The Ice Age
Based on W. Peltier's paleotopography and ice sheet extent and paleotopography
for the past 21,000 years.
(University of Toronto)
The Earth's climate is constantly changing in response to changes in atmosphere
and ocean currents, or the ongoing process of plate tectonics. The last few million
years of Earth's history have been characterized by particularly wide, short term
swings in climate. These resulted in huge continental glaciers that advanced and
receded across the northern latitudes. If the interpretations of the sedimentary
record of deep sea cores are correct, there may have been over twenty glacial
ages in the past 1.8 million years of the Pleistocene Epoch. These glacial events
in turn triggered other climate changes and greatly affected the distribution
and evolution of animals and plants.
This module only concerns the most recent major climate shift, the end of the
last glacial interval. However, its important to realize that our world is still
very much in a glacial mode. We're simply living during an interglacial interval,
a time when glaciers are at a relative minimum. Other interglacial intervals,
some even warmer than the present, have occurred during the last few million
years. These have alternated with periods of cooler climate - glacial maxima
when ice covered over 30% of the Earth's land surface. Unless human-induced
global warming artificially extends the present interglacial, the Earth's climate
will likely begin to shift back towards a glacial maximum sometime in the next
thousand years. Although it takes thousands of years to build a continental
ice sheet, some computer models suggest that the first significant shift towards
glacial expansion could occur within the next two hundred years.
Software/Plug-in Requirements
To view the Quicktime movies and Quicktime VR visualizations throughout
this resource you will have to have Quicktime 4.0 or higher installed.
Quicktime 4.0 can be downloaded and installed from Apple Computer free
of charge. Click
here to go to the Apple Quicktime download page.
You will also need Microsoft Internet Explorer 3.0 or Netscape Communicator
3.0 or better.
BUG NOTES:
- You may have problems viewing the Quicktime visualizations if you are using
Netscape 4.5. Movies may load incorrectly, take a long time to play or may
be slow to respond when manipulated. There is nothing at present that can
be done about the slow response/play but reloading a page can fix "broken"
or miss-loaded movies.
- Pages with multiple movies may load slowly. Also playing both movies at
the same time may result in frames being skipped. Other than assigning your
browser more memory there is no way to fix this if it occurs.
- Note that if you enlarge the visualizations, you should use the 'Back' link
to return to the previous page, rather than the 'Previous' link in the bottom
banner.
1. The Ice Age
1.1 The Last Glacial Interval
The image above shows the interpreted extent of glacial ice during the last glacial
maximum 21,000 years ago. Use the link above to learn about manipulating the image.
You can use the mouse to change the tilt of the globe's axis, or to rotate the
globe about its axis.The visuals in this module consist of moveable globes, movies,
photographs and maps. Apart from the photographs and maps, most of the visual
elements can be manipulated in some manner, but they may have to be triggered
by the viewer. Note that if you enlarge the image, you should use the Internet
browser's 'Back' button to return to the text, rather than the 'Previous' button
in the bar below. This will be true for most of the module's visualizations.
At the North Pole the image does not attempt to show the extent of sea ice, just
continental ice. Rotate the globe north to south and about its axis to see the
distribution of glaciers at this time. As you manipulate the globe, also pay close
attention to shorelines. Notice how the shape of peninsulas, islands and continents
vary from the present. Florida was nearly twice as wide at this time and North
America was connected to Eurasia by the Beringian landmass. At this time so much
water was locked up on land as glacial ice that global sea level was nearly 120
meters lower than at present
Click Here
for Larger Image
1.2 Polar Ice Caps
The interpreted distribution of ice over the last 21,000 years is shown to the
right. Clicking the left button will activate an interpretation of how ice volume
and distribution have changed through time. As you run through the movie, be sure
to notice both the change in ice distribution and the consequent shift of coastlines
as sea level rises.
Question:
Both the north and south polar areas supported extensive ice sheets, but one pole
had a much greater impact on global sea level change over the last 21,000 years
than the other. Which polar area would have had the greatest effect on global
sea level?
