Lab 9 _Glaciers_LLL
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Lab 9—Glacier dynamics and sea level changes
12 Questions
Lab introduction and purpose
Of the various spheres (the atmosphere, hydrosphere, cryosphere, lithosphere, and biosphere) the cryosphere is perhaps the least familiar to many people. Not many people live at high altitudes or latitudes where glaciers tend to be, which can be cold and unpleasant places at times, although also stunningly beautiful. However, as a geologic force, glaciers have shaped major parts of our earth in significant ways. This is, in part, because glacial ice used to be so much more extensive during past Ice Ages, at times extending all the way from the interior of what is now Canada down into Kansas. But, the waxing and waning of the glaciers has affected all latitudes, mainly by causing changes in sea level, a topic we will look at in some detail. The waxing and waning of glaciers is also related to global climate change. Glaciers are a significant
part of the water cycle. Sea ice also strongly influences weather and, in the long run, climate. Therefore, this lab course would be deficient if it did not introduce you to glaciers and associated earth system science behavior.
This lab has several goals: 1) to familiarize you with glaciers, their attributes and behavior, 2) to learn how imagery can be used to track how glaciers have changed in recent times, and 3) to explore how glacier volume and sea level are connected. You will also gain additional appreciation for how even simple mathematical models that are approachable at the introductory college level can provide insights into how the earth behaves. As with previous labs, there is an explanatory and training section first, and then you will apply what you have learned.
Image from NASA’s Earth Observatory of
the Arctic area showing several important
elements of the cryosphere – the sea ice
covering parts of the Arctic Ocean, the
Greenland continental ice cap, and snow
cover on the land. Abundant smaller
alpine glaciers also occur in this view but
are too small to resolve. Image source: http://earthobservatory.nasa.gov/IOTD/view.php?id=45766
.
What are glaciers? (1.5 pts)
Some of you may be familiar with glaciers and maybe even fortunate enough to have walked on
one, but most of the students in this class will not have been so fortunate, and it is reasonable to
start with the simple question – what are glaciers? Where more snow falls than melts in a year, 1
on average, the snow starts piling up. The snow lower down gets compressed and refrozen
into ice, and the “pile” of snow and ice begins to move. If the ice is in a mountain valley it flows down
the valley forming an alpine glacier (such as are very common in Alaska). If the ice forms a cap
that mantles the landscape, it flows outward from the central and thicker part of what is called an ice cap or continental glacier
(e.g. Greenland and Antarctica). Probably one of the best ways to understand a glacier as a dynamic feature is to see one move.
However, most glaciers move slowly enough that one would have to be very patient indeed to witness the movement, which is evident over time scales of days to months, not to mention the difficulty of getting to the glacier. Luckily several people have made time lapse movies available on the web. The event where a piece of the front of a glacier at the water’s edge falls off and into the water is known as a calving
. This is one way that glaciers lose ice and of course icebergs result, which in turn can sink unwary ships. View these YouTube clips to get an idea of how glaciers behave (sorry for any associated ads). >Stunning examples of calving events around the world:
https://www.youtube.com/watch?v=xLFWV0d3-d0
>Extreme Ice Video of Columbia Glacier in Alaska:
http://www.youtube.com/watch?
