, being preceded by the Neogene. The Quaternary contains two Epochs: the Pleistocene (2.59-0.12 Ma ago) and the Holocene (0.12 Ma ago to present). In this period, specifically the Pleistocene, ice sheets were able to form in Greenland and Antarctica and the continents were formed to their present shape. As glaciers formed and later retreated, thousands of lakes and rivers were created all over the world. As the glaciers retreated the sea level rose and the amount of biological diversity in the oceans increased.

The Pleistocene glaciation is divided into a series of glacial and interglacial periods (also known as stadial and interstadials, respectively). In terms of Marine Isotope Stages, it spanned from MIS 102 all the way up to MIS 1. This also means that there were over 50 different individual glacial periods. The (arguably) most well-known of these is the Last Glacial Period, from 115 to 11.7 thousand years ago and corresponding with MIS 4 and MIS 2. MIS 3 was a local interglacial, therefore making MIS 5 still the last proper interglacial. THe LGP is also characterized by the Last Glacial Maximum, roughly 26,000 years ago, when glaciation hit a maximum. Towards the end of the LGP, around 14,000 years ago, temperatures began climbing during the Bølling and Allerød interstadials. The dates for these two events, as well as the Older Dryas Event, a period of re-glaciation, are not well defined, but the entire process lasted from approximately 14,000 to 12,900 years ago, with the Older Dryas lasting about 200 years. With the Younger Dryas Event, temperatures briefly cooled down again starting from 12,900 years ago, but temperatures never got close to reaching LGM levels. Eventually, the Pleistocene glaciation ended 11,700 years ago.

Global Effects of Glaciers
Beyond leaving landforms everywhere they travel, glaciers also have many more wide-reaching impacts, some of which can affect the entire planet.

Climatic Effects
Snow and ice have very high reflectivity. When gigantic masses of them cover the surface of the planet, as glaciers and ice sheets do, they can reflect a substantial amount of solar radiation back into space. Therefore, more, larger glaciers means more incoming sunlight gets reflected back, resulting in an overall cooling trend. Since glaciers develop in cooler climates, this becomes a positive feedback mechanism. However, the reverse is also true. Warmer temperatures which melt glaciers also expose more land and open water, both of which absorb sunlight very well. This leads to a total increase in the amount of absorption, warming the climate, and furthering melting.

Isostatic Effects
Glaciers weigh millions and millions of tons, billions in the case of some of the large ice caps and ice sheets. Tectonic plates are also not fixed, as they float on the (relatively) fluid mantle. This means that the weight of glaciers can actually depress tectonic plates further down into the mantle. When they melt, the plates resume their original level in a process known as isostatic or postglacial rebound.

Hydrologic Effects
Glaciers, being made of water, have a fairly substantial impact on the world's water. Even though freshwater makes up 2.5% of all of the world's water, 69% of it is trapped within glaciers. Glaciers, by expanding or melting, can have substantial impacts on the both regional and global bodies of water.

Glaciers often supply streams and rivers in mountains. Their meltwater provides the basis for all the drinking and irrigation water used by communities downstream, as well as those reliant on reservoirs that they feed. Their rapid decline due to global warming poses a huge threat to said communities. In the short-term, the water level will rise and the risk of floods will increase. Once the glacier has disappeared, the entire source of water will be lost, causing even more devastation.

These very same rivers and streams are also important on a local ecological scale. Many species of fish, both freshwater and saltwater, are dependent on the cold meltwater for survival and reproduction.

Glacial Outburst Floods (Jökulhlaups) and Glacial Lake Outburst Floods are also worth mentioning. Jökulhlaups originate from within or on top of the glacier as glacially-based lakes of various types. When an ice dam is broken, an outburst flood is released, creating massive damage downstream. A Glacial Lake Outburst Flood (GLOF) is similar, but comes from proglacial lakes such as moraine-dammed lakes.

The melting of all the glaciers and ice sheets in the world would lead to a sea level rise of 70 meters. The majority, 61 meters, would come from the Antarctic Ice Sheets alone, with 55 from the East Antarctic Ice Sheet, 6 from the West Antarctic Ice Sheet. The Greenland Ice Sheet would contribute about 7 meters. At the current rates of melting, this would take thousands of years. However, this is no excuse, as a rise of even a few meters threatens billions of people worldwide.

During the Last Glacial Periods, sea levels were as much as 110 meters lower than present-day, opening up various land bridges and other shallow embankments for life to cross.

