Ice-sheet model

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In climate modelling, Ice-sheet models use numerical methods to simulate the evolution, dynamics and thermodynamics of ice sheets, such as the Greenland ice sheet, the Antarctic ice sheet or the large ice sheets on the northern hemisphere during the last glacial period. They are used for a variety of purposes, from studies of the glaciation of Earth over glacial–interglacial cycles in the past to projections of ice-sheet decay under future global warming conditions.

Contents

History

Beginning in the mid-18th Century, investigation into ice sheet behavior began. [1] Since the Journal of Glaciology's founding, physicists have been publishing glacial mechanics. [1]

Barnes Ice Cap Barnes Ice Cap 432 pan merge crop 15 (31109130474).jpg
Barnes Ice Cap

The first 3-D model was applied to the Barnes Ice Cap. [1] In 1988, the first thermodynamically coupled model incorporating ice-shelves, sheet/shelf transition, membrane stress gradients, isostatic bed adjustment and basal sliding using more advanced numerical techniques was developed and applied to the Antarctic ice sheet. [1] This model had a resolution of 40 km and 10 vertical layers. [1]

When the first IPCC assessment report came out in 1990, ice sheets were not an active part of the climate system model, their evolution was based on a correlation between global temperature and surface mass balance. [2] When the second IPCC assessment report came out in 1996, the beginning of both 2D and 3D modelling was shown with ice sheets. [2] The 1990s heralded several more computational models, bringing with it the European Ice Sheet Modelling Initiative (EISMINT). [1] [3] The EISMINT produced several workshops throughout the 1990s of an international collaboration, comparing most models of Greenland, Antarctic, ice-shelf, thermomechanical and grounding-line. [3]

The 2000s included integrating first-order approximation of full Stokes Dynamics into an ice-sheet model. [1] The fourth IPCC assessment report showed ice-sheet models with projections of rapid dynamical responses in the ice, which led to evidence of significant ice loss. [2]

In 2016, part of the Coupled Model Intercomparison Project Phase 6 (CMIP Phase 6) was the Ice Sheet Model Intercomparison Project, which defined a protocol for all variables related to ice sheet modelling. [4] The project allowed for both improvement in numerical and physical approaches to ice sheets. [5]

Modelling

Ice-Flow

Shallow Ice Approximation

Shallow Ice Approximation (SIA) is a simple method to model ice flow without having to solve full-Stokes equations. [6] The approximation is best applied to ice sheet with a small depth-to-width ratio, without many sliding dynamics and a simple bed topography. [7] SIA does not include many forces on an ice sheet, and can be considered a 'zero-order' model. [7] The model assumes that ice sheets are mostly split up by basal sheer stress, and it is not necessary to consider the other forces. [8] It also assumes that the basal shear stress and the gravitational driving stress of the grounded ice balance one another out. [7] The method is computationally inexpensive. [8]

Shallow Shelf Approximation

Shallow Shelf Approximation is another method to model ice flow, in particular a membrane-type flow of floating ice, or of sliding grounded ice over a base. [9] Also known as a membrane model, they are similar to free-film models in fluid dynamics. [10] As opposed to Shallow Ice Approximation, Shallow Shelf Approximation models ice flow when longitudinal forces are strong; sliding and vertical forces. [7] SSA can also be considered a 'zero-order' model. [7]

Full Stokes Equations

It is considered advantageous to model ice using Navier-Stokes equations as ice is a viscous fluid and these capture all forces exerted on the ice. [6] As these equations are computationally expensive, it is important to include many approximations to reduce running time. [6] Because of their computational expense, they are not easily used at a large scale and can be used in specific sections or scenarios, such as at grounding lines. [11]

A diagram of some of the aspects of an ice-sheet model Illustration-of-the-mass-change-of-ice-sheets-and-key-processes-that-are-specific-to-ice-sheet-model-evaluation-or-forci.png
A diagram of some of the aspects of an ice-sheet model

Interactions with other climatic components

Ice sheets interact with the surrounding atmosphere, ocean and sub-glacial earth. [12] All of these interactive components need to be included to be able to have a comprehensive ice-sheet model. [12]

Basal Conditions play an important role in determining the behavior of ice sheets. The basal thermal state (if the ice is thawed or frozen) and the basal topography are difficult to map. [12] The most favored method is to apply mass conservation constraints. [12] For long-term projections, it is important to project the topography onto the continental shelf or into the fjords, and this can be difficult when the sub-glacial topography is not well-known. [12]

Summer Insolation drive temperature responses that have an effect on the rate of melting and mass balance of the ice sheet. [13] For example, the dependence of ice volume on summer insolation can be represented with , where I is ice volume, is the rate of change of ice volume per unit of time, T is the response time of the ice sheet and S is the insolation signal. [13]