v=6dFbuaz130c
>Time lapse of Chilean glacier: http://www.youtube.com/watch?v=hRhnLtFZxso
>BBC Earth Lab on glacial features
https://www.youtube.com/watch?v=ghC-Ut0fW4o
> Extreme Ice video of massive calving event in Greenland filmed as part of James Balog’s efforts (this is spectacular footage): http://www.youtube.com/watch?v=hC3VTgIPoGU
Given what you saw in these videos, describe the relationship between glacial ice movement forward and the rate of melting or calving at the glacier front, and describe how the front (or terminus) of the glacier changes in position over time. Question 1:
Description of glacial movement:
You may have noted in the videos dark stripes and patches on the glaciers. This is rock sediment that fell off the mountainsides onto the glaciers. You can also see that the moving ice of glaciers also carries a tremendous amount of sediment within and on top of the ice. As the ice
melts, the rock debris gets deposited in front and on the sides of the glacier to form features called moraines
. The mixture of boulders, sand, and mud that gets deposited is known as till
. In some of the videos large open cracks known as crevasses can also be seen to form. The position and orientation of crevasses provides insight into how the glacier is moving. In the simplest case the ice is moving perpendicular to the crevasse length, but more complicated relationships also exist. On the surface, or at the bottom of glacier (beneath the ice), and continuing out in front, are a great array of melt-water streams and rivers
. The more plentiful 2
these are, the greater the rate of ice removal from the glacier at that time. In some parts of the world, these glacial melt-water streams and rivers represent an important water resource for people downstream. Sometimes small ponds and lakes form on glaciers. Because they are darker and can absorb more sunlight, such ponds can localize heating and be self-perpetuating (a positive feedback loop). With time, as the ice moves and grinds over the rocks, the glaciers scour distinctive U-shaped valleys
, and where the glaciers melt and retreat and the sea invades the valley, fjords
are formed. Fjord walls in Norway are over a mile high to give you some idea of the size these features can attain. You will be asked to identify moraines, crevasses, and meltwater features in
images provided later in this lab. Glacial features are labeled and described in the images below
to help train your eye. Study them carefully – they are not simply pretty illustrations, but sources of information. Most of these images come from the Arctic archipelago of Svalbard (link to info on Svalbard: http://en.wikipedia.org/wiki/Svalbard
if you want more info) in the extreme North Atlantic (where one of the authors of this lab has been lucky enough to spend a lot of time). Glaciers are very diverse in their characteristics, so you will need to generalize from these specific examples to other instances. Above is a view across a fjord in northwest Spitsbergen, Norway, of a rather typical mountain glacier. You can see the upper portion is smoother and whiter. This is where last year’s snowfall
remains (this photo was taken in August), and will likely be covered by next year’s snowfall. Eventually (unless continued summer melting comes to dominate the entirety of the glacial surface), some of this snow will become part of the glacial ice. Further down the glacier you can see a rougher glacial surface with a grayish tint. This is where older glacial ice has been exposed by melting, and the roughness is caused by uneven melting of the ice. The glacier is losing mass over this portion of the glacier. Out in front of the glacier is a jumble of brown debris, which consists of a chaotic mix of boulders, sand, silt, and mud. This is where the glacier
has dropped a lot of the rock debris it was carrying at a stable ice front position, forming what is 3
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known as a terminal moraine. The ice front no longer reaches to this moraine because in the last century, melting at the front has dominated and the glacier’s ice front has retreated, leaving the moraine stranded behind. In between the ice front and the terminal moraine can be seen a brown and muddy lake, which, because of its position, is known as a proglacial lake.The mud was part of the sediment the glacier was carrying and was picked up by the melt waters that run
into this lake. Such lakes quickly fill with sediment, often seasonally layered. The image above is another view in the same part of Svalbard. Here we are standing on the terminal moraine of one glacier, looking across the fjord waters to the front of another large glacier that has a calving front that ends in the seawater, and is several miles wide. The glacier that built the moraine in the foreground is to the viewer’s back. Note how the moraine material in
the foreground includes large boulders and is quite uneven in its topography. This hummocky character is typical of moraines and produces lots of depressions and small lake and ponds, such as the pond seen here. The hummocky character is a result of the irregular way in which the sediment gets deposited as the ice melts. This moraine is old enough that vegetation has begun to establish itself. Looking at the larger glacier in the background one can also see how it
is linked to and fed by a smaller trunk glacier in the center of the photo coming out of the mountains. This is similar to how a smaller stream feeds into a bigger one, and one can think about a glacier system of ice drainage with glacial tributaries. Other smaller glaciers can be 4
seen perched in small valleys cut into the flanking rocky ridges. These relatively sharp ridges are known as arêtes and this landscape is very typical of alpine topography carved by glaciers. The image above is a view from a boat on the fjord waters of the front of a calving and crevassed glacier front. All the small chunks of ice floating in the water are small icebergs. Look
carefully and you can see a three-masted schooner right in front of the glacier to give you a sense of scale. You can also see the numerous crevasses on this glacier – these large cracks form as the glacier moves. The sharp rock ridges in the background are typical of alpine topography carved by glaciers.