Click Here for
Larger Image
1.3 Glacial Evidence
The landscape across much of the northern latitudes bears mute testimony to the
presence of now-vanished glaciers. Glaciers eroded U-shaped
valleys and broad flattened plains. Glacial deposits formed sinuous low hills
and lake-dotted, irregularly drained landscapes.
Other evidence for these glacial periods has been known since the early 1800's,
including:
Till
Erratics
Striations
Drainage Patterns
Crustal Rebound
Glacial valleys have a distinctive U-shaped cross-section due to their erosion
by glacial ice. Water is less viscous than ice and tends to cut deep V-shaped
mountain valleys. If a glacier builds and moves down a mountain valley, the moving
ice tends to widen the valley's cross-section. The difference between ice and
water erosion is also reflected in the way the valley floors meet. River valleys
tend to meet at a common elevation, but glacial ice is viscous enough that tributary
and main glaciers do not necessarily cut down to the same level. If the glaciers
recede, the tributary valley floor may be perched well above the main valley floor.
Rivers that subsequently occupy these "hanging valleys" often form spectacular
waterfalls.
U-shaped valley in
Yosemite National Park
(Photo by Dr. Karen Kleinspehn)
Although glacial erosion sculpted much of this area, some of the more distinctive
features resulted from glacial deposition. As the glaciers melted they left their
load of sediment behind - creating an irregular, poorly drained landscape. Thick
accumulations of sediment formed moraines that mark still stands in the glacial
retreat. Rainfall and groundwater accumulated in low areas to form marshes and
lakes. The resulting mixture of low ridges, wetlands and lakes created a heterogeneous
resource base that is characteristic of many recently glaciated areas. As post-glacial
river systems continue to cut through the terrain, the area will progressively
be drained and leveled, resulting in a more homogenous plain.
Classified LandSat
image of the Twin Cities Metro Area
moraine till in Svalbard,
Norway
(photo by Dr. Karen
Kleinspehn)
Although landforms composed of deposited glacial material are called moraines,
the sediments that form the moraines are called till. Till is characterized by
its poorly sorted nature. Compared to water or wind, ice tends to do a poor job
of sorting the material it carries. Large blocks are carried along at the same
speed and in the same manner as fine clay. When the glacier melts and recedes,
any material that is not reworked by water or wind is simply left behind as a
chaotic mixture of grain and sizes and composition.
Their heterogeneity makes glacial tills very distinctive. Before coming up with
the idea of past glacial ages, scientists spent decades trying to understand how
such deposits could form. Early ideas focused on cataclysmic events. Some scientists
proposed that there were periods of intense volcanism that hurled large blocks
for kilometers and caused entire mountainsides to collapse. Others thought that
tremendous floods had carried these strange deposits high up into mountain passes.
Many of the latter scenarios attempted to tie these deposits to Noah's Flood.
Other scientists developed a less cataclysmic, though equally inaccurate, explanation.
By the early 1800's it was well known that icebergs did not consist solely of
ice, but also carried sediment and even large blocks. Some scientists proposed
that these mixed deposits originated from melting icebergs that drifted over the
land area at a time when it was covered by shallow seas. This led to the deposits
being called 'drift', a term that is still in use today, even though the idea
behind it has been abandoned.
By the 1840's careful mapping and study of glacial deposits through much of Europe
and North America led to the recognition that continental and alpine glaciers
were once far more extensive than at present. Once proposed, the idea of past
glacial ages had a relatively rapid acceptance, largely because so much was already
known about these distinctive deposits.
Superior Lobe till
of central Minnesota
(photo by Dr. Karen
Kleinspehn)
Erratics are boulders that have been transported long distances and deposited
by glaciers. Their composition is different than the local bedrock - hence the
term `erratic'. Glacial erratics comprise much of the fieldstone that has plagued
generations of northern farmers. In many ways, these boulders are simply part
of the glacial till, no different in origin than the associated clay, silt and
sand. We only tend to view them differently because of their size. From a human
perspective, there's something about an immense boulder on a landscape that sparks
human curiosity. Even trained geologists will often lavish attention on a particularly
imposing erratic while ignoring the soil at its base, which is economically a
far more important glacial legacy.
