The University of Alberta, the University of Tromso and the University of the Arctic invite you to explore this four week course that examines the environment and climate of the circumpolar North. This course is the result of an international collaboration and provides you with an insight into our planet's North. Following an overview of regional geography, we will focus on the cryosphere (ice), as well as the atmosphere and ocean of the region. We will learn why the Arctic is cold and ice covered, and how that impacts its climate and ecosystems. We will also consider how the Arctic is connected to the rest of the world. Finally, we will examine present day climate change, the processes driving it, and evidence for it in the Arctic, before looking at the implications in the rapidly evolving North. Watch a preview of the course here: https://uofa.ualberta.ca/courses/arctic-climate
3.3A Ice Sheets and Permafrost
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来自 University of Alberta 的课程
Introduction to the Arctic: Climate
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从本节课中
The Cryosphere
In this lesson, we’ll be learning about the Arctic cryosphere, which is the portion of the Earth’s surface where water occurs in solid forms. Such as snow and ice! In Lesson 2, we talked about atmospheric and oceanic circulation. Now we will learn about when and how ice forms on the surface of the Earth, including such topics as: sea ice, river ice, glaciers , and permafrost. How all this ice formed on the ocean and on (and in) the land? How it relates to the atmosphere and ocean?
与讲师见面
Paul Myers, Ph.DProfessor
Department of Earth and Atmospheric Sciences
The whole life cycle of sea ice, from formation through growth and
eventual melting, is confined to the ocean.
In contrast, all land-based ice is composed of frozen fresh water.
Some types of land-based ice include,
A, glaciers, B, ice sheets, C,
ice caps, and/or, D, icebergs.
More than one answer might be correct, so check all that you think apply.
All of the answers are correct.
Land-based ice includes glaciers, ice sheets, and ice caps.
Where glaciers reach the sea, they can float on the surface of the coastal ocean
in the form of vast ice shelves.
Grounded glacier ice and
ice shelves reaching the sea eventually break off, calving in chunks.
These can drift with wind and ocean currents and
are called icebergs, which we'll come back to later in this lesson.
Importantly, icebergs are only made of glacier ice and not of sea ice.
Glaciers and ice sheets are a crucial part of the Arctic cryosphere.
But what are they exactly?
Put simply, glaciers, ice caps, and
ice sheets form where winter snow is not completely melted away in the summer.
That is, the mass of snow that falls during the winter
is greater than the mass of water that is melted in summer.
This concept is mass balance, and is measured in meters of water equivalent.
On the surface of a glacier, the region with net annual accumulation of snow and
ice is known as the accumulation zone.
And the region with net annual loss, or ablation of snow and
ice, is known as the ablation zone.
The line separating the two zones is the Equilibrium Line Altitude or ELA.
At the end of the summer melt period, the ELA is seen on a glacier surface
as the boundary between bare glacial ice and snow covered ice.
If mass balance is positive, the glacier is growing.
Whereas if it is negative, the glacier is melting and shrinking.
You probably know that glaciers are primarily ice, so
how does snow that falls on a glacier become ice?
Typically, snow that falls in the accumulation zone
is buried each year by more and more snow.
With the weight of new snow, the older snow is compressed and
eventually solidified into ice.
In addition, ice does not simply remain solid, but deforms and
flows under its own weight.
If the bed of the glacier is frozen, this creep is the only way that the ice flows.
Such ice is known as cold-based ice.
Creep is normally proportional to the temperature of the ice to the third power.
Many smaller Arctic glaciers and ice caps are cold-based as they rest on permafrost.
An important exception is the Greenland ice sheet,
which has both cold-based and warm-based areas.
If the bed of the glacier is unfrozen, the glacier will slide across its bed.
This is known as warm-based ice.
The rate of sliding depends on the roughness of the bed and
the amount of water available at the bed.
What do you think it means for a glacier to be in equilibrium?
A, mass balance is in equilibrium,
meaning that gains equal losses over the course of a year.
B, t2he glacier is neither growing nor shrinking over the course of a year.
C, the glacier is growing, accumulating more snow.
And/or, D, the glacier has no accumulation and no ablation.
More than one answer might be correct, so check all that you think apply.
Both answers A and B are correct.
Equilibrium means that annual accumulation and ablation equal each other.
Scientist who study glaciers are glaciologists.