5
The image on the left above shows an overview of the front of yet another glacier in the same area. The strip of distinctive grey rock debris on the far side is a lateral moraine, sediment that was carried and deposited by this glacier. Note how the moraine extends farther to the right and
much higher than the present position of the glacier. This indicates that in the past this glacier extended much farther to the right and was also much thicker. The great majority of glaciers in Svalbard show clear signs of getting smaller, with their fronts retreating. The image on the right above is a close up of a glacier in summer with meltwater pools on the glacier, in this case located in an old crevasse. The ice is grayer, because as the glacier top melts rock debris stays behind and becomes concentrated. In some places this process occurs to such a degree that the glacier top is covered by rock debris hiding (and shielding from melting) the underlying ice. This is a close-up view of the same glacier as in the photos above. Degraded and partly snow-
filled crevasses can be seen here. Note the slightly grey character of the ice. This is clearly a part of the glacier that has seen significant melting and ice loss. The sediment is exposed and accumulates on the glacier surface as the ice melts away, creating the darker streaks and coloration. The darkening of the glacial surface can promote even more melting. However, once
6
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a mantle of debris forms over the ice (a not uncommon occurrence) then the rock debris tends to insulate the underlying ice from further melting. 7
Glaciers as the canaries of climate change
You have probably heard of the Ice Ages. For some of the Ice Ages, glacial ice extended from the north all the way down into Kansas. The white area on the small map below is the approximate area covered by the continental glacier that covered a good bit of North America as
recently as 15,000 years ago (source: http://pubs.usgs.gov/gip/ice_age/ice_age.pdf
). Clearly glaciers grow and shrink. They do so mainly as a function of climate, shrinking during warmer times and growing during colder times. From a system perspective this makes perfect sense. The mass input for glaciers is snowfall, and the colder it is, the more the
precipitation in an area will be in the form of snow, and the less
melting will occur in the summer. A mass loss (an outflow) for
glaciers is melting, and so the warmer it is, the more melting
occurs, and the more precipitation falls as rainfall, which also
helps melt the ice. Historical studies of glaciers indicate that
they are quite sensitive to local climate changes. Bonus:
While due to time constraints it is not required, this
would be a good opportunity to practice your system diagram
construction skills by creating such a diagram for the ice mass
in a glacier. You can earn up to 3 points if you submit such a
diagram of your own construction (you can use PowerPoint as
you have done before).
Glacier front changes from various parts of the world (7.5 pts)
Google Earth is the crucial resource for this part of the lab. As you hopefully now appreciate, Google Earth shows detailed surface imagery from around the earth and therefore provides an unparalleled opportunity to learn about our world without ever leaving your computer. In addition, for some places it shows imagery taken at different times. This means you can use it to
track the change of surface features with time. In this section of the lab we will use it to track changes in the position of the front (the terminus) of the glacier. By now you should be familiar with Google Earth, but if you want a refresher, tutorials on using this software can be found at: http://support.google.com/earth/bin/answer.py?hl=en&answer=176576
. You will choose five of the seven glacial sites listed in the table below and complete the following tasks for each site. You will basically be making an annotated image that shows glacial-related features and documents change in the glacier front. Read on for instructions and an example.
a)
Navigate to the glacier using the latitude and longitude given for the site. You can copy and paste them into the location box in the upper left part of the Google Earth window. Then re-size the Google Earth viewing window
so that it has the approximate horizontal extent given in the table below, and so that the end of the glacier is approximately centered. You may have look carefully to see where the terminus of the 8
glacier is in some cases. Under the view option in the menu bar at the top of the Goggle Earth window, select the scale option so that a scale bar appears in the lower left of the image window. If you units are not in m/km, change that setting now.
b)
Under the “View” option in the top menu bar of Google Earth
is a “Historical Imagery” option you should select (see
screen shot below). A small horizontal scroll bar will appear
in the upper left of the image screen. Using the slider, you
can select imagery for the different times for which imagery
is available in Google Earth. Note that the year of the
imagery is given. In this way you can detect changes
through time. Go through the sequence of imagery available
for the glacier of interest to see the differences from one
time period to another.
