A glacier is a large body of ice that moves slowly over land under the pressure of its own weight and the force of gravity. A common feature of the Earth’s cryosphere, glaciers are formed by the gradual accumulation and compaction of snow, a process which takes many years, even centuries. Most glaciers are found in high altitude mountain ranges and across the polar ice sheets in Greenland and Antarctica. They usually flow into a mountain lake or the ocean and are quite different – in size, content and behavior – from the thin ice that typically forms on water.
Why Do We Have Glaciers?
Because of natural climatic conditions. As long as snow falls in winter and doesn’t melt away in the summer before more snow arrives to compress the first layer, we’ll have glaciers. Well, we would have, were it not for global warming, which is causing widespread ice melt.
Glacial ice is very sensitive to variations in climate – including rising temperatures, rainfall/snowfall, mean temperature, and cloud cover – so glaciologists now play a key role in assessing the rate of global warming and comparing it to earlier fluctuations in Earth’s temperature, in terms of carbon dioxide concentrations, sea levels and other metrics.
According to the latest satellite data, glaciers (like Totten glacier) in East Antarctica , an area previously thought to be gaining ice mass, are systematically shedding ice, albeit not quite as rapidly as glaciers in West Antarctica (e.g. Thwaites glacier). Events in Antarctica are particularly important, because when it comes to our climate crisis, the southern polar region is the No 1 danger area. Its glaciers and ice sheets hold so much water that any form of runaway ice melt could have unimaginable consequences for sea level rise around the world.
- How Many Glaciers are there?
- How Much Land is Covered by Glacial Ice?
- Are Glaciers Growing or Shrinking?
- How Big Are Glaciers?
- Which Are the World’s Biggest?
- How Thick Are They?
- Do They Need a Particular Climate?
- Where Are Glaciers Found?
- Does the Arctic Contain Glaciers?
- How Do Glaciers Form?
- How Do Glaciers Retreat?
- What are the Most Common Types?
- Effects of Climate Change on Glaciers
How Many Glaciers are there?
There are roughly 198,000 glaciers in the world, covering a total of 726,000 square kilometers. The largest ones are found on the Greenland ice sheet or the Antarctic ice sheet, with others in Alaska and Iceland. In the United States, glaciers occupy about 90,000 square kilometers (35,000 square miles), mostly in Alaska which (according to a 2011 survey) has about 27,000 glaciers. The number of glaciers, however, is less important than total glacial land coverage.
How Much Land is Covered by Glacial Ice?
At the height of the last ice age, about 20,000 BCE, glacial ice covered about 32 percent of Earth’s total land area. Today, that figure is down to 10 percent, with glacierized areas (including glaciers, ice caps, and ice sheets) covering about 15 million square kilometers (5.8 million square miles). 1 In total, glacial ice stores approximately 69 percent of the world’s fresh water.
Are Glaciers Growing or Shrinking?
Glaciers are shrinking. Worldwide, most glaciers are dwindling in size or disappearing altogether. According to a study led by the University of Zurich, which employed classical glaciological field measurements along with a wealth of satellite data, glaciers from 19 different glacierised regions around the world lost 9.6 billion tonnes of ice, between 1961 and 2016. 2
How Big Are Glaciers?
Some glaciers are as small as football fields, while others grow to be hundreds of kilometers in length. In a relatively recent study of glacial ice, which surveyed 1,100 glaciers, using a variety of measuring techniques including ground penetrating radar (GPR) and airborne/terrestrial radio-echo sounding, 26 percent of glaciers investigated have a size of less than 5 sq km; 29 percent cover an area of 5-50 sq km; and 30 percent cover 50–500 sq km. 3
Why is glacier ice colored blue?
Many glaciers and icebergs often appear blue. This is because water molecules absorb other colors more efficiently than blue, and also because of the lack of air bubbles, which usually impart a white color to ice. Because glacier ice is dense and compacted, most of the air is squeezed out.
Which Are the World’s Biggest?
According to data from GLIMS (Global Land Ice Measurements from Space), the three largest glaciers in the world are Vatnajokull Glacier in Iceland, the Flade Isblink Ice Cap in Greenland, and the Seller Glacier in Antarctica. The RGI (Randolph Glacier Inventory) database differs: it lists Flade Isblink Ice Cap in Greenland, Carney Island Ice Cap in Antarctica, and Seward Glacier in Alaska as the top three. Antarctica’s Pine Island Glacier is the world’s largest ice stream. 4
How Thick Are They?
Approximately 83 percent of glaciers have a mean thickness of less than 100 m (330 ft). 3
Do They Need a Particular Climate?