They traditionally have measured mass balance by placing stakes in a glacier and
measuring the thickness and
the density of snow added to the surface in the accumulation zone.
As well as the decrease of thickness and
the density of ice lost from the surface in the ablation zone.
By converting thickness and
density to water equivalent, glaciologists can measure mass balance.
If mass balance is positive, the glacier is growing.
Whereas if it is negative, the glacier is melting and shrinking.
Today, glaciologists have many more tools at their disposal
to measure mass balance and other glacier characteristics.
Using satellite-remote sensing, it is now possible, in a given season,
to identify the regions of the glacier that have melted and those that have not.
In addition to this, the use of glacier mounted base stations using GPS,
GLONASS, or Galileo Satellite Systems allows accumulation and
ablation to be measured with much greater accuracy and precision than ever before.
Satellites allowed glaciologists to measure glacier
calving rates with relative accuracy.
Can you think of any other ways that glaciers might lose mass?
What happens when glaciers enter a body of water?
That's right, icebergs.
An iceberg famously sunk the ocean liner Titanic in 1912.
Glaciers that end in either a lake or
in the ocean lose mass as blocks of ice fracture and float away.
This process is known as calving.
Calving is a relatively efficient means of mass loss, as immense blocks of ice
are quickly removed and transported away from a glacier.
The largest known icebergs have been formed by calving glaciers in Antarctica.
In the Arctic, icebergs as large as 260 square kilometers, or four times the area
of Manhattan Island in New York, have calved from the Greenland ice sheet.
Icebergs are regularly observed along the eastern seaboard of Canada and
the northeasternmost United States.
In 1958, a 168-meter high iceberg originating from
Greenland was observed in the North Atlantic by a US Coast Guard ship.
That's the height of a 55-story skyscraper.
Understandably, icebergs are a concern for maritime shipping and travel.
In order to protect ships and crews,
many nation states have invested heavily in studying icebergs and
monitoring their location through the famous International Ice Patrol.
Icebergs that calve from an ice sheet can be massive in size.
What do you think happens to these ice masses once they get dropped into
the ocean?
A, they sink, melting on the seabed.
B, they float with the winds and currents, melting as they reach warmer latitudes.
C, they get grounded, or stuck on the seabed.
D, they roll and overturn in the water.
More than one answer may be correct, so check all that you think apply.
Answers B, C, and D are correct.
Icebergs will drift with the winds and
currents of the ocean, melting as they float along.
Because ice is less dense than water, it will float rather than sink.
If icebergs are in shallower waters,
they may get stuck, their bottom grounding on the seabed.
As icebergs melt, their center of gravity can shift, and the berg can flip over.
In 2012, 97% of the Greenland ice sheet's 1.7 million
square kilometers experienced melting during the year.
This was dramatically higher than the long-term average of about 30%.
Roughly 20% more of the Greenland ice sheet melts each year
today compared to initial measurements in 1979.
Although these measurements are deeply concerning,
they are unfortunately the tip of the metaphorical iceberg.
The Greenland ice sheet contains some 2.5 million cubic kilometers of ice.
Were the entire ice sheet to melt tomorrow,
sea level would rise by 7.2 meters, leaving many coastal cities underwater and
displacing over 600 million people.
This scenario will not happen in the short term, but meltwater from the Greenland
ice sheet is responsible for fully 30% of the 20th century's sea level rise,
which has increased by about 20 centimeters since 1900.
Overall, the Greenland ice sheet loses close to the same amount of mass
annually by calving as by melting.
The IPCC now recognizes it to have an overall negative mass balance.
Current IPCC predictions indicate that the entire surface of the Greenland
ice sheet may well be melting annually by 2100, with global sea level
rise expected to be between 30 and 100 centimeters over that time.
Even 30 centimeters represents a dangerous increase, particularly for low-lying
islands where any sea level increase may force entire populations to relocate.
What effect do you think surface meltwater has on the Greenland Ice Sheet?
A, it causes speeding up of ice streams and outlet glaciers
because increased meltwater reaches the bed of these features.
B, it causes lower albedo, promoting more melt and less reflectivity.
C, it causes enhanced calving of ice into the sea.
D, enhanced calving will enable warm ocean water to get to
the base of tidewater glaciers, promoting more ablation.
More than one answer might be correct, so check all that you think apply.
All four answers are correct.
