What is an ice sheet?
Together, the Greenland and Antarctic Ice Sheets contain more than 99 percent of the freshwater ice on Earth.
An ice sheet is a mass of glacial land ice extending more than 50,000 square kilometers (20,000 square miles). The two ice sheets on Earth today cover most of Greenland and Antarctica. During the last ice age, ice sheets also covered much of North America and Scandinavia.
Together, the Antarctic and Greenland ice sheets contain more than 99 percent of the freshwater ice on Earth. The Antarctic Ice Sheet extends almost 14 million square kilometers (5.4 million square miles), roughly the area of the contiguous United States and Mexico combined.
The Antarctic Ice Sheet contains 30 million cubic kilometers (7.2 million cubic miles) of ice. The Greenland Ice Sheet extends about 1.7 million square kilometers (656,000 square miles), covering most of the island of Greenland, three times the size of Texas.
How do ice sheets form?
Ice sheets form in areas where snow that falls in winter does not melt entirely over the summer. Over thousands of years, the layers of snow pile up into thick masses of ice, growing thicker and denser as the weight of new snow and ice layers compresses the older layers.
Ice sheets are constantly in motion, slowly flowing downhill under their own weight. Near the coast, most of the ice moves through relatively fast-moving outlets called ice streams, glaciers, and ice shelves. As long as an ice sheet accumulates the same mass of snow as it loses to the sea, it remains stable. The critical factor of the melting of the Ice sheets.
The Antarctic Ice Sheet covers an area larger than the U.S. and Mexico combined. This photo shows Mt. Erebus rising above the ice-covered continent. Credit: Ted Scambos & Rob Bauer, NSIDC US National Snow and Ice Data Centre ( part of Colorado University )
Why are ice sheets important?
Ice sheets contain enormous quantities of frozen water. If the Greenland Ice Sheet melted, scientists estimate that sea level would rise about 6 meters (20 feet). If the Antarctic Ice Sheet melted, sea level would rise by about 60 meters (200 feet).
The Greenland and Antarctic ice sheets also influence weather and climate. Large high-altitude plateaus on the ice caps alter storm tracks and create cold downslope winds close to the ice surface.
In addition, the layers of ice blanketing Greenland and Antarctica contain a unique record of Earth’s climate history.
Has climate change started to affect Earth’s ice sheets?
The mass of ice in the Greenland Ice Sheet has begun to decline. From 1979 to 2006, summer melt on the ice sheet increased by 30 percent, reaching a new record in 2007. At higher elevations, an increase in winter snow accumulation has partially offset the melt.
However, the decline continues to outpace accumulation because warmer temperatures have led to increased melt and faster glacier movement at the island’s edges. To learn more about research on the Greenland Ice Sheet, visit former CIRES Director Konrad Steffen’s research Web page (http://cires1.colorado.edu/science/groups/steffen/).
Most of Antarctica has yet to see dramatic warming. However, the Antarctic Peninsula, which juts out into warmer waters north of Antarctica, has warmed 2.5 degrees Celsius (4.5 degrees Fahrenheit) since 1950.
A large area of the West Antarctic Ice Sheet is also losing mass, probably because of warmer water deep in the ocean near the Antarctic coast. In East Antarctica, no clear trend has emerged, although some stations appear to be cooling slightly. Overall, scientists believe that Antarctica is starting to lose ice, but so far the process has not become as quick or as widespread as in Greenland.
A researcher works with an ice core drill during the 2003 Antarctic Megadunes expedition. Credit: Ted Scambos & Rob Bauer, NSIDC
What can ice sheets tell us about Earth’s climate history?
Scientists extract ice cores from ice sheets and ice caps, studying them to learn about past changes in Earth’s climate. Ice sheets are made up of layers of snow and ice that collected over millions of years. Those layers contain trapped gases, dust, and water molecules that scientists can use to study past climates.
Like a glacier, an ice sheet forms through the accumulation of snowfall, when annual snowfall exceeds annual snowmelt. Over thousands of years, the layers of snow build up, forming a flowing sheet of ice thousands of feet thick and tens to thousands of miles across. As the ice thickens, the increasing height of snow and ice causes the ice sheet to deform and begin to flow.