Right: Screen shot of Google Earth View tab with
various options including the Historical Imagery
option which you should check. c)
Use the Add Path tool to trace the end of the glacier from the
earliest time period where you can see the glacier front.
Notice in the window that appears when you select the Add
Path tool that there is a color-style tab. You can use this to make the glacier terminus that you trace a distinctive color. You can also increase the width of the line you draw so it is easier to see. Note the time period.
d)
Now switch to the image for the most recent time. The original tracing should still be there. You can now see how far the front of the glacier has retreated or advanced. Use the ruler tool in Google Earth to measure how far it has advanced or retreated in meters (change your setting if you are in miles). e)
Now, using the path or polygon tool, trace out parts of the glacial features described above (terminal moraines, lateral moraines, crevasses, and proglacial lakes) that are present in the image. Give the line traced for each different feature a different color (by selecting the appropriate tab in the path window and choosing a color) so that in your explanation you can clearly distinguish among them. When you are finished, go to Edit, Copy Image and paste your labeled image into your assignment. f)
For each of the 5 sites you choose from the list of 7 possibilities in the table below, write up a short report that includes the following: i)
the name of the site, ii)
an image that shows a recent position of the glacier front, with the tracing of the glacier front from a previous time, and with the colored tracings of other glacier features you identified, iii)
the time span of the change, iv)
the amount of retreat or advance in meters, v)
the rate of retreat in meters per year, and vi)
an explanation identifying what the various traces 9
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are by color (e.g. the red line tracing shows a terminal moraine). One way to gauge your product from this portion of the exercise (and throughout this lab) is to ask the question –
“could another student in this course learn from what is displayed?”
To help, an example of this type of analysis is provided below for Exit Glacier, near Seward, Alaska, a common tourist destination. You can use this example as a guide for your reports on the other glaciers. The image to the right shows a
1996 image of Exit Glacier, and
the ice margin at that time has
been outlined in pink. Note the
scale bar and the 1996 imagery
date. Switching to the 2005
image available in Google Earth
and then tracing out other
features of interest led to the
next image below. The caption of
the image below includes the
type of information requested
above. Below is the finished
product, with the 2005 imagery. 10
Example of explanation for the above image
: The Google Image above shows Exit Glacier as it was in 2005. The pink line shows where the front of the ice was in 1996. The blue line shows the present position of a melt-water stream. The tan lines on the glacier trace two examples of crevasses on the glacier (and there are many more). The turquoise line shows the approximate extent of some lateral moraines. Note how new moraine material is also apparent on the south side of the glacier. As measured by the Google Earth ruler tool, the ice front has retreated about 100 m over the 9-year period from 1996 to 2005, indicating an average retreat rate of 11 m/year. To get a better idea of what you are looking at in Google Earth, you may also want to zoom in and change the perspective so that you are looking obliquely instead of straight down. You may also want to look at the glaciers from different directions (grabbing the north arrow on the top Google Earth navigation tool and rotating it also rotates your view direction). The nature of a distinctive feature may become clearer as you zoom in to take a closer look, and then zoom out to capture the larger context. Below is the table of the 7 localities we have identified where the imagery for glaciers at multiple
times exists in Google Earth, suitable for this exercise. You are encouraged to find appropriate new sites of your own, if you wish.
Locality latitude
longitude
horizontal window
extent
Solheimfjell glacier, Iceland
63 32.270 N
19 21.859 W
1.2 km Columbia Ice Field, Alaska
61 8.482 N
147 4.731W
8-10 km Uppsala, Argentina
49 55.544 S
73 15.424 W
10 km
Kenai, Alaska
59 24.17 N
150 49.989 W
5 km
Northwestern Glacier, Alaska
59 50.83 N
150 4.32 W
4 km
Bear Glacier, Alaska
59 57.703 N
149 34.942 W 5 km
Insert your 5 reports here.