Air Temperature is needed in a model as it informs surface melt and runoff rates. [14] For example, surface air temperature can be expressed with latitude 'lat', surface elevation h and mean temperature to provide an estimate of annual mean temperatures: [14] . This example assumes the ice shelf 's surface is as cold as at 1000m altitude. [14]

Precipitation is directly tied to air temperature, and also depends on moisture above and around the ice sheet. [14] Precipitation plays an important part in ice-sheet melting and accumulation. [14]

Calving

Calving is still an active area of investigation in ice-sheet modelling. [12] A total picture of calving will include many different aspects, including but not limited to tides, basal crevasses, collisions with ice bergs, thickness and temperature. [15] The recent development of the concepts of Marine Ice Sheet Instability and Marine Ice Cliff Instability have contributed to more accurate results of ice-sheet calving processes. [16]

Examples

CISM

The Community Ice Sheet Model is part of the Community Earth Systems Model funded by the National Science Foundation and models ice dynamics. [17] [18] It is written in Fortran 90 and is open-source. [17] The US Department of Energy has begun to contribute to CISM. [18] The CISM project works on other adjacent projects in developing a cirriculum to expand knowledge on ice sheets, and engaging a broader community in ice-sheet modelling. [18] Many ice-sheet modelling softwares have influenced CISM, including the Parallel Ice Sheet Model (PSIM) and Glimmer. [19] [20]

seaRISE

Sea-level Response to Ice Sheet Evolution (SeaRISE) is a subcommunity of CISM that sets out to estimate the upper limit of sea level rise from ice sheets. [21] The project sets out to develop a set of experiments and assessments for ice sheet and sea level rise modelling, as well as make a unified input dataset for ice sheet models. [21]

Glimmer

Glimmer (GENIE Land Ice Model with Multiply-Enabled Regions) is an ice-sheet model initially made to contribute to a more comprehensive earth system model, GENIE. [22]

PISM

The Parallel Ice Sheet Model is an open-sourced 3D ice sheet model capable of high resolution. [23] PISM is written in C++ and Python, and takes NetCDF files as input for the model. [24] PISM uses a "SIA+SSA hybrid" model, using both the shallow shelf approximation and shallow ice approximation models as stress balance models and does not solve full Stokes equations. [23] The model gets climatic information from an external General Circulation Model, and needs information like boundary temperature, mass flux into the ice, precipitation and air temperature. [25]

A horizontal grid of equal distance is used, with a variable vertical axis, and runs on a year timescale. [26] [27]

See also

Ice-sheet models on the web

Related Research Articles

<span class="mw-page-title-main">Iceberg</span> Large piece of freshwater ice broken off a glacier or ice shelf and floating in open water

An iceberg is a piece of freshwater ice more than 15 m long that has broken off a glacier or an ice shelf and is floating freely in open (salt) water. Smaller chunks of floating glacially-derived ice are called "growlers" or "bergy bits". Much of an iceberg is below the water's surface, which led to the expression "tip of the iceberg" to illustrate a small part of a larger unseen issue. Icebergs are considered a serious maritime hazard.

<span class="mw-page-title-main">Ice shelf</span> Large floating platform of ice caused by glacier flowing onto ocean surface

An ice shelf is a large floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves are found in Antarctica and the Arctic. The boundary between the floating ice shelf and the anchor ice that feeds it is the grounding line. The thickness of ice shelves can range from about 100 m (330 ft) to 1,000 m (3,300 ft). The world's largest ice shelves are the Ross Ice Shelf and the Filchner-Ronne Ice Shelf in Antarctica. When a large piece of an ice shelf breaks off, this can lead to the formation of an iceberg. This process is also called ice calving.

<span class="mw-page-title-main">Ice sheet</span> Large mass of glacial ice

In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than 50,000 km2 (19,000 sq mi). The only current ice sheets are the Antarctic ice sheet and the Greenland ice sheet. Ice sheets are bigger than ice shelves or alpine glaciers. Masses of ice covering less than 50,000 km2 are termed an ice cap. An ice cap will typically feed a series of glaciers around its periphery.

<span class="mw-page-title-main">West Antarctic Ice Sheet</span> Segment of the continental ice sheet that covers West (or Lesser) Antarctica

The Western Antarctic Ice Sheet (WAIS) is the segment of the continental ice sheet that covers West Antarctica, the portion of Antarctica on the side of the Transantarctic Mountains that lies in the Western Hemisphere. It is classified as a marine-based ice sheet, meaning that its bed lies well below sea level and its edges flow into floating ice shelves. The WAIS is bounded by the Ross Ice Shelf, the Ronne Ice Shelf, and outlet glaciers that drain into the Amundsen Sea.