Yes. Glaciers need very specific climate conditions: namely, high snowfall in winter followed by a cool summer, in order to ensure that winter snow isn’t lost during the summer. Such conditions typically occur only in polar and high-altitude regions, although there are areas in Antarctica where the temperature is perfectly cold but snowfall is too light. As a result, it can be thousands of years before enough snow accumulates to form glacial ice.
Where Are Glaciers Found?
Because of its special climate needs, about 99 percent of all glacier ice is found within the vast ice sheets (referred to as “continental glaciers”) that cover the Antarctic region and parts of the Arctic, notably Greenland. The remaining glaciers are found in mountainous ice fields and alpine biomes, such as the Swiss Alps, the Himalayas, Tien Shan, Andes, the Zagros mountains in Iran, the Chitral Valley of Pakistan, and a number of sites in East Africa, Mexico, New Guinea and New Zealand.
Does the Arctic Contain Glaciers?
Yes. There are numerous glaciers within the Arctic Circle, such as Jakobshavn Glacier, Helheim Glacier and Kangerlussuaq Glacier in Greenland, Bering Glacier in Alaska and Yelverton Glacier on Ellesmere Island, and thousands of others.
Unfortunately, many Arctic glaciers are losing ice rapidly due to rising temperatures across the circumpolar region, and because of the loss of Arctic sea ice, which causes greater mixing of seawater and faster underwater ice melt. In addition, winter sea ice and pack ice also acts as a buttress against glacier ice flow, slowing the flow speed.
What is an Ice Sheet?
Ice sheets are vast continental masses of glacial ice and snow that cover more than 50,000 sq km. The Greenland ice sheet is one example, the Antarctic, another. Actually, Antarctica has three ice sheets: the Antarctic Peninsula ice sheet, the West Antarctic ice sheet and the East Antarctic ice sheet. They cover the entire terrain except for the Trans-Antarctic Mountains. The Greenland ice sheet is far smaller than the overall Antarctic ice sheet, which contains about 17 times more ice.
Ice sheets also used to cover most of Canada and Scandinavia, but these have now gone, leaving behind only a few ice caps and alpine glaciers. Ice sheets are typically drained by numerous glaciers according to the topography of the region.
What is an Ice Cap?
Found mainly in polar and sub-polar regions, ice caps are miniature ice sheets, measuring less than 50,000 sq km (19,300 sq mi). Their typical dome-shaped ice mass flows downward in all directions. An example is the ice cap on Ellesmere Island in northern Canada.
How Do Glaciers Form?
The formation of a glacier begins as snow piles up over time, partially melts, refreezes, becomes compacted and turns to ice. As more layers of snow are added, the lower layers are compacted even further. This compression squeezes out most of the air pockets, adding to the snow’s density.
Generally, if the compressed snow survives one full year, it forms a dense, granular layer of ice, known as firn or névé. This is an intermediate type of frozen material between snow and glacier ice, about two-thirds as dense as water.
This firnification process continues for dozens or perhaps hundreds of years, during which the ice grows thicker and denser. Once it reaches about 50 m (160 ft) thick, the body of ice begins to move outwards and downwards under the pressure of its own weight, and in the process becomes a glacier.
As it moves, the huge mass of ice behaves like a very viscous liquid, oozing forward over uneven surfaces until it covers everything in its path. Unlike a liquid, a glacier is extremely heavy and powerful, gouging and bulldozing everything in its path.
Glaciers are often so heavy and create so much pressure, that they generate their own heat. In fact, they create three types of heat: dissipative heat due to ice deformation; frictional heat at the glacier base; and frictional heating from flowing water down englacial channel walls. As a result, some glacial ice melts without any increase in temperature. The resulting meltwater lubricates the bottom of the glacier making it easier to flow across the ground. 5
As a glacier moves, especially if it has to move around a rocky outcrop or similar obstacle, internal stresses build up inside the ice body. This is compounded by the fact that movement along the underside of a glacier is slower and more difficult than movement at the surface due to contact and friction with the ground.
These stresses lead to frequent cracks and crevasses on the glacier surface. During the summer, as ice and snow begin to melt on the surface of the glacier, meltwater flows through these cracks and vertical drainage shafts known as moulins. Moulins are normally much deeper than crevasses, penetrating right to the bottom of the glacier.
Sometimes, even a thin layer of meltwater at its base causes the glacier to slide down a slope. This basal sliding accounts for a good deal of the movement of thin glaciers on mountainous slopes. Thicker glaciers that cover gentle slopes rarely experience this type of movement.
