Download Past Climates

Past climates
NGEN03 HT-2014
This module has emphases on:
1. The Quaternary Period and Quaternary geology (incl. the
Holocene epoch).
2. Reconstructions of glacial/interglacial cycles and changes in
the size of major carbon reservoirs.
3. Feedbacks in the climate system and non-linear responses.
Mats Rundgren, Nov. 2014, based on Ian Snowball, Nov. 2012
Cover of Science magazine, April 27th 2001.
In 1565, Pieter Bruegel the Elder painted this
frigid northern European landscape in his work
Hunters in the Snow.
Much of the discussion about contemporary
climate change concerns whether or not the
observed warming of the 20th century was part
of a ”natural recovery” from the Little Ice Age.
How many ”big” and ”little ice ages” have there
been and how often do they occur? How do we
know about them?
Is contemporary climate change different to
past (natural) climate change?
Snowball Earth: an extreme example of
positive feedback in the climate system
Earth’s climate system
and interaction of components
© M. Calner, Lund University
The natural thermostat…
…is driven by the Earth’s endogenic processes and the carbonate-silicate cycle
…two main processes control the concentration of CO2 in the atmopshere
Uplift weathering
hypothesis
Chemical weathering
of carbonate and silicate rocks
Cooling effect
BLAG hypothesis
Volcanism
Warming effect Atmospheric reservoir = CO
2
-2
Ocean reservoir = -CO3
CO2
Ocean
Land reservoir = living
organisms
Earth’s crust = calcium carbonate,
fossil fuels
© M. Calner, Lund University
The natural thermostat…
BLAG-hypothesis (1983): Plate tectonic processes drive the climate
system over timescales of hundreds of million years
…CO2 is released from bedrock
and put into the atmosphere through
two tectonic processes
2. Along subduction zones
1. Along midocean ridges
Ocean floor spreading:
The rate of spreading controls the rate of
release of CO2 into the atmosphere
© M. Calner, Lund University
The natural thermostat…
Hydrolysis:
80 % of the 0.15 Gt C which is
deposited in the ocean each year
Calcium carbonate rich
sediments in the Pacific
Ocean, near Honolulu
(IODP)
Si4+
HCO3-
Ca2+
© M. Calner, Lund University
The natural thermostat…
Long term reduction in atmospheric CO2 concentration over the last 500
million years is due to the net production of carbonate rocks
Orbital parameters and continental configuration:
the last 70 Ma
Long term climate development
involves changes in:
1. Tectonics and the positions of the
continents
2. Ocean circulation
3. Orbital parameters
4. Evolution in the biosphere
Published by AAAS
J. Zachos et al., Science 292, 686-693 (2001)
Global temperature, major climatic, tectonic
and biotic events: the last 70 Ma
Temperature of the deep ocean
Published by AAAS
J. Zachos et al., Science 292, 686 -693 (2001)
The Quaternary world
What is the “Quaternary?”
• The Quaternary:
– is the most recent major subdivision (period) of
the geological record. It includes the present
day and, most likely, the near future.
– is synonymous with the label “Ice-age.”
– has lasted c. 2.6 million years, so far.
– Note that tectonic processes are of minor
importance
What caused the start of
the Quaternary?
Closing of the Central
American isthmus was
complete 4 Myr ago.
Although the northwards
flow of warm, salty water
warmed the northern
regions, it also transported
more moisture, which was
needed to produce the
continental ice sheets.
Ice-sheets have a high
albedo, and promote a cool
regional climate.
A positive feedback.
Quaternary ice-sheets in the
Northern Hemisphere
Position at the last glacial maximum (LGM) at c. 20 Kyr
Tundra in central Europe and a very different mammalian fauna
North-south migration of vegetation during
the Quaternary
How do we know?
Quaternary Geology
Why?
1. Meteorite?
2. Flood?
3. Humans?
4. Ice!
Different geological archives of the
Quaternary world
• Q. What is a geological archive?
– In the best case, an easily dated continuous,
“natural” record of past environmental changes,
where events can be placed in stratigraphic
(temporal) order.
– A geological archive contains so-called “proxy
indicators”, where inferences must be made on the
premise of “the present is the key to the past”.