<span class="mw-page-title-main">Larsen Ice Shelf</span> Ice shelf in Antarctica

The Larsen Ice Shelf is a long ice shelf in the northwest part of the Weddell Sea, extending along the east coast of the Antarctic Peninsula from Cape Longing to Smith Peninsula. It is named after Captain Carl Anton Larsen, the master of the Norwegian whaling vessel Jason, who sailed along the ice front as far as 68°10' South during December 1893. In finer detail, the Larsen Ice Shelf is a series of shelves that occupy distinct embayments along the coast. From north to south, the segments are called Larsen A, Larsen B, and Larsen C by researchers who work in the area. Further south, Larsen D and the much smaller Larsen E, F and G are also named.

<span class="mw-page-title-main">Antarctic ice sheet</span> Earths southern polar ice cap

The Antarctic ice sheet is one of two ice sheets on Earth and covers about 98% of the Antarctic continent. It is the largest single mass of ice on Earth, with an average thickness of over 2 kilometres (1.2 mi). It is distinct from the Antarctic sea ice. The Antarctic ice sheet covers an area of almost 14 million square kilometres and contains 26.5 million cubic kilometres of ice. The other ice sheet on Earth is the Greenland ice sheet.

<span class="mw-page-title-main">Last Glacial Maximum</span> Most recent time during the Last Glacial Period that ice sheets were at their greatest extent

The Last Glacial Maximum (LGM), also referred to as the Last Glacial Coldest Period, was the most recent time during the Last Glacial Period that ice sheets were at their greatest extent 26,000 and 20,000 years ago. Ice sheets covered much of Northern North America, Northern Europe, and Asia and profoundly affected Earth's climate by causing a major expansion of deserts, along with a large drop in sea levels.

<span class="mw-page-title-main">Thwaites Glacier</span> Antarctic glacier

Thwaites Glacier is an unusually broad and vast Antarctic glacier located east of Mount Murphy, on the Walgreen Coast of Marie Byrd Land. It was initially sighted by polar researchers in 1940, mapped in 1959–1966 and officially named in 1967, after the late American glaciologist Fredrik T. Thwaites. The glacier flows into Pine Island Bay, part of the Amundsen Sea, at surface speeds which exceed 2 kilometres (1.2 mi) per year near its grounding line. Its fastest-flowing grounded ice is centered between 50 and 100 kilometres east of Mount Murphy. Like many other parts of the cryosphere, it has been adversely affected by climate change, and provides one of the more notable examples of the retreat of glaciers since 1850.

<span class="mw-page-title-main">Retreat of glaciers since 1850</span> Shortening of glaciers by melting

The retreat of glaciers since 1850 is well documented and is one of the effects of climate change. The retreat of mountain glaciers, notably in western North America, Asia, the Alps and tropical and subtropical regions of South America, Africa and Indonesia, provide evidence for the rise in global temperatures since the late 19th century. The acceleration of the rate of retreat since 1995 of key outlet glaciers of the Greenland and West Antarctic ice sheets may foreshadow a rise in sea level, which would affect coastal regions. Excluding peripheral glaciers of ice sheets, the total cumulated global glacial losses over the 26-year period from 1993 to 2018 were likely 5500 gigatons, or 210 gigatons per yr.

<span class="mw-page-title-main">Totten Glacier</span> Iceberg in Antarctica

Totten Glacier is a large glacier draining a major portion of the East Antarctic Ice Sheet, through the Budd Coast of Wilkes Land in the Australian Antarctic Territory. The catchment drained by the glacier is estimated at 538,000 km2 (208,000 sq mi), extending approximately 1,100 km (680 mi) into the interior and holds the potential to raise sea level by at least 3.5 m (11 ft). Totten drains northeastward from the continental ice but turns northwestward at the coast where it terminates in a prominent tongue close east of Cape Waldron. It was first delineated from aerial photographs taken by USN Operation Highjump (1946–47), and named by Advisory Committee on Antarctic Names (US-ACAN) for George M. Totten, midshipman on USS Vincennes of the United States Exploring Expedition (1838–42), who assisted Lieutenant Charles Wilkes with correction of the survey data obtained by the expedition.

