Transcript Image: W Berner/University of Bern - Cal State LA

Climate of Past—Clues to Future
► Climate
has changed in past
 Humans not present
 So, why worry about present –
►Earth
goes through cycles
►Always recovers
►Earth warming faster than previously
►More CO2 in atmosphere than in previous 600 k yrs
Examples of Past Climate Change
► Glacial
periods
 See major cycles
 Smaller cycles on
large cycles
► How
do we
know?
 Rocks
 Fossils
Other methods for determining
climate of the past
► Ice
Cores
► Tree Rings
► Isotopes
Researchers choose between weatherports (the
semi-cylindrical structures consisting of tubular
metal frames overlaid with insulatory tent
materials) or tents. Weatherports equipped with
plywood floors and oil heaters, shared with up
to 15 other scientists
Gathering
Ice Cores
Vostok ice core drilling site, Antarctica
Todd Sowers, LDEO – Columbia University
Ice core drilled at Russian Vostok station in East Antarctica reaches depth of more
than two miles and provides information about climate cycles over hundreds of
thousands of years. (Schematic courtesy of John Priscu research group, Montana
State University)
http://www.globalchange.umich.edu
•Ice forms in contact with
atmosphere
•Voids in ice
•Voids contain air from atmosphere
•With burial, voids closed, gas
trapped
Raynaud, 1992
Ice Cores
► Show
annual layerss
 Winter – darker
 Summer Lighter
► Must
be able to date
 Count layers to get
years
► Core
must be complete
 Isotopes
 Ash layers
Determining Climate From Cores
► Look
at thickness of layers
 Thick lighter layers = longer summers
► Examine
CO2 in cores
 CO2 content changes with climate
 Impacts greenhouse effect
► More
dark layers during cold periods
 More wind blowing, more dust
► We
will concentrate on CO2
NOAA
►
Thin cut of a polar ice
core sample as seen
through two polarising
filters. The dark areas are
gas bubbles enclosed in
the ice (Image: W
Berner/University of
Bern)
CO & Glacial Periods
2
►
More CO2 during warm
periods
 Less during ice ages
 Reflect greenhouse effect
►
Cycles are about 50-150 k
years
 Shorter cycles in
between
►
►
►
Oceans warmer hold less
CO2
Oceans warmer release
methane
More plants and decay
Vostok Ice Core Record
►
►
►
►
From 120,000 to about 20,000 years ago, there was a
long period of cooling temperatures, but with some
ups and downs of a degree or two. This was the last
Great Ice Age. From about 18,000 or 19,000 years
ago to about 15,000 years ago, the climate went
through another warming period to the next
interglacial, - the current one.
Fig 1 and Greenland ice cores indicate climate
oscillations lasting 7,000 - 15,000 years during the
Last Great Ice Age (110-16 kBP). These are known as
�Heinrich events� and are also evidenced by ocean
sediments. A more detailed ice core analysis shows an
occasional abrupt change of climate during the last
interglacial (the Eemian, at 120 kBP), changing by as
much as 10K during only 10 -30 years. Such changes
may be due to switchings of flows in the northern
Atlantic. Similar changes have been observed at the
end of the last glaciation.
Fig 1 also shows that carbon dioxide and methane
(main greenhouse gases) occur in higher
concentrations during warm periods; the two
variables, temperature and greenhouse gas
concentration, are clearly consistent, yet it is not clear
what drives what. The correlation coefficient is 0.81
between CO2 content and apparent temperature, on
the whole. During deglaciation the two varied
simultaneously, but during times of cooling the CO2
changed after the temperature change, by up to 1000
years. This order of events is not what one would
expect from the enhanced greenhouse effect.
Finally, Fig 1 shows that high concentrations of dust
occur at the same times as the colder periods. The
most likely reason is that the ice sheets were more
extensive during colder periods, and therefore the sea
level lower, thus there would have been more
exposed, bare land.
http://www-das.uwyo.edu
Dust concentration, mean temperature (as estimated from the
oxygen isotope ratio), CO2 and CH4 concentrations plotted
against time, estimated from the analysis of an ice core drilled
at the Russian station, Vostok, on the Antarctica plateau
Past climates
► http://www.globalchange.umich.edu/globalc
hange1/current/lectures/kling/paleoclimate/i
ndex.html
Dendrochronology-Tree Rings
http://www.sonic.net/bristlecone/dendro.html
http://web.utk.edu/~grissino/principles.htm
►
Dating of past climate change
through tree rings
 Ist used early 20th century
►
Wide rings = wet periods
 Narrow rings = dry periods
►
New wood grows between old
wood and bark.
 In spring, moisture plentiful,
tree producing new growth
cells.
►
►
first new cells are large,
as summer progresses their
size decreases
in fall, growth stops and cells
die,
 no new growth appears until
next spring.

Extending Dates
►
►
►
►
►
Trees unknown aged matched with tree sequences of known age
Can extend ageing to the past
Oldest known living thing are Bristlecone Trees
By overlapping samples extend dating to 9000 yrs
Problem: Ring width depends on environmental factors
 If environmental factors constant, no ring variability
 The more variable the environment the more the tree rings vary
Problem with Ice Cores and Tree
Rings
► Cannot
see to far into past
 Ice cores 650,000 yrs
 Tree rings 9000 yrs
► Earth
is 4.6 billion years
 Missing a lot of history of climate
► Must
use another techniques
What is an isotope
► Element
with same number of protons but with
different number of neutrons
► Examples: O18, O16, C12, C13, C14
http://education.jlab.org/faq/index.html
Oxygen Isotopes
Copyright Cushman Foundation
► O2
isotopes: 16O (99.789%), 17O (0.037%),
(0.204%)
► 18O\16O ratio built into shells, minerals.
Foraminifers especially useful
► Important for Quaternary & Late sediments
► First developed in 1955 (Emiliani)
180
Foraminifera in Sediments
►
►
►
►
►
Drill in ocean to retrieve
cores of sediments
Pick the foraminifera out
Analyze oxygen isotopes
Foraminifera must be well
preserved
Core must be dated
 Forams can be used for
dating
 Other microfossils
 Volcanic ash
Compare oxygen isotope ratio to a
standard
► 18O/16O dependent on temp & ratio in water
► Deviation
of ratio compared to standard
 More common standard SNOW = Standard Mean Ocean
Water
Evaporation, lighter 16O removed,
heavier 18O remains in water
► Precipitation, heavier 18O returned
to oceans near coast
► Vapor depleted in 18O
► Also temp dependent, polar regions
more 16O locked in ice
►
 Oceans depleted in
18O
16O,
enriched in
Warmer period, more 16O in water
Therefore, changes in 18O reflect
changes in ice volume, sea level
► Decreased temperature also causes
more 18O in shells.
►
►
► Oxygen,
carbon,
sulfur, strontium vary
through time
► Ocean mixing time
about 1000 yrs,
variations considered
instantaneous
► Timing of variations
must be established