The photo to the right shows the Three Maidens at the Pipestone National Monument.
These five boulders appear to have been part of a single large erratic that broke
into several pieces. If this interpretation is correct, the original erratic was
one of the largest glacially transported rocks known in the northern United States.
George Catlin, the explorer and artist that helped record many North American
Indian cultures, visited the Pipestone quarry in 1836. Catlin recognized that
these granite boulders were foreign to the valley and unlike any known local bedrock.
In his journals, Catlin correctly linked these blocks to thousands of exotic rocks
he had previously encountered in his travels across the northern states and tried
to explain their possible origin. At the time, he knew of no other option than
to attribute them to a past catastrophic flood.
Catlin was only the first person of European descent to record his thoughts of
the Three Maidens. The Dakota religion is an oral tradition and by Dakota custom,
their religious traditions should not be presented in written form. It can be
pointed out, however, that native tribes had recognized the Maidens' exotic nature
centuries before European contact
Three-Maidens Erratics
(photo by Kent Kirkby)
Striations are simply scratches or grooves on glacially eroded rock surfaces.
Even under immense pressure, glacial ice cannot cut the underlying rock - but
a glacier does not consist solely of ice. Rocks and sand caught in the moving
glacier cause it to act as an immense form of sandpaper, eroding the underlying
bedrock. The movement of larger rocks leaves striations in the eroded surface.
Striations formed at roughly the same time tend to parallel one another, recording
the movement of the glacier. However there can be more than one set of striations
carved in a surface if the glacier shifted direction, or if multiple glaciers
moved across the surface at different times.
View looking down
on a flat striated surface (sub-parallel lines running from bottom to top are
striations, the dark lines are fractures)
(photo by Dr. Karen Kleinspehn)
As the ice sheets spread over the land surface they widened and deepened pre-glacial
valleys. After the ice sheets retreated, these expanded valleys filled with water
to form the Great Lakes and New York's Finger Lakes. Glacial erosion also reduced
much of the northern landscape to a relatively flat lying plain. As the ice sheets
retreated, the sediment load left behind formed a very irregular, poorly drained
surface. Between each glacial advance, rivers cut through this irregular topography,
carving channels and slowly draining the landscape. As another ice sheet advanced
and retreated, glacial deposits largely filled the old river channels, and the
next generation of rivers had to carve a new set of channels. The legacy of this
interrupted drainage history was a series of buried river channels that honeycomb
many glaciated areas. The present river systems across much of the northern continents
are relatively recent features (<20,000 years) that often have a markedly different
geometry than any of their predecessors.
Another major effect on drainage patterns was the sometimes catastrophic draining
of large glacial lakes. As the ice sheets first began to melt and recede, they
released a tremendous amount of water. In some areas, this water was trapped
by a line of hills or by the glaciers themselves. When this occurred, the melt
waters formed large glacial lakes. Lake Agassiz was a huge glacial lake that
straddled the current US-Canada border. Lake Superior rose almost 150 meters
above its present level, because ice blocked its drainage to the east. Eventually
these lakes filled to a spill point or the ice dams melted. The subsequent drainage
of these lakes could be spectacular. In the Midwest area, the Minnesota and
St. Croix River valleys were carved out within a relatively short time by the
drainage of Lake Agassiz and Glacial Lake Superior respectively. Further west,
a lobe of glacial ice blocked the ancestral Clark Fork River roughly 13,000
years ago to form Glacial Lake Missoula. Collapse of this ice dam and the subsequent
catastrophic draining of Lake Missoula formed the Channeled Scablands of eastern
Washington.
Pre-glacial and interglacial
river valleys(gray) in the Minneapolis-St. Paul area, identified from well records.
Lakes located over the valleys formed in depressions that were created from
the melting of buried ice masses. The modern Mississippi and Minnesota Rivers
are shown as dotted lines. Modifed from Wright (1972)
(Geologic History
of Minnesota Rivers, Minnesota Geological Survey Educational Series - 7)
Take a moment to run through the visualization a few times. The first time through,
your eye will probably tend to focus on the retreat of the ice sheets, but as
you go through it again, note the shift in coastlines as the ice melts.