Question 2:
Question 3:
Question 4:
Question 5:
Question 6:
11
Estimating sea level rise from glacial ice mass loss (6 pts)
If ice on land melts, eventually that water finds its way to the sea, and, everything else being equal, sea level will be a little bit higher. How much higher, i.e. if we were to melt all, or some significant percentage of glacial ice on land, how much would sea level increase? We can address that question here in a realistic way with just a few numbers. It will be a first-order approximation but will provide some distinct insight (and the magnitude of sea level change is a surprise to some).
Remember that the melting of ice floating in the ocean will not change sea level. Because the ice is floating, the volume of water the ice displaces as it floats is equal to the volume of water released when the ice melts. You can try this at home if you like and see if the water level changes as an ice cube melts in a glass (make sure there is no evaporation). For this reason, we do not need to worry about floating ice, only the ice resting on land.
If you pour a certain amount of water into a container with water in it already, the amount
the water level rises depends not only on the amount of water added, but also on the shape of the container (imagine a small slender glass versus a baking pan). Specifically, it depends on the area of the container you are pouring it into. If the walls are vertical, you know enough to calculate how much rise there should be if a given volume of water is added and the surface area of the container is also known. However, if the container walls are sloping, then some of the added water goes to flooding the sloping area, increasing the water’s surface area in addition to raising the water level of that water body, as shown in the diagram below.
The diagram above shows the geometry for estimating sea level increase in a basin with sloping
boundaries. Remember that this is a schematic diagram, and a simplified situation. In the case of the world’s oceans, the water volume before the sea level increase is obviously much, much larger than that involved in sea level increase. The main focus of the diagram is to show the two
different volumes involved in estimating sea level increase.
The numbers needed to make a first-order estimate how global sea level might change are the volume of ice in glaciers available to melt, the surface area of the world’s oceans, the length of coastlines that would be flooded, and the average slope of those shorelines. All these numbers are available. Estimates for these do vary some, and you can try different numbers to see what difference it makes in the results if you are interested in doing so. The section below develops the math. The only tricky bit is that we end up with what is known as a quadratic equation, which you may remember having seen in high school, and 12
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probably thought (or hoped) you would never see again. Luckily, the general solution for a quadratic equation has been developed, and it is a simple chore to insert the numbers into the general equation. This has been captured in the Excel sheet you will use for this lab. While you don’t need to directly work with the math (since the Excel sheet does the calculations for you) it is very important to understand how the model works. Otherwise the model is like a magic black
box where results come out, but because the user doesn’t understand how the model works and
what the assumptions and limitations of the model are, the results could be seriously misunderstood and misused. Let:
X = sea level increase in meters. This is what we want to compute.
= average slope of coastal shores
K
o
= area of ocean in square meters.
K
m
= volume of ice melt * 0.9 in cubic meters
K
cl
= coastline length in meters
The three Ks above can be treated as constants for a given situation. We can then set up the equation based on the idea that the volume of water that results from the glacial ice melting (K
m
) equals the volume of water involved in a sea level increase, including
that needed to raise the level over the existing area of the sea (= K
o
* X) and the component needed to flood the sloping shores ( = X
2
*K
cl
)/(2*tan(
)). This later part comes from treating the area flooded as a right triangle and using the tangent trig function to find another side of the triangle and then calculating the area of the triangle and multiplying by the coast line. K
m = (K
o
* X) + (X
2
*K
cl
)/ (2*tan (
))
We can then rearrange terms and let K5 = K3 * K4 to simplify, ending up with the following equation.
