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3.3B Ice Sheets and Permafrost

Course video 14 of 18

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?

Introduction to the Arctic: Climate

Альбертский университет

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

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Преподаватели

Paul Myers, Ph.D
Professor

Текст видео

Why do you think permafrost is still present today

on the shallow continental shelves of the Arctic ocean?

Is it because A, sea ice formation and consolidation keep the seabed cool?

B, iceberg activity is minimal on these shallow continental shelves?

C, the overlying water is relatively cool.

And/or D, the cool temperature of the overlying water preserves the permafrost.

More than one answer may be correct.

So check all that apply.

Both answers C and D are correct.

As we learned in the previous lesson Arctic Ocean surface waters

are typically about minus two degrees celsius throughout the year.

This cold surface water preserves the permafrost and prevents its melting.

However warming ocean temperatures could put this permafrost at risk.

If the permafrost melts, other frozen materials, such as gas hydrates,

could be released.

As we'll see in lesson four.

Although the Earth receives most of its heat from the Sun.

Heat produced within the Earth radiates outward as well.

This results in decreasing temperatures,

moving from the Earth's core to the Earth's surface.

This is called geothermal heat.

Because of geothermal heat,

the formation of deep permafrost requires very long periods of time

during which air temperature is substantially below freezing.

The deeper the permafrost is, the more time is required.

It probably took more than one million years to

form the deep permafrost found in Siberia.

Where permafrost is present and

summer temperatures rise above the freezing point.

The uppermost layer of the permafrost will thaw and

re-freeze annually, forming the active layer.

The active layer can vary between a few centimeters in the coldest areas,

to as much as four meters near the southern limit of permafrost.

The base of the active layer is known as the permafrost table.

When the active layer occurs in soil or

soft sediment, especially if snow is present it tends to be rich in moisture.

This aids in the process of cryoturbation,

which is the mixing of permafrost soils due to freeze and thaw processes.

Cryoturbation tends to create relatively uniform soil within the active layer.

And contributes to some of the interesting landforms in permafrost areas.

In forested permafrost regions one striking feature is the so

called drunken forests of black spruce, where trees appear to topple over.

Having just talked about permafrost features.

What do you think causes this phenomenon?

Is it that, A.

Trees are not able to extend their roots below the permafrost table.

B, roots are within the active layer.

C, trees will grow at an angle since the active layer is frozen during the summer.

And or D, trees tip over in summer when the active layer thaws.

More than one answer might be correct.

So check all that apply.

Answers A, B and D are correct.

As tree roots cannot penetrate beneath the permafrost table

they can only grow within the active layer.

During summer,

when the active layer is unfrozen, these trees will often partially tip over.

Sometimes even due to wind.

Within permafrost regions, many of the interesting and

famous landforms are the result of ground ice.

This is ice that is separated from the surrounding sediment.

Some of the simplest forms of ground ice are ice wedges.

These form in winter, at temperatures below minus seven degrees Celsius.

When thermal contraction of the ground surface results in a crack forming

up to several meters in length, and in depth.

When surface snow begins to melt in the summer.

The resulting melt water will drain into the crack

freezing when it reaches the permafrost table.

Water expands when it freezes, this leads to the expansion of the wedge and

the displacement of adjacent sediment upwards.

Thermal contraction will typically occur atleast once a year at a given site.

So scientists can estimate the minimum age of an ice wedge

by counting the ice layers within it.

Over many years,

intersecting ice wedges can form nets at the surface called ice wedge polygons.

This is one of several types of patterned ground in permafrost regions.

In addition to ice wedges, patterned ground may be formed by the sediment

sorting processes resulting from cryoturbation.

During repeated freeze-thaw cycles,

coarser material is forced to the surface and finer material fills its place.

As a result sorted circles of material can form on flat ground

typically with the coarse material at the outside of the circle.

On low angle slopes, cryoturbation leads to the formation of sorted stripes.

As the finer grained sediment tends to be more heavily vegetated.

Patterned ground can often be recognized as much by the covering vegetation

as the sediment grain size.

In addition to ice wedges, the conditions within permafrost can lead to

the formation of segregated ice lenses and massive ground ice.