Most glaciers move very slowly, hence the phrase “glacial speed”. Usually they advance only a few centimetres to a few meters each day. Occasionally, however, glaciers speed up. This is known as “surging” or “galloping”. A surging glacier is capable of covering dozens of meters a day.
In 1986, the Hubbard Glacier in eastern Alaska reached a speed of around 10 meters (32 feet) per day as it blocked the outlet of Russell Fjord, creating “Russell Lake”. But the fastest known glacial surge was achieved by the Kutiah Glacier in Pakistan. In 1953, it advanced more than 12 kilometers (7.5 miles) in three months, averaging approximately 112 meters (367 feet) per day. 6
Glaciers don’t just contain ice and snow. They also contain rocks and sediments. As the glacier passes, debris and pieces of bedrock are pulled from the surface and carried along in the ice. Rocks transported for long distances by glaciers are known as glacial erratics. When finally jettisoned, they may be quite different from the surrounding landscape. An interesting example is the massive 15,000-ton quartzite boulder called “The Big Rock”, which is located near Okotoks, Alberta. It was uprooted from a site about 1,640 kilometers (500 miles) away by a glacier during the last ice age.
Debris is frequently deposited to one side of the glacier as it slowly bulldozes its way forward. These ridges of sediment are called lateral moraines. Deposits dumped at the front of a glacier are called terminal moraines.
In addition, the surfaces of many glaciers – in Iceland, Svalbard, Alaska, Venezuela and Chile – have been found to contain colonies of fuzzy, green moss balls, known as “glacier mice”. The moss balls can persist for years and they move around in a coordinated, herdlike manner that scientists are unable to explain. 7
All this can make glaciers look extremely dirty.
Alpine glaciers typically start in bowl-shaped hollows called cirques. Snow and firn fill up the cirque and then flow down into the valley. A number of cirque glaciers can merge to form a single valley glacier. When valley glaciers flow down from the mountains, they spread out and join to form a piedmont glacier.
Continental glaciers don’t follow a single valley or direction of flow. In fact, continental glaciers quite often don’t flow downhill at all, because the areas over which they flow are usually flat. Instead, glacial ice flows from the region where it is thickest – areas where snowfall and ice accumulation are highest – towards the parts where it is thinnest. As a result, in the thickest parts, the ice descends almost vertically towards the base, while in the peripheral areas, it flows laterally to the edges. 8
How Do Glaciers Retreat?
Glaciers lose ice due to various factors, including surface melting, reduction in albedo due to microbial activities, evaporation, wind erosion, the calving of icebergs and ice shelf melt. But ice loss (known as ablation) is a natural (mostly seasonal) part of glacier life. It’s all down to mathematics. As long as the amount of snow accumulation equals or is greater than ice loss, glaciers remain in equilibrium or grow. But the moment winter snowfall is exceeded by summer melt, the glacier will retreat.
When a glacier retreats does it actually reverse course and flow uphill? No. The “retreat” we talk about, only refers to the location of the terminus of the glacier. The glacier continues to move down the slope under pressure of its own weight, even when in retreat. It merely melts faster than it flows, in which case the front edge of the glacier retreats back up the slope as it melts. Materials left behind by a retreating glacier – a mixture of rock, soil and gravel, called till – are known as ground moraines.
What is an Ice Field?
An ice field is an expanse of interconnected glaciers (less than 50,000 sq km), typically located in mountainous terrain. What’s the difference between an ice field and an ice cap? The answer is: ice flow in an ice field is regulated by the underlying topography. Ice fields usually have visible peaks and ridges that poke out above the surface and influence the flow of ice, whereas ice caps submerge the topography beneath their dome-shaped mass.
What are Ice Streams?
Ice streams are typically ribbon-shaped glaciers bordered by ice that is flowing more slowly, rather than by slopes or rocks. Perhaps the most famous ice stream is Antarctica’s Pine Island Glacier.
What is an Ice Shelf?
Ice shelves form when ice sheets extend into the sea and float on the water. Ice shelves fringe most of the Antarctic continent, varying in thickness from 100 m to over 1 km (0.62 mile).
What are the Most Common Types?
These form in high mountainous regions, typically draining icefields that extend over several peaks. The largest mountain glaciers can be seen in Alaska, Canada, the Andes, and the Himalayas.
These glaciers are named after the rounded hollows they first occupied. Like mountain glaciers, they are found on mountain slopes at high elevations and are usually wider rather than long.
Typically emanating from mountain glaciers, these ice formations flow down valleys, often advance beyond the snow line, occasionally reaching sea level. Sometimes, alpine glaciers deepen valleys by pushing dirt, rocks, and other debris out of their way, creating U-shaped valleys out of V-shaped ones.