0 = a
*
X
2
+ b
*
X + c: where a = K
cl
/ (2 * tan (
)), b = K
o
, and c = - K
m
There is a simple reason we went to all this trouble to get it into this form. We would like to solve
for X. Luckily this is in the form of the well-known (at least in mathematical circles) quadratic equation, for which there is a general solution, which is provided below. X = (-b +/- (b
2
– 4 *
a * c)
.5
) / (2 *
a)
In this way, we can solve for X, the sea level rise, given
the input Ks above. 13
You can now open the companion Excel sheet where these numbers and equations have been captured for you. Read through this sheet and then use it to answer the questions listed below. To right is a screen shot of part of the Excel sheet you will use. The green is where you have your input variables, and the yellow shows an estimate of the global sea level increase given the
percentage of melting input. The necessary volumes, areas, and lengths mentioned above are provided in your Excel sheet, along with an estimate of the volume of the Greenland Ice Sheet. You may have to do some simple calculations to turn the numbers into the simple percent value input required. How much sea level increase could be expected if all the Greenland Ice Sheet melted (Hint: what percentage of the total volume
of the world’s glaciers does Greenland represent—these volumes are given in the Excel sheet? Note that E is exponential notation such that 2.84E+16 would be 2.84 x 10^16).
Question 7:
What is the estimated sea level increase in meters from the melting of Greenland Ice Sheet? Copy and paste cells A2 to B15 from your Excel sheet here.
The volume of all the ice in the other smaller glaciers around the world (excluding Antarctica and Greenland) is also estimated, and the number can be found on your Excel sheet. Question 8:
How much sea level increase could be expected if all this ice melted (all glacial ice
except Greenland and Antarctica)? Copy and paste cells A2 to B15 from your Excel sheet here.
Impact of slope—
remember that there are two places the melting water goes: the water volume needed to raise sea level over the area already spanned by the oceans, and the water volume needed to cover the land flooded by a certain amount of sea level rise. In the Excel model you can change the slope amount. At a slope angle (one of the input parameters you can
vary) of 90 degrees, the shores would be vertical and there would be no second component. So,
seeing how the resulting sea level changes as you change the slope provides insight into what proportion of the melting water goes into flooding land. Question 9:
Write two or three sentences on the amount of sea level increase change as you change the shoreline slope component, and what you conclude from this?
Question 10:
If all the glacial ice on earth were to melt, how much sea level increase would there be? Copy and paste cells A2 to B15 from your Excel sheet here.
Question 11:
If the glacial ice was melting at 1% per year, what would be the corresponding rate of sea level rise in meters? Copy and paste cells A2 to B15 from your Excel sheet here.
Question 12:
Write two to three sentences on how your answer from Q11 compares with the observed, present rate of global sea level rise of roughly 2-3 mm/year (which is equal to .002 to .003 m/year), and what you conclude from this.
14
There are all sorts of additional considerations that can be used to refine the estimate of sea level increase, and that we encourage you to learn about if you have interest and time. Consider
for example, what happens to the shoreline length as flooding occurs (it gets smaller – think of a
shrinking island). This is not accounted for in this model. Consider also isostasy that was discussed in a previous lab and the fact that some of the glacial ice mass is below sea level. What type of effect will this have as ice melts? However, the relatively simple model above that we have explored in this introductory course is within roughly 80-90% of the sea level rise predicted by much more sophisticated models that take these additional considerations into account, which is not bad at all for such a simple model.
Sea level rise and sustainability
What is the broader significance of sea level rise? Consider that the world’s human population is
concentrated along coast lines (for good reasons – they tend to be resource-rich environments).
How can we live sustainably? Rising sea levels due to global warming indicates that conditions along the coast will always be changing for people in these areas. They will change for some coasts much more than others and that needs to be considered in each specific instance. The figure below shows the areas that will be flooded for different levels of sea level rise and demonstrates the extent of the challenge. However, in general, rising sea level indicates that it is impossible to sustain many coastal communities exactly as they are now. Instead, coastal communities and peoples will have to adapt to changing conditions. Sustainability requires adaptation to changing conditions
in this case (and in most cases), and a very important related question is what is it that one desires to sustain? Sustaining specific structures (such as the hotel on the beach front), is not possible in the long run, but sustaining a shifting community may be. Feel free to add any comments on these thoughts here. We will find them of interest. Image from NOAA showing in red the
areas that will be flooded with a 1 m,
2 m, 4 m, and 8 m rise in sea level:
image source - http://www.gfdl.noaa.gov/index/knutson-
climate-impact-of-quadrupling-co2
.
15
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