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Much of climate history is recorded in four
climate archives: |
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(1)
Sediments |
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(2)
Ice |
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(3)
Corals |
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(4)
Trees |
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How are those records dated? |
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Hoe much of Earth’s history each archive spans? |
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What is the resolution of climate history
yielded by each? |
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Sediments are the major climate archive on Earth
for over 99% of geological time (and on all time scales), primarily as
continuous sequences deposited by water. |
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Rainfall and the runoff it produces erode rocks
exposed on the continents and transport the eroded sediments in streams and
rivers. |
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The sediments are deposited in quieter waters
where layer upon layer of sediments can be laid down in undisturbed
succession. |
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For intervals before the last 170 million years,
all surviving sedimentary records come from continents. |
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Sediments in the deeper parts of some lakes
contain annual-layer couplet called varves. |
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Varve couplets usually result from seasonal
alternations between deposition of light-hued mineral-rich debris and
darker sediment rich in organic material. |
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The degree of resolution of climate records in
sediment archives is related to the rate of deposition (and burial) of
sediment and to the amount of activity of organisms burrowing into the
sediments. |
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Ice cores retrieve climate records extending
back thousands of years in small mountain glaciers to as much as hundreds
of thousands of years in continental sized ice sheets. |
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The antarctic ice sheet has layers that extend
back over 400,000 years. |
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The Greenland ice sheet has layers that extended
back 100,000 years. |
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The most visible forms of annual layering in ice
are the alternations between darker layer, containing dust blown in from
continental source regions during the dry windy season, and lighter layers
from other seasons with little or no dust. |
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Annual layer of snows are visible at the surface
of many mountain glaciers and rapidly deposited ice sheets. |
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As snow is buried and slowly recrystallized into
ice, annual layers remain resolable to a depth. Below this depth, the
layering is lost. |
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Ice cores from mid-latitude ice sheets such as
the one on Greenland, where deposition of snow is rapid, the annual
layering may remain visible tens of thousands of year into the past. |
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Ice cores from Antartica, where only a small
amount of snow accumulates each year, annual layering may not occur even at
the ice surface. |
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Trees are climate archives for the interval of
the last few tens and hundreds of years. |
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The outer softwood layers of many kinds of trees
are deposited in millimeter-thick layers that turn into hardwood. |
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These annual layers are best developed in middle
and high latitudes, where seasonal climate changes are larger. |
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In regions of marked seasonal variations of
climate, trees produce annual layers call tree rings. |
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These rings are alternations between layers of
lighter, thicker wood tissue formed by rapid growth in spring and much
thinner, darker layers marking cessation of growth in autumn and winter. |
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Corals form annual bands of CaCO3 that hold
several kinds of geochemical information about climate. |
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Individual corals may live for time spans of
years to tens or hundreds of years. |
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Coral archives are located at tropical and
subtropical latitudes. |
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In tropical oceans, corals record seasonal
changes in the texture of the calcite (CaCO3) incorporated in their
skeletons. |
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The lighter parts of these coral bands are laid
down in summer, during intervals of fast growth, and the darker layers
during winter, when grow slows. |
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The major boundary conditions that have driven
climate changes during the last 21,000 years have been the changes in: |
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(1)
size of ice sheet |
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(2)
seasonal insolation |
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(3)
level of greenhouse gases in the atmosphere. |
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Seasonal insolation levels 21,000 years ago were
nearly identical to those today. |
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The only factors that can explain the colder and
drier glacial maximum climate 21,000 years ago are: |
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(1)
the large ice sheets |
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(2)
the lower values of greenhouse gases. |
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During the deglaciation interval between 17,000
and 6,000 years ago, climate changes were driven by rising summer
insolation and by increased concentrations of CO2 in the atmosphere. |
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By 10,000 years ago, the angle of tilt of
Earth’s axis had reached a maximum at
the same time that Earth’s precessional motion moved it closest to
the Sun on June 21. These combined orbital effects produced a summer
insolation maximum at all latitudes of the northern hemisphere. |
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As the ice sheets shrank, their ability to
influence climate diminished. |
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Radiocarbon dating of organic remains shows that
the ice sheets in North America disappeared completely shortly after 6,000
years ago. |
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Submerged corals off Barbados, in the Caribbean,
show the deglacial history of the rise in sea level caused by the return of
meltwater from the ice sheets to the ocean. |
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The mid-deglacial pause in ice melting was
accompanied by a brief climate osscilation in records near the subpolar
North Atlantic Ocean. |
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Temperature in this region has warmed part of
the way toward interglacial levels, but this reversal brought back almost
full glacial cold. |
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Because an Arctic plant called “Dryas” arrived
during this episode, this mid-deglacial cooling is called “the Younger
Dryas” event. |
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During the last 6,000 years, with the ice sheets
melt and CO2 level stabilized near interglacial levels, the gradual change
in solar insolation is the main orbital-scale factor that affect the
climate since deglacation. |
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Millennial-scale climate oscillations are
referred to climate variations with time scales as short as 1,000 years. |
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These climate variations are rapid enough to be
relevant to human concern. |
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These changes were large when glacial ice sheets
extended, but smaller during interglacial climate like today. |
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The millennial-scale changes in the North
Atlantic is characterized by short cooling cycle 1,500 years in length. |
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These oscillations gradually drift toward colder conditions and
occasionally cumulate in major ice-rafting episodes, followed by an abrupt
return to warmer conditions. |
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Millennial-scale oscillations are found in many
places in the northern and southern hemisphere, such as those in |
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(1)
Greenland ice cores |
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(2)
North Atlantic sediments |
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(3)
European soil properties |
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(4)
CH4 concentration |
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(5)
amounts of sea slat, dust….. |
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It is still not fully understood what caused the
millennial-scale climate changes. |
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Any successful hypotheses have to explain |
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(1)
What initiated these oscillations? |
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(2)
How are they transmitted to various part of the climate system? |
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(3)
Why are they stronger during glaciations than during interglaciations? |
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Three possible hypotheses have been suggested: |
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(1)
The natural oscillations inherent in the internal behavior of northern
hemisphere ice sheets |
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(2)
The result of internal interactions among subcomponents of the climate
system |
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(3)
A response to solar variations external to the climate system. |
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This hypothesis argues that northern hemisphere
ice sheets are the source of millennial climate oscillations as a result of
their own natural interannual variations. |
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This hypothesis emphasizes the marine margins of
ice sheets, where ices are thin, to provide the rapid interactions with
their surroundings. |
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This hypothesis argues that millennial
oscillations were produced by the internal interactions among various
components of the climate system. |
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One most likely internal interaction is the one
associated with the deep-water formation in the North Atlantic. |
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Millennial oscillations can be produced from
changes in northward flow of warm, salty surface water along the conveyor
belt. |
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Stronger conveyor flow releases heat that melts
ice and lowers the salinity of the North Atlantic, eventually slowing or
stopping the formation of deep water. |
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Weaker flow then causes salinity to rise,
completing the cycle. |
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This hypothesis argues that the strength of Sun
may have changed on the millennial scales. |
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