Unlike a glacier, which generally flows in one direction, an ice field flows outward in all directions from the center. If an ice field covers more than 50,000 square kilometers (20,000 square miles), it is defined as an ice sheet. Although ice sheets covered much of the Northern Hemisphere during a series of Pleistocene Ice Ages, the Earth now has just two major ice sheets, one on Greenland and one on Antarctica.
Ice sheet structure, flow, melting, and fracture
The Greenland and East Antarctic Ice Sheets are roughly 3,000 to 4,000 meters (10,000 to 13,000 feet) high at their summits. The West Antarctic and the Antarctic Peninsula Ice Sheets are about 2,500 meters (8,200 feet) high.
Ice sheet flow is a function of surface slope and ice thickness. Near the summit of the ice sheet, where the slope is the lowest, flow speeds are generally a few centimeters to a few meters per year. Along fast-flowing outlet glaciers, ice speeds can reach hundreds of meters or even several kilometers per year.
Ice sheet components: Multiple factors, such as snowfall, ablation, underlying topography, ocean water, even simple gravity, all interact in shaping ice sheets. Image from LIMA: Meet Antarctica.
Ice sheets flow outward from their dome-like centers, where they are generally thickest, and push ice outward until they encounter ocean, or where climate is warm enough to melt the ice faster than the combined flow rate and winter snowfall. In areas where summer surface melt exceeds winter snowfall, old interior layers in the ice sheet are exposed. The ice sheet becomes thin, meltwater runs off the surface of the ice, and the ice sheet may terminate on land.
However, for much of Greenland and Antarctica, ice flow terminates at the ocean, as a tidewater glacier (not fully afloat) or an ice tongue or ice shelf(fully floating thick permanent ice above the ocean). In these areas, the location of the edge of the ice sheet is very sensitive to both ocean condition and the amount of ice fracturing (crevasses or rifts).
Areas with some ocean heat can rapidly melt the floating ice from the underside, thinning the ice sheet and making it weaker. Stresses from ice flowing over bedrock or around islands causes fracturing, and at the front edge of the ice this fracturing leads to iceberg calving.
Climate, weather, and ice sheets
Blue ice: This satellite image shows a variety of ice types, including blue ice, along the Antarctic coast near Mawson Research Station. Image courtesy NASA Earth Observatory.
Ice sheets and cyclonic storms (low pressure cells) have a complex interaction. Most of the moisture and energy in a storm is in the lower part of the atmosphere. As a storm approaches an ice sheet, it encounters the steep slopes of the ice sheet edges, and the air is lifted and cooled.
This leads to heavy blizzards and snowfall along the ice sheet margins. By the time the air masses reach the center of the ice sheet, they are stripped of most of their moisture. As a result, snow accumulation is typically very low near the summit of an ice sheet. In addition, the large bulk of the ice sheet, like a wall or building in the wind, can redirect storms around the ice sheet.
The high-elevation center of an ice sheet also plays a role in driving a peculiar kind of local weather created by the ice sheet itself. Over the center of an ice sheet, the air is typically dry, and skies are clear. Heat radiates to space from the ice sheet surface. This chills the surface and the layer of air just above the ice, creating an inversion of cold, dense air near the ice surface and warmer air above.
Gravity then pulls the dense layer of cold air downhill. As it flows down the flanks of the ice sheet, the cold air layer picks up speed. By the time it reaches the coast, hurricane-force winds, known as katabatic winds, result.
In contrast to coastal storms, katabatic winds can be bone dry. Cold, dry winds over the surface can lead to ice evaporation of snow and exposure of ice. “Blue ice” seen in many satellite images results from this process, as do the famous “dry valleys” of Antarctica, where the ice sheet has been completely evaporated away.
Thus, there are four ways that ice in the ice sheet may be lost: ablation (evaporation of the ice), surface melt, calving at the interface with the ocean, and melting from contact with the ocean. Mass gain occurs almost entirely by snowfall, although in a few areas rainfall on the snow can add a small fraction to the mass input.
Ice sheet mass balance
A key area of glaciological study in recent years is ice sheet mass balance. The mass balance of an ice sheet is the difference between its total snow input and the total loss through melting, ablation, or calving. So long as an ice sheet gains an equal mass through snowfall as it loses through melt, ablation, and calving from glaciers and ice shelves, it is said to be in balance.
Because ice sheets contain so much ice and have the potential to raise or lower global sea level so dramatically, measuring the mass balance of the ice sheets and tracking any mass balance changes and their causes is very important for forecasting sea level rise. Scientists monitor ice sheet mass balance through a variety of techniques. No measurement method is perfect, however, and ice sheets’ sheer size makes exact measurement difficult.