The increased melting appears to serve as a positive feedback on calving,
leading to enhanced calving and increased melting of tidewater glaciers.
The warming ocean water further enhances this.
When a larger area of the ice sheet melts, it decreases the albedo of
areas that do melt, increasing melting as a positive feedback.
At the same time, several major outlet glaciers such as Jakobshavn Isbrae or
Helheim have dramatically accelerated due to increased meltwater
reaching their beds and the influx of warmer ocean water.
This is facilitating increased calving and
overall retreat of several tidewater glaciers.
In addition to sea ice, river and lake ice and glaciers,
the cryosphere also consists of frozen ground or permafrost.
What do you think permafrost is?
Is it, A, ice below the Earth's surface.
B, parts of the Earth's surface that are permanently frozen.
C, regions with ice sitting on the Earth's surface.
And/or D, frost that forms after a very cold night.
More than one answer might be correct, so check all that you think apply.
Answers A and B are correct.
Some people imagine permafrost is consisting of ice below the Earth's
surface, but it is more than this.
In the simplest form, permafrost refers to that portion of the Earth's surface
that is permanently frozen.
This means that the temperature is below freezing throughout the year.
Permafrost occurs within all soil and sediment types and even within solid rock.
The southern limit of the Arctic is sometimes defined by the extent of
discontinuous permafrost.
Because of this, frozen ground is an important reality
to the peoples of the Arctic and for all economic activity.
We will see that permafrost also has important connections to climate and
climate change.
For permafrost to be present, the mean annual temperature in a region must be
below the freezing point of freshwater, which is 0 degrees Celsius.
Even so, most areas with mean annual temperatures with between 0 and
-2 degrees Celsius only rarely have permafrost,
typically only on north-facing slopes and in shaded areas.
This is often due to the aspect of a site or the direction in which it faces.
Sites that are exposed to direct sunlight receive energy directly from the Sun
as well as from the air, which can prevent the formation of permafrost.
If permafrost is found over less than 50% of the area of a region,
it is considered to have sporadic or patchy permafrost.
Sporadic permafrost is typically found as isolated,
disconnected bodies of frozen ground.
Where mean annual temperatures drop because of increased latitude or
increased altitude, the proportion of permafrost normally increases.
Looking at the characteristics below,
which ones do you think describe discontinuous permafrost?
A, it occurs in regions where 50 to 90% of the area is frozen throughout the year.
B, it occurs where the mean annual temperature is above 0 degrees Celsius.
C, it occurs when the mean annual temperature is typically colder
than -2 degrees Celsius.
D, it is usually connected, surrounding unfrozen areas of ground.
More than one answer might be correct, so check all that you think apply.
Answers A, C, and D are correct.
The next section discusses the various kinds of permafrost.
Discontinuous permafrost, as we learned in lesson one,
occurs in regions where 50 to 90% of the area is frozen throughout the year.
And the mean annual temperature is typically colder than -2 degrees Celsius.
Within this discontinuous permafrost zone, permafrost is usually connected
with unfrozen areas forming islands surrounded by frozen ground.
Unfrozen ground may be found on south-facing slopes,
receiving additional solar radiation, and also,
under lakes and ponds that do not fully freeze during the winter.
Unfrozen ground in permafrost regions,
especially when surrounded by other permafrost, is known as a talik.
Where mean annual temperatures are below -5 degree Celsius,
even the influence of aspect is insufficient to maintain on frozen ground.
This zone of continuous permafrost is only broken by taliks formed
under large lakes and rivers which haven't completely frozen during the winter.
This can happen even though the surface is completely frozen.
Small lakes may have unfrozen ground immediately beneath them, but
permafrost below that.
Within this continuous permafrost zone,
the ground can be frozen to greater than 1,500 meters below the surface.
These areas, such as parts of Eastern Siberia, coincide with
the coldest annual air temperatures in the entire northern hemisphere.
Cold-based glaciers are also underlain by continuous permafrost.
Permafrost also occurs on the sea floor, particularly North of Siberia.
This subsea permafrost was formed during previous Ice Ages, when global sea level
was up to 120 meters lower than present, because all that water was
stored on land in vast ice sheets covering western Europe and northern North America.
The lowered sea level exposed the wide continental shelves, North of Siberia,
to extremely cold temperatures, and allowed the aggradation of permafrost.