<span class="mw-page-title-main">Glacier morphology</span> Geomorphology of glaciers

Glacier morphology, or the form a glacier takes, is influenced by temperature, precipitation, topography, and other factors. The goal of glacial morphology is to gain a better understanding of glaciated landscapes and the way they are shaped. Types of glaciers can range from massive ice sheets, such as the Greenland ice sheet, to small cirque glaciers found perched on mountain tops. Glaciers can be grouped into two main categories:

<span class="mw-page-title-main">Polar amplification</span>

Polar amplification is the phenomenon that any change in the net radiation balance tends to produce a larger change in temperature near the poles than in the planetary average. This is commonly referred to as the ratio of polar warming to tropical warming. On a planet with an atmosphere that can restrict emission of longwave radiation to space, surface temperatures will be warmer than a simple planetary equilibrium temperature calculation would predict. Where the atmosphere or an extensive ocean is able to transport heat polewards, the poles will be warmer and equatorial regions cooler than their local net radiation balances would predict. The poles will experience the most cooling when the global-mean temperature is lower relative to a reference climate; alternatively, the poles will experience the greatest warming when the global-mean temperature is higher.

<span class="mw-page-title-main">East Antarctic Ice Sheet</span> Segment of the continental ice sheet that covers East Antarctica

The East Antarctic Ice Sheet (EAIS) is one of two large ice sheets in Antarctica, and the largest on the entire planet. The EAIS lies between 45° west and 168° east longitudinally.

<span class="mw-page-title-main">Ice-sheet dynamics</span> Technical explanation of ice motion within large bodies of ice

Ice sheet dynamics describe the motion within large bodies of ice such as those currently on Greenland and Antarctica. Ice motion is dominated by the movement of glaciers, whose gravity-driven activity is controlled by two main variable factors: the temperature and the strength of their bases. A number of processes alter these two factors, resulting in cyclic surges of activity interspersed with longer periods of inactivity, on both hourly and centennial time scales. Ice-sheet dynamics are of interest in modelling future sea level rise.

<span class="mw-page-title-main">Ice calving</span> Breaking of ice chunks from the edge of a glacier

Ice calving, also known as glacier calving or iceberg calving, is the breaking of ice chunks from the edge of a glacier. It is a form of ice ablation or ice disruption. It is the sudden release and breaking away of a mass of ice from a glacier, iceberg, ice front, ice shelf, or crevasse. The ice that breaks away can be classified as an iceberg, but may also be a growler, bergy bit, or a crevasse wall breakaway.

Deglaciation is the transition from full glacial conditions during ice ages, to warm interglacials, characterized by global warming and sea level rise due to change in continental ice volume. Thus, it refers to the retreat of a glacier, an ice sheet or frozen surface layer, and the resulting exposure of the Earth's surface. The decline of the cryosphere due to ablation can occur on any scale from global to localized to a particular glacier. After the Last Glacial Maximum, the last deglaciation begun, which lasted until the early Holocene. Around much of Earth, deglaciation during the last 100 years has been accelerating as a result of climate change, partly brought on by anthropogenic changes to greenhouse gases.

<span class="mw-page-title-main">Climate change in Antarctica</span> Impacts of climate change on Antarctica

Temperature change due to climate change in Antarctica is not stable over the whole continent. West Antarctica is warming rapidly, while the inland regions are cooled by the winds in Antarctica. Water in the West Antarctic has warmed by 1 °C since year 1955. Further increase in temperature in water and on land will affect the climate, ice mass and life on the continent and have global implications. Present-day greenhouse gas concentrations are higher than ever according to ice cores from Antarctica, which indicates that warming on this continent is not part of a natural cycle and attributable to anthropogenic climate change.

<span class="mw-page-title-main">Frank Pattyn</span> Belgian glaciologist

Frank Jean-Marie Léon Pattyn is a Belgian glaciologist and professor at the Université libre de Bruxelles. He is best known for developing ice-sheet models and leading model intercomparisons.

<span class="mw-page-title-main">Lake Washburn (Antarctica)</span>

Lake Washburn is a lake that formerly existed in the Taylor Valley, McMurdo Dry Valleys, Antarctica. It formed when climatic changes and an expansion of ice caused the flooding of the valley, between 23,000 and 8,340 radiocarbon years ago. Its extent and elevation are unclear but Lake Bonney and Lake Fryxell are considered to be its present-day remnant.

Ruth Mottram is a British climate scientist who is a researcher at the Danish Meteorological Institute. Her research considers the development of climate models and the dynamics of glaciers and ice sheets in the climate system.

References

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  20. ↑ "Documentation for PISM, the Parallel Ice Sheet Model". pism-docs.org. Retrieved 28 October 2021.
  21. 1 2 "SeaRISE Assessment – Interactive System for Ice sheet Simulation". websrv.cs.umt.edu. Retrieved 28 October 2021.
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  26. ↑ "Spatial grid – PISM, a Parallel Ice Sheet Model 1.2.1 documentation". pism-docs.org. Retrieved 18 October 2021.
  27. ↑ "Model time – PISM, a Parallel Ice Sheet Model 1.2.1 documentation". pism-docs.org. Retrieved 18 October 2021.
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