What is happening in the Hudson Bay area? Note that Hudson Bay actually shrinks
in size, even though global sea level is rising! This isn't an error in the model.
At present a series of low ridges encircle much of Hudson Bay. These ridges are
beach deposits that mark the position of former shorelines.
Question:
There is a great deal of physical evidence supporting the idea that Hudson Bay
has shrunk in area over the last 10,000 years. Yet this is a time period when
global sea level rose and most coastlines retreated towards the continent interiors.
What other factor(s) could cause Hudson Bay to decrease in area at a time when
global sea level was rising?
Answer
Click Here for
Larger Image
1.4 Oceanic Evidence
Incomplete Land Records
Surprisingly, some of the best records of past glaciers are found in the sea and
not on land. During interglacial intervals, the land surface was exposed and actively
eroded. While deposition occurred in lakes and along river valleys, it tended
to be sporadic as rivers migrated and lakes dried up or filled in. Rivers and
lakes are also sensitive to small changes in climate that greatly affect their
sedimentary records. Finally, it can be very difficult to accurately date terrestrial
records, and this makes it difficult to correlate terrestrial records from one
area to another area - at least on the time scale necessary to study glacial cycles.
Even if terrestrial sedimentation was more continuous, each advancing ice sheet
modified or removed part of the previous record. Older glacial deposits, interglacial
river sediments and soils were eroded by the weight of the new advancing ice sheet
nd mixed together to form the next layer of glacial deposits. Generally, the deposits
of different glacial events can only be clearly separated along the ice sheets'
margins, where successive ice lobes might have moved in a different directions,
or reached different extents. This is the reason that the visualizations in this
module only model the most recent glacial event. There simply isn't as much available
data for older glacial events.
Marine Records
While the terrestrial sedimentary record is inherently incomplete, a more complete
record exists in the deep ocean. As microscopic shallow water plankton die, their
shells settle to the ocean floor creating a slow, continuous deposition of fine
sediment. Hence sediment cores taken from the deep sea floor contain a succession
of shells that formed in the overlying shallow water environments. Deep-sea cores
taken in high latitude areas exhibit a vertical alternation of warm and cold water
communities, reflecting a succession of glacial and interglacial events. Some
of these planktonic foraminifera also have shells that coil in different directions
depending on the water temperature in which the organism lived. In cooler waters
(less than 8¡ C) shells of Globoratalia truncatulinoides will coil to the left
(clockwise), while in warmer waters (8¡ to 10¡ C) the shells coil to the right
(counter-clockwise). So both the type and shape of plankton shells can provide
clues to the climate that existed at the time the shells formed.
1.5 Oxygen Isotopes Record
Water vapor evaporated from the ocean tends to have a slightly higher ratio of
16O to 18O than that of the original ocean water. This means
that snow precipitated from the water vapor is also slightly richer in 16O
relative to 18O than the original ocean water, as are the glaciers
that eventually form from this snow. The oxygen isotope ratio of ocean water remains
relatively constant as long as there is no net change in the global amount of
glaciers or groundwater. However, as the Earth's climate cools and glaciers expand,
more 16O-rich precipitation is stored on land as glacial ice, and the
16O to 18O ratio of the remaining sea water drops slightly.
The change in oxygen isotope ratios is subtle, but measurable. The shells of plankton
that grew during glacial intervals have slightly lower 16O to 18O
ratios than shells of plankton that grew during interglacial intervals. During
warmer intervals, glaciers melt and release their stored water back to the ocean
- raising the ratio of 16O to 18O in ocean water. Hence
the oxygen isotope composition of plankton shells indirectly reflects the amount
of glacial ice stored on the land.
Ice Cores
Similar oxygen isotope changes also occur in ice cores. Ice cores taken from existing
glaciers show that snow precipitated during past glacial maximums had higher 16O
to 18O ratios than snow precipitated during glacial minimums. So the
ratio of 16O to 18O in preserved ice appears to reflect
the size of continental ice sheets. However, the oldest ice recovered from glaciers
so far only dates back to 160,000 years ago, so the temporal record of ice cores
is somewhat limited compared to deep-sea cores.