Within a water saturated active layer of sediment or

porous rock, water will preferentially accumulate in porous layers.

When such porous layers freeze, addition water

is drawn to the freezing layer which is known as the freezing front.

Expanding the already formed ice and eventually leading to the fracture of

the sediment or rock and the formation of an ice lens.

Where conditions favor this process ice masses up to

tens of meters of thickness in diameter maybe formed over several years.

If exposed to summer air temperatures ground ice is particularly susceptible

to melting and can lead to rapid and catastrophic landscape changes.

Even over a single season.

Which of these features or

land forms Do you think were formed by permafrost processes?

A, Pingo.

B, Palsa.

C, Moraine.

And or D, Drumlin.

More than one answer might be correct.

So check all that apply.

Both answers A and

B show landforms that might be formed by permafrost related processes.

We will look into these two landforms, pingos and palsas, in the next section.

Moraines and drumlins are formed through glacial processes.

Perhaps the best known permafrost land forms of all are pingos.

The inuvialuit word for a type of ice chord hill.

Classical closed system pingos are formed by the segregation of ice

in saturated sediment in a talik after the drainage of a small lake.

As permafrost degrades into the talik from the surface and the subsurface.

Water is drawn towards the freezing front leading to large segregated ice bodies.

As a result of ice formation.

The former lake basin is heaved upwards

into a large ice-cored hill called a pingo.

Such pingos can be up to 70 meters in height and

several hundred meters in diameter.

Pingos may rise by several centimeters per year and

take tens to hundreds of years to form.

The largest pingos often crack at the surface due to over expansion.

This exposes the ice core to air temperature and

ultimately melts the core and causes the demise of the pingo.

Another ice ridge permafrost land form is the Palsa.

Finnish for an ice-cored mound in a bog.

Typically less than two meters high and a few tens of meters long.

Palsas form in the highest points within low,

boggy, vegetated areas, usually in discontinuous permafrost terrain.

More limited snow cover leads to earlier and deeper freezing in winter.

Similarly, water drainage from this relative high points

may insulate the underlying promofrost during the summer.

When temperatures drop in the fall,

water is drawn into the freezing front of the palsa.

Leading to the formation of ice lenses

up to several centimeters thick underneath the vegetation.

Palsas are a useful landform for

understanding the interplay between snow and permafrost.

As we saw in the first lesson,

many Arctic regions receive relatively little precipitation throughout the year.

Particularly during the Winter.

Like in the case of pulses, minimal or no snowfall helps permafrost to grow.

As snow acts as an effective insulator of the ground from the cold winter

atmosphere.

How do you think permafrost processes are impacted by the presence of snow?

Is it that?

A, the permafrost will be thickest where the annual snowpack is thin or absent.

B, strong winds in the areas with little vegetation will promote

permafrost growth as snow is blown away from these areas.

C, the snow will be turned into permafrost over an extended period of time.

And or D, the temperature of the permafrost is strongly

affected by the thickness of winter snow cover.

More than one answer might be correct.

So check all that apply.

Answers A, B and D are correct.

Snowfall, as well as vegetation and wind are important for permafrost.

In forested parts of Northern Fennoscandia,

abundant snowfall prevents the formation of permafrost.

Even though mean annual temperatures are cold enough to support permafrost.

In many tundra areas where vegetation is minimal.

The wind can carry away and redistribute snow.

Ensuring that the surface is exposed to the cold atmosphere and

keeping permafrost relatively thick.

More insulating snow, generally means thinner permafrost.

Persistent changes to the amount of winter snowfall

can have significant effects on the integrity of permafrost in a given region.

Increasing the thickness of the active layer with a positive feedback

On the overall temperature of the permafrost itself.

Such affects can be particularly profound

in areas of the southern limits of permafrost.

The active layer is also particularly important to the maintenance and

decay of permafrost.

When water saturated the active layer can slide on the permafrost table,

especially on slopes.

Leading to exposure and melt of deeper permafrost.

Similarly any disturbance of the active layer, particularly by human activity.

Can introduce additional heat into the active layer and the permafrost itself.