The melting of the polar ice sheets also could severely disrupt the Thermohaline circulation (THC), or ocean circulation patterns. These patterns are reliant on the temperature (thermo) and salinity (haline), which factor into the density of seawater. Near the poles, warm sea water is cooled down, making it denser. Additionally, the formation of sea ice does not keep the same proportions of water and salt, leaving extra salt in the water, making it even denser. This denser water sinks to the ocean floor, where further circulation moves it away to eventually well back up to the surface. The melting of the ice sheets provides an excessive amount of less dense freshwater, which makes it difficult to sink, thereby hindering THC. Unless this water is cooled far enough to make up for the difference in density from its lack of salt content, it will eventually be able to stop THC altogether. Thermohaline circulation is extremely critical to maintaining several climates around the globe. The most notable example is Europe, which is warmed by the Gulf Stream from the Gulf of Mexico. If THC were to stop as a result of Global Warming, Europe would, counterintuitively, be severely affected by colder temperatures. There are various other places around the world where the end of ocean circulation would also spell disaster.

Recent Changes
Since the beginning of the Industrial Revolution in the 1800s, greenhouse gases and global temperatures have been on the rise. The retreat of glaciers has affected the availability of freshwater and sea levels. Mountains in the midlatitudes (e.g. Himalayas, Alps, Rockies) and tropics (Kilimanjaro) have shown the largest proportional losses. During the Little Ice Age (1550-1850), there were relatively cooler temperatures than present, but from then until 1940, glaciers retreated as the climate warmed. There was a brief cooling period from 1950 to 1980, but since then, increased global warming has created a widespread retreat of glaciers, such that many have disappeared entirely.

Midlatitude Glaciers

Midlatitude glaciers are between either the Tropic of Cancer and Arctic Circle or Tropic of Capricorn and Antarctic Circle. They generally have mountain glaciers, ice fields, and even small ice caps, and are larger the closer they are to the poles. They have been the most widely studied in the past 150 years, and virtually all of them are in retreat.

Tropical Glaciers

Tropical glaciers are found between the Tropics of Cancer and Capricorn. They are the least common types of glacier, as they are in the warmest area on earth, lack the seasonal variations for a cold accumulation season, and few tall mountains exist there. Like midlatitude glaciers, they too are in retreat. They are particularly sensitive to temperature changes.

Polar Glaciers

Polar glaciers are found within the Arctic and Antarctic Circles. They make up the majority of glacial ice in the world, with 99% of all freshwater ice contained in the Antarctic and Greenland Ice Sheets. While these are generally less affected by the recent warming patterns than mountain glaciers, an increased rate for the outlet glaciers which drain them has been observed. While their proportional losses may be smaller, they are also huge in their own right, and will have the largest individual contributions to sea level rise if they do melt.

Studying Glaciers
Oftentimes the glacier's mass balance must be recorded on the ground, although satellites are sometimes used for rudimentary recording.

The two main processes used to determine ablation or accumulation are probing and crevasse stratigraphy, which can give accurate measurements of snowpack thickness.

Probing: researchers will place poles in the ice at various points, at the beginning of the melt period or accumulation period. After a few months the researchers will return and look at the changes in levels of ice, by looking at the height of the ice along the pole.

Crevasse stratigraphy: researchers will find crevasses, then observe the number of layers that formed. Based on the layers the researchers will be able to determine how much snow accumulated. The layers are almost like layers in a tree trunk.

Besides field-monitoring of mass-balance, measurements of movement, large-scale, and long-term changes are made through extensive satellite, airplane, and ground-based photography. Dozens of satellites and routine aerial photography missions occur to document the movement and thickness of glaciers, among other things, as well as search for subglacial lakes and identify underlying topography.

Glaciers are extremely useful tools for discovering past climate conditions as well. By drilling an ice core from a glacier or ice sheet, two important sets of information can be used. First, Oxygen isotope data can be used to determine the average temperature at the time (See Glacial Periods - Oxygen Isotope Analysis) Additionally, glaciers encase air pockets from millions of years ago in the ice. When drilled out, it is possible to study their air inside. The air can indicate what the atmospheric conditions and composition were like in the past, how the temperature variated, and different types of vegetation that were present millions of years ago.

Glaciers can also indicate current climate change depending on where the snow line (firn line) is on a glacier and based on ice shelves and how they are retreating. If the snow line on a glacier continues to move up the glacier then the ablation is greater than accumulation of snow and the glacier is retreating. This causes more water to be released from the glacier and to add to sea level and to form new lakes and rivers. Ice shelves are able to indicate global climate change because if they continue to shrink and retreat that indicates that the global temperatures are increasing because the ice is melting. Retreat from any glacier can indicate climate change, but ice shelves are better at indication because they are located in places where the temperatures are normally colder.



Vocabulary
For a list of vocabulary relating to glaciers, please see Dynamic Planet/Glacier Vocabulary.

Resources

 * http://en.wikipedia.org/wiki/Glacier
 * http://scioly.org/w/images/7/78/Dynamic_planet_glaciers_smith.pdf
 * http://nsidc.org/glaciers/
 * http://www.hanksville.org/daniel/geology/glerosion.html
 * http://www.backyardnature.net/g/ice-ages.htm
 * CrazyPunyMan's Dynamic Planet Notes