Deep-Sea Sediment Cores
The chart to the right shows a composite record of 16O:18O
ratios as compiled from deep-sea cores. To date, these cores provide the most
detailed record of glacial changes over the past few million years. Unlike the
terrestrial record, sedimentation is more continuous in the sea than on land.
Although erosion occurs in marine environments, erosion is far less pervasive
than on land. Deep-sea cores are used because the deeper environments tend to
be less sensitive to changes in sedimentation rates than shallow marine areas
and contain fewer burrowing organisms. Animals that burrow through sediment homogenize
the vertical succession of deposits, destroying the section's usefulness as a
climate change record.
Note that the visualizations only model the last 21,000 years of this two million-year
record. Since it is the most recent change, it is the one with the best record.
However, realize that this was only the most recent climate swing in a long series
of climate changes.
2. Ice Age Legacy
How did these continental ice sheets affect our modern world? Some effects, such
as landforms, lakes and sculpted mountain valleys were already mentioned. These
features provided much of the early evidence for past glacial ages. Arguably though,
the most important legacy of the ice ages may have been their effects on the evolution
of life. Glacial ages can affect the evolution and distribution of life directly
by climate change and shifting environments, or indirectly by changing sea level
and subsequently affecting animal migration.
The direct impact of glacial ages on life is reflected in the record of recent
shifts in flora distribution. Most plant parts decay very quickly, and fossils
of complete plants are relatively scarce. However plants produce pollen in huge
quantities and pollen is surprisingly durable. If pollen is blown over wetlands
or a lake, it can settle to the bottom to become part of the fossil record. Consequently,
sediment cores taken from wetlands and lake bottoms provide a record of the sequence
of local plant communities that thrived in the area over time. By dating and correlating
pollen records from different areas, climate researchers can reconstruct the large-scale
migration of flora that occurred in response to climate change.
2.1 Pollen Records
This visualization displays pine pollen records from North America. Each circle
represents a separate lake or wetland record. Both the size and color of the circle
indicate the quantity of a particular type of pollen. As lakes are not uniformly
distributed and not all lakes have been sampled and studied, the data is rather
scattered. In particular, there is a general lack of data across western North
America - due to the relative paucity of western lakes and wetlands. Hence you
have to interpolate between the data points to discern patterns of flora migration.
As you run the visualization, notice that the changes in flora distribution are
not restricted to the margins of the ice sheets. Although glaciers only extended
to the northernmost part of the United States, they affected the distribution
of flora and fauna well to the south.
Notice that 18,000 years ago pine trees were largely restricted to the eastern
coast and were common in Florida. By 16,000 years ago, a northward shift began
that accelerated over the next few thousand years. Six thousand years later extensive
pine forests covered most of northern Minnesota, Wisconsin, Michigan and southern
Canada. These forests supported a succession of native Woodland Indian societies
for several thousand years, but much of this pine belt was cleared in the late
1800's, when European settlers and explorers reached the upper Midwest. This northern
pinery was essentially logged off within a few decades (1860-1910). At the time,
this logging operation was easily the largest planned deforestation in human history
and in retrospect it remains one of the more dramatic examples of human impacts
on the global environment. Rerun the pine records and notice the rapid decrease
in pine pollen in the last few hundred years. Realize that these records are not
exactly correlated to time. There are only a few points in each pollen record
that are carbon dated. The remainder of the record is simply interpolated between
these points. So don't try to correlate the dates shown specifically to a modern
calendar, just notice the patterns of pine pollen distribution toward the end
of the record. The impact of human activity is easily seen in both the general
decrease of pine across the logged northern belt and the recent reestablishment
of pine in some southern areas.
Notice what was happening to the pine belt during the period of Woodland Indian
Culture (roughly 1000 BC up until European contact). Did it remain in the same
location or migrate? How might this have affected Indian cultures that depended
on the woodland environment?
Question:
In general, pines dominate the present northern latitude forests, yet pine trees
can certainly survive in more equatorial climates. Why isn't pine more dominant
in warmer climates?
Although
the two visualizations above cover the same time interval, note that they run
at different rates and in different time steps.