Examples include, stripping reflective material off the surface,

thereby decreasing the albedo.

Much like insulation by snow,

many land disturbances lead to an increase in absorbed heat.

A thicker active layer, and eventually the decay of the permafrost.

Where ice rich permafrost is exposed to increased temperatures,

melting can be rapid and sometimes even catastrophic.

Typically the melt of ice rich permafrost leads to low,

water filled depressions called thermokarst.

Areas subject to thermokarst can be extremely unstable and

can destroy overlying infrastructure.

A recent example of probable thermokarst in Siberia

formed a 30 meter wide, 70 meter deep crater.

As the ice melted methane was released,

building up pressure until it violently escaped forming the crater.

Along coast lines,

ice rich permafrost is especially susceptible to erosion by waves.

With decrease summer sea ice, some ice-rich coastlines in Northern Canada

are being eroded by several tens of meters annually.

Permafrost is very sensitive to disturbance and global climate warming.

Human activities are also leading to increased disturbances year over year.

Which of these changes do you think might be occurring in the Arctic?

A, reductions in summer sea ice caused by warmer summer temperatures.

B, Arctic winters are now shorter and warmer than before.

C, in some regions increased snowfall is insulating the permafrost.

D, a changing Artic is becoming more and more the site of economic activity.

More than one answer may be correct.

So check all that you think apply.

If you selected all of these changes, you would be correct.

Now let's discuss these changes in detail.

Most Arctic infrastructure built prior to the 1960s didn't have permafrost in mind.

Many buildings were built on the surface transferring heat to the permafrost.

This led to increased melting and sometimes to the dislocation and

even destruction of the structures themselves.

More recent infrastructure was built with permafrost in mind.

In order to protect the underlying permafrost,

buildings are placed on stilts or over gravel pads up to several meters thick.

In arctic towns and cities sewer, water, and

electrical utility lines are often built above ground.

Sometimes in utilidor systems to insulate the permafrost from their heat.

Even so,

most of these systems have been designed with steady state conditions in mind.

With the effects of warming temperatures, active layer thicknesses are increasing.

Often beyond what infrastructure was designed for.

In addition, economic activity in the Arctic often relies on

winter transportation of equipment over ice roads build over permafrost,

frozen lakes, and frozen rivers.

Increased Arctic temperatures has led to later freeze up and

earlier melt of the ice roads.

Limiting the viability of communities, mines and

other facilities relying upon them.

This increases the danger to those using them.

Why do you think permafrost thaw should be a concern to all of us?

A, there are vast amounts of carbon stored within the permafrost,

which will add greenhouse gases to the atmosphere if released.

B, if permafrost thaws, methane will be released due to the breakdown of organic

matter and methane is a more powerful greenhouse gas than carbon dioxide.

C, permafrost thaw causes the active layer to become detached

which destabilizes slopes.

D, permafrost thaw prevents sediments washing into

freshwater supplies via active layer detachment.

More than one answer might be correct.

So check all that you think apply.

Answers A, B, and C are correct.

D is incorrect,

as permafrost thaw actually promotes the washing in of sediment.

Now, let's consider the impact of permafrost thaw.

Perhaps the most troubling aspect of melting permafrost

is the sheer volume of carbon stored within it.

Soils are the largest carbon reservoir on land worldwide.

And permafrost soils contain as much as 50% of the worlds soil carbon.

It is stored in the form of frozen, organic matter.

But also as carbon dioxide created by aerobic microbial decomposition.

And methane,

created by anaerobic microbial decomposition frozen within permafrost.

With permafrost melting, organic matter may be subject

to microbial decay, releasing further CO2.

But with melting also releasing trapped CO2,and methane.

Arctic regions have the potential to dramatically worsen

the human induced greenhouse affect.

Leading to dramatic climate change,

which will continue to disproportionately affect the Arctic.

Permafrost thaw can also lead to the detachment of the active layer,

destabilizing slopes, and

washing sediments into fresh water supplies for arctic communities.

We've reached the end of lesson three.

Come back for lesson four.

Where we'll talk about the impacts of climate warming on

the Arctic landscape, biology, and the cryosphere.

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