A piedmont glacier occurs when one or more valley glaciers spill out onto relatively flat plains, forming a distinctive bulb-like shape. Malaspina Glacier in Alaska is a famous example of a piedmont glacier, as is the Canada Glacier, located in the McMurdo Dry Valley, Antarctica.
A tidewater glacier is a valley glacier that reaches the coast. When a glacier meets the sea, its front edge lifts and floats in the water, forming cliffs of ice that may be hundreds of meters high. In some areas, they provide ecologically important breeding habitats for krill and seals. Sometimes, substantial chunks of ice break off the tidewater glacier in a violent process known as calving. These chunks of glacial ice are called icebergs.
Tidewater glaciers may be grounded or floating and often lead to calving ice walls. In a fjord setting, the width of the ice front is usually regulated by the sides of the valley, but may be broader where unconstrained by topography.
When a major valley glacier retreats, it sometimes leaves isolated tributary glaciers in smaller valleys high above the old glacier surface. These tributary ice bodies are known as hanging glaciers.
Effects of Climate Change on Glaciers
Since the early 1900s, with few exceptions, glaciers across the world have been losing ice at unprecedented rates, even in East Antarctic, the heart of the cryosphere. In fact, a number of ice caps, glaciers and ice shelves have vanished altogether during this time. Many more are melting so fast that they may disappear completely within decades. 9 A typical example is the set of alpine glaciers on Mount Kilimanjaro in Tanzania. In 1912, they occupied 12 sq km (4.6 sq mi). By 2009, they covered a mere 2 sq km (0.8 square miles).
And the situation seems to be deteriorating faster than ever. According to the American Meteorological Society, the cumulative net loss of glacial ice, during the period 1980-2018, is 21.7 meters: that’s the same as cutting a 24 meter (79 ft) thick slice off the top of the average glacier. 10 11
The widespread nature of these glacier retreats strongly suggests that climate change is the principal culprit, which is bad news because global temperature projections indicate that things are likely to get significantly worse.
Other related causes of glacier retreat, include increased levels of air pollution, including atmospheric dust and black carbon aerosols from forest fires – like the circumpolar Arctic fires of 2019 and the Australian bushfires (2019-2020). Haze clouds that form over parts of South Asia can end up depositing soot over the Arctic. See for example, the Asian brown cloud, some of whose pollutants fall to earth in the Arctic Circle.
The aerosols created by these events eventually fall to earth. If they land on glaciers or ice sheets, their dark color can reduce the albedo effect of the ice, thus reducing the amount of sunlight reflected back into space. This climate feedback leads to higher temperatures in polar regions.
There are three main consequences of glacier retreat.
First, it damages the water supply. This is because many glaciers store water as ice during the winter and release it as meltwater in the summer. In the process, they create a water source of particular importance for plants, animals and humans when other sources may be scarce or non-existent. As glaciers retreat, this precious reservoir of freshwater becomes increasingly depleted.
A recent example is the Chacaltaya Glacier in Bolivia, which was the largest single source of fresh water and electricity for the country’s capital city, La Paz. In 2009, Chacaltaya Glacier melted entirely, triggering a humanitarian crisis.
Second, glacier melt is likely to add to the warming of the polar regions. In turn, this is almost certain to weaken the thermohaline circulation that drives deep-water currents around the world, with unpredictable consequences for ocean temperature, the marine food web and levels of carbon dioxide in the atmosphere.
The second consequence is rising sea levels. If all the glaciers were to melt, sea levels would rise by 41cms. This may not sound too bad, but if all the glaciers melted then we might see the beginning of the meltdown of the ice caps and ice sheets. It might take centuries for the ice sheets to disappear completely, but the process could rapidly become irreversible.
It’s worth remembering that 93 percent of global warming has been absorbed by the oceans, the ramifications of which remain unclear. Will ocean thermal expansion lead to more rapid warming of the ice? We’ll find out soon enough, I’m sure.
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- “Global glacier mass changes and their contributions to sea-level rise from 1961 to 2016.” Zemp, M., Huss, M., Thibert, E. et al. Nature 568, 382–386 (2019).
- “A database of worldwide glacier thickness observations.” I.Gartner-Roer, et al; Global and Planetary Change Volume 122, November 2014, Pages 330-344.
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- “Thermodynamics of Glaciers.”
- Dickson, James H.; Johnson, Robert E (2014). “Mosses and the beginning of plant succession on the Walker Glacier, southeastern Alaska“. Lindbergia. 37 (2): 60–65.
- “Physical Geography”
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- “Climate Change: Glacier Mass Balance.”
- “State of the Climate 2018” WMO.