Click Here
for Larger Image
2.2 Ragweed Pollen Records
Question:
Ambrosia (ragweed) is a relatively minor component of the pollen record until
the last 0.1 ka when ambrosia pollen fairly explodes across the landscape. Consider
the timing of this rapid expansion. It takes place well after the glacial retreat,
yet represents a remarkably rapid transition in flora communities. What factor
or force might have been responsible for the dramatic increase in ambrosia?
Click Here for
Larger Image
2.3 Migration Routes
Ever consider the modern distribution of moose
and caribou? These are remarkably cosmopolitan animal species, with ranges
that include much of North America and Eurasia. In contrast, lower latitude species
tend to be far more provincial, with ranges limited to a single landmass and faunas
in the Southern Hemisphere tend to be even more localized. How can the greater
range of northern species be explained?
In part, this is another legacy of the glacial ages. The visualization below
is centered on the Bering Sea. Running the model back in time to the last glacial
maximum reveals a major land connection, called Beringia or the Bering Land
Bridge. Twenty-one thousand years ago, Eurasia and North America were a single
continent. Their separation into two continents depends upon the amount of glacial
ice present. During glacial maxima, Beringia is exposed and North America and
Eurasia are connected. During glacial minimums, a shallow sea covers the area
separating the two areas into discrete landmasses. With two million years of
glacial cycles, land animals (including humans) have had many opportunities
to move back and forth between the present Eurasian and North American continents.
Click Here
for Larger Image
2.4 The Origin of the Wallace Line
There are over 13,000 islands in the Malay Archipelago and some of these played
a major role in the development of the theory of evolution. In 1854, eighteen
years after Darwin had finished his Beagle voyage, a young naturalist named Alfred
Russel Wallace began collecting and studying the fauna of these islands. Combined
with his earlier observations on the flora and fauna of the Amazon basin, these
studies led Wallace to propose the concept of natural selection. Wallace's work
was a major innovation, providing the first plausible mechanism to explain the
process of evolution. His work also had the beneficial effect of spurring Darwin
to finally publish his own work on evolution and the origin of species.
In the course of his studies, Wallace recognized that the Malay Archipelago
comprised two very different broad communities. Some islands had faunas that
were related to Australian forms, while the faunas on other islands were related
to Eurasian communities. On a map, Wallace could separate the two groups of
islands by a line (later called the Wallace line) but he was not certain why
the communities were so clearly distinct. In Letters & Reminiscences (1916),
Wallace wrote:
|
In this archipelago there are two distinct faunas rigidly circumscribed
which differ as much as do those of Africa and South America and more than
those of Europe and North America; yet there is nothing on the map or on
the face of the islands to mark their limits. The boundary line passes between
islands closer together than others belonging in the same group. I believe
the western part to be a separated portion of continental Asia while the
eastern part is a fragmentary prolongation of a former west Pacific continent. |
First try to guess where the Wallace
Line lies on the modern image, then run the model back in time. How did glaciation
lead to the development of Wallace's separate communities?
Click Here
for Larger Image
2.5 North Atlantic Routes
Great Britain and Ireland also furnish examples of the effects of continental
glaciation on the distribution of modern faunas. Since the Cretaceous, Australia
has been an isolated continent and its terrestrial fauna has evolved independently
of other continental communities. This helps to explain Australia's very distinct
land community. Although there has been some communication with the African mainland,
Madagascar provides another, smaller scale, example of the evolution of distinct
faunas on isolated islands. Yet the faunas of Ireland and Great Britain are very
similar to European communities. Why? Even though the English Channel is a relatively
narrow body of water, its breadth (32 kilometers wide at its narrowest point)
is still an effective barrier to any non-swimming, non-flying animal. Run the
North Atlantic visualization to find a reasonable explanation for the similarity
of these island faunas to the mainland communities.