Measurement techniques for ice sheet mass balance
Scientists have adopted three general approaches to ice sheet mass balance measurement: comparing outflow and melt to snowfall accumulation (the mass budget method), observing changes in glacier elevation (volume change or geodetic method), and detecting changes in the Earth’s gravity field over the ice sheet (gravimetric method).
The study of ice sheet mass balance underwent two major advances, one during the early 1990s, and again early in the 2000s. At the beginning of the 1990s, scientists were unsure of the sign (positive or negative) of the mass balance of Greenland or Antarctica, and knew only that it could not be changing rapidly relative to the size of the ice sheet.
Advances in glacier ice flow mapping using repeat satellite images, and later using interferometric synthetic aperture radar SAR methods, facilitated the mass budget approach, although this still requires an estimate of snow input and a cross-section of the glacier as it flows out from the continent and becomes floating ice.
Satellite radar altimetry mapping and change detection, developed in the early to mid-1990s allowed the research community to finally extract reliable quantitative information regarding the overall growth or reduction of the volume of the ice sheets. By 2002, publications were able to report that both large ice sheets were losing mass (Rignot and Thomas 2002).
Then in 2003 the launch of two new satellites, ICESat and GRACE, led to vast improvements in one of the methods for mass balance determination, volume change, and introduced the ability to conduct gravimetric measurements of ice sheet mass over time.
The gravimetric method helped to resolve remaining questions about how and where the ice sheets were losing mass. With this third method, and with continued evolution of mass budget and geodetic methods it was shown that the ice sheets were in fact losing mass at an accelerating rate by the end of the 2000s (Veliconga 2009,Rignot et al. 2011b).
Satellite technology has made a significant difference to scientific estimates of ice loss both in Greenland and Antarctica. What ice sheet loss is so critical to sea water rise is its melt water that’s actually added to the ocean which helps increase overall sea level.
Arctic sea ice and ice shelves float on the water so they don’t add to sea level rise when they melt. However when ice shelves break off from the land they surround it allows increase flows of the glaciers into the surrounding sea and this does increase sea level.
As the surrounding ice shelves water warm they help melt and thin out those surrounding barrier ice shelves. It’s a positive feed back loop. Less ice the greater the absorption of energy into the surrounding water with reduced albedo effect. ( Albedo means less refraction of the incoming solar radiation)
Dark water absorbs solar radiation where ice (white) reflects solar radiation back up into the atmosphere. More solar energy is absorbed into the oceans the warmer they get. Species migration is great indicator such as fish mackerel and sea birds. They need to move further north due to fish species moving north to cooler waters.
In conclusion
The climate has gone through changing climatic conditions over tens of thousands of years, from warm tropical conditions to frozen conditions. The scientist can identify that from the ice cores. Why this current climatic change is so serious it’s happening at such an accelerated rate of change in 100’s of years not in 10,000 of years.
I remember as a small boy in my home time of Wallasey the road I lived in I could count on one hand the cars parked in the road. My family never owned a car, none of my friends parents owned cars. My teacher owned a car but her husband was business man. He owned a number of butcher shops in Wallasey.
Most people those days, either walked, biked or used the bus or the train. Cars in the 1950’s were a luxury item.
Not so many years ago in Indian, China and Asian people cycled everywhere. Last time I was in India the great majority of the population rode on small 125cc motorbikes, they were ubiquitous. Two stroke engines produce loads of exhaust fume.
The conversion of petrol/diesel in ICE ( internal combustion engine) into work propulsion/motion is incredibly inefficient only 25 to 40% of fuel produces real work or motion. Rest is lost in heat that goes up into the atmosphere.
Think about it the technology of ICE engines is now over 100 years old. In modern electronic terms it’s archaic! Hence the reason for drive train factories closing down, here in the UK and abroad.
The writing is on the wall for ICE propelled mobility and the auto manufacturers unfortunately know it! I’m sure some, if not many, will have a Kodak moment.( Kodak failed to adapt to digitised technology). The future of automobiles will be with Silicon Valley and the digitised silicon chip world.
https://youtu.be/f7sEhuSbQo8https://youtu.be/AV2DvfWKyC0https://youtu.be/YRe1ymYR45k