Run the movie backwards from 21,000 years ago to the present. To do this you have
to use the buttons on the right. Starting around 10,000 years ago notice what
happens to the Baltic Sea coastlines. At the same time that the English Channel
starts to flood, the Baltic Sea is actually shrinking in size. This is due to
the uplift of Scandinavia as it adjusts to the removed weight of a melted ice
sheet. Uplift of this area has continued to the present and highlights the complexity
of changes related to glacial advance and retreat. Even within the same region,
you can have some coastlines recede while others advance - depending upon the
relative magnitude of local rebound and global sea level rise.
Click
Here for Larger Image
2.6 South American Routes
Can the ice ages explain the distribution of all faunas? How about Darwin's famous
Galapagos Islands? The distribution of finches and tortoises on these islands
provided Darwin with a historically important example of evolution. Unlike turtles,
tortoises can not swim, so how did they get to the Galapagos Islands? Would a
drop in sea level reveal a connection to the mainland that would allow tortoises
to migrate to the islands? Run the visualization back 21,000 years to try to explain
the tortoises' present distribution.
Click Here for
Larger Image
2.7 Present Population
What about the effects of future sea level changes on the world's fauna - particularly
human communities? This visualization shows the present distribution of the world's
human population. On a global scale, it is relatively easy to see the impact that
geological forces play on this distribution. Note the relative paucity of humans
in two belts centered over 30 degrees north and south of the equator. How can
you explain this pattern?
Click Here
for Larger Image
2.8 North Atlantic Current
There is a less obvious, but equally important geologic tie to population density
in northern Eurasia. London is at the same latitude as Calgary (Alberta), yet
unlike Calgary London tends to have mild winters. The effect of climate on the
population density of these two cities is pronounced. In 1995 only three-fourths
of a million people lived in Calgary, compared to London's nine million. Obviously
there are important differences is history and industry between the two cities,
but these population trends also hold for the two countries overall. In 1993,
Canada had an estimated population of over 27.5 million people spread over nearly
10 million square kilometers of land. In contrast, Great Britain had an estimated
population of 58 million people living in only 244,000 square kilometers. The
Arctic-centered view of population density above illustrates the skewed population
distribution across the northern latitudes. Across northern Europe and into eastern
Russia there are far more people living per square mile than anywhere else at
the same latitudes. This population distribution effectively mirrors regional
climate.
Northern Europe's hospitable climate is due to the North Atlantic current. Fed
in part by warm saline Caribbean waters, the North Atlantic current originates
around 35¡ north of the equator and flows northward off the northwest coast of
Great Britain. Water has a remarkable heat capacity, and the North Atlantic Current
is still 8¡ C when it reaches 60¡ north. As the current flows between Great Britain
and Iceland it cools, releasing its heat to the air. Wind patterns then distribute
this heat across northwestern Eurasia, allowing a much more dense human population
in this area than in any other area of comparable latitude.
Future Change?
Ocean currents are among the Earth's most important means of heat transport,
and any change in their pattern can have an immediate impact on climate. If
the North Atlantic Current shifted to the west it would increase precipitation
in the Baffin Bay area between Greenland and North America. This could tip the
climate balance in favor of a new ice advance. The effect of this current migration
on the climate of northern Europe would be of far more immediate concern. Without
the warming from the North Atlantic Current, the climate of northern Europe
would dramatically cool. Consider the impact of such a change on world population
and politics. The implications are immense and unlike the slow buildup of a
glacial advance, the effects would take place very quickly.
2.9 'Super-Interglacial' Alternative
For a moment, consider the effects that an alternative scenario might have on
human populations. Rather than having the Earth shift back into a glacial mode,
what if humans instead manage to push the Earth into a super-interglacial interval?
If human activities artificially increase greenhouse warming (by changing the
present concentration of greenhouse gases in the atmosphere), we might see an
extended interglacial interval - a warmer global climate. This could cause the
present ice sheets to melt. If this did occur, the rise in sea level would impact
human society as much as a glacial advance.
If greenhouse warming melted all the glaciers on Earth, global sea level could
rise an estimated 65 to 80 meters. Use the visualization above to imagine the
consequences this sea level rise might have on the Earth's present population
and resources. Present population density and an exaggerated topography are shown
on the globe, which can be rotated by using the mouse in a horizontal direction.
Moving the mouse vertically will change global sea level. The magnitude of the
sea level rise is shown at the bottom of the image. Raise the sea level by 65
to 80 meters to see the effects that melting the present ice sheets might have
on many of the world's most populated areas. In particular, note the effect of
this sea level change on India and China, two of the most densely populated areas.
Question:
On the image, you can further raise sea level 200 meters above its present position.
This is far beyond the rise expected if all the world's present glaciers were
melted. Yet marine Cretaceous sedimentary deposits cover large areas that are
well over 2000 meters above sea level. If melting all the present glaciers would
only raise sea level a maximum of 80 meters, how could sea level have been higher
during the Cretaceous?
Click Here for
Larger Image
2.10 The Future
So which scenario lies in our planet's immediate future - a glacial advance or
a major sea level rise? At present we simply do not know enough about the Earth's
climate system to be able to accurately predict the consequences of natural and
human processes. This ignorance should be a concern in a world where the only
certainties are that climate will change, and that any climate change will have
a significant impact on our present population of nearly six billion people. Under
such circumstances, ignorance is not bliss.
Click Here
for Larger Image
3. Visualizations
4. Ice Age Module CD Screen Shot
Editor's Note:
The following image is a screen shot of a typical page as it appears on the
original Ice Age
Module CD.
Please use this area for comments about
the layout and design of the Ice Age Module CD.
Click
here to open in a separate window
5. Navigation Toolbar
Editor's Note:
The following toolbar is a nonfunctioning representation of the navigation toolbar
as it appears on the Ice Age Module CD. On the CD, this toolbar appears at the
bottom of each page (view
screen shot from the original CD). The menu on the left is used to jump
directly to a particular section. The buttons on the right bring you to the
previous page, the next page or the beginning, respectively.
Please use this area for comments about the navigational
features of the Ice Age Module CD.
6. Credits
Text/Concept:
- Kent Kirkby University of Minnesota
- kirkby@umn.edu
Visualizations and Animations:
- Paul Morin University of Minnesota
- lpaul@umn.edu
Web Page Design/Layout:
- Heidi Kamp University of Minnesota
- kamp0063@umn.edu
Photos:
- Dr. Karen Kleinsphen University of Minnesota
- Dr. Kent Kirkby University of Minnesota
Data:
- Ice Sheet Data
| |
Dr. W. Peltier University of Toronto |
| |
Peltier, W.R., 1993, Time Dependent Topography Through Glacial Cycle.
IGBP PAGES/World Data Center-A for Paleoclimatology Data Contribution
Series # 93-015. NOAA/NGDC Paleoclimatology Program, Boulder CO, USA.
Peltier, W.R. 1994. Ice Age Paleotopography, Science,265, 195-201
Peltier, W.R. 1996. Mantle Viscosity and Ice Age ice sheet topography,
Science, 273, 1359-1364
Peltier, W.R. 1998. Postglacial variations in the level of the sea:
Implications for climate dynamics and solid earth geophysics, Reviews
of Geophysics, 36, 603-689.
|
- Pollen Data
- Oxygen Isotopes
| |
E. Barron, 1994, Climatic Variation in Earth History, Module #108
of Understanding Global Change: Earth Science and Human Impacts, National
Center for Atmospheric Research. |
- Moose & Caribou Ranges
| |
K. Whitehead, 1993, The Whitehead Encyclopedia of Deer, Swan Hill
Press, Shrewsbury, England |
- World Population Data
- Galapagos Island Maps
Other References:
| |
Jackson, M, Galapagos: A Natural History, University of Calgary Press
Marchant, J, Alfred Russel Wallace Letters and Reminiscences, Harper
& Brothers Publishers, New York and London, 1916
Oosterzee, P, Where Worlds Collide: The Wallace Line, Cornell University
Press, 1997
Geologic History of Minnesota Rivers, Minnesota Geological Survey
Educational Series - 7
Wright, H.E., Jr., 1972, Quaternary History of Minnesota, in
Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A Centennial
Volume: Minnesota Geological Survey, p. 515-547
|
Funding:
| |
National Science Foundation Grant EAR-9809817 University of Minnesota,
Department of Geology & Geophysics |