Common examples of “proxies”
Direct, “First Order” proxies of an environmental, or climatic,
parameter:
1. Concentration of atmospheric gas (e.g. CO2, CH4) found in a bubble
of air enclosed in old ice.
Indirect, “Second Order” proxies of an environmental or climatic
parameter:
1. Ratio of stable isotopes, e.g. 18O/16O found in ancient ice stored in
ice-sheets. A measure of isotopic fractionation, which is controlled
by temperature at the time of evaporation and condensation, plus the
ratio of the moisture source.
2. Abundance of pollen grains in lake sediments and ratios between
different species. A proxy for the composition of past vegetation
communities, controlled by (for example) temperature, precipitation
and more recently, human impact.
3. The abundance of ice-rafted-debris (IRD) in ocean sediments. A
proxy for the abundance of icebergs.
Quaternary records of global change:
1. Deep-sea sediments
Ocean sediment layers extend
further back in time than most
lake sediment sequences and ice
cores.
Studies of the oxygen isotope
composition of fossil foraminifera
deposited on the ocean floor
produced a new picture of how
the Earth’s climate varied during
the Quaternary period.
Details later
Quaternary records of global change:
2. Ice-cores (e.g. Greenland & Antarctica)
Photos: NEEM ice core drilling project, www.neem.ku.dk
.
Ice core stratigraphy
The ice cores have provided:
1. Records of atmospheric gas concentrations over glacial
cycles.
2. Records of (sub) millennial-scale climate change within the
glacial and interglacial stages.
Ice core stratigraphy
The lowermost sections of
the GISP2 ice core.
At a depth of 3053.51 m the drill
struck the basal rock, having collected
a 200,000 year long record of the
Earth's atmospheric composition.
Ice-core record
Marine record
The last glacial-interglacial cycle
Quaternary records of global change:
3. Continental lake sediments
Lake sediment record of the Holocene
Lake Windermere, UK
Today
Early Holocene
Younger Dryas
stadial
Bølling/Allerød
interstadial
Glacial
Quaternary records of global change:
4. Tree rings
Dendrochronology in northern Sweden: the last 7400 years
Fossil trees were found in lakes and peat bogs.
Through cross-correlation of 880 sampled trees
a 7400 year chronology has been constructed.
Summer temperatures for the last 7,400 years in
northern Sweden based on dendrochronology
(Grudd et al. 2002)
Quaternary stratigraphy & correlation
Stratigraphy
Biostratigraphy: =
observable variations in fossil content
Climatostratigraphy = geological-climatic units based on
inference from, for example, fossil
content
Chronostratigraphy = Classification of stratigraphic units
according to inferred, relative or absolute
age
The detail and complexity of Quaternary archives has lead to problems of
classification, interpretation and correlation that are not encountered in
investigations of the earlier geological record.
Quaternary stratigraphy & correlation
Quaternary climatostratigraphy =
Glacials =
Long (several 10,000 years) cold phases characterized by major expansions of
continental ice sheets and glaciers (e.g Weichselian, the last glaciation).
Interglacials =
Warm intervals separating glacial phases. Climate similar or warmer than
present (e.g Eemian, the last interglacial).
Stadials =
Relatively short-lived cold episodes within glacials (e.g. Younger Dryas).
Interstadials =
Relatively short-lived warm episodes within glacials. Climate not as warm as
during interglacials (e.g. Allerød).
The Younger Dryas in the
North Atlantic region as
defined by:
Morphological evidence:
Biostratigraphy:
Climatostratigraphy:
Using isotopes to reveal
global cycles and climate change
Two molecules of a particular compound that
contain different isotopes of a given element
behave differently as they flow through the Earth
system
Fractionation between isotopes can be caused by,
for example.
Photosynthesis,
Temperature,
Evaporation.
The most commonly used isotopes in studies of the
Quaternary period are those of oxygen and carbon.
A marine oxygen isotope record
How was this record constructed, and how is it interpreted?
Oxygen isotope stratigraphy
Why measure the ratio between
different oxygen isotopes?
1.
16O
and 18O are non-radioactive “stable”
isotopes of oxygen that occur naturally in
Earth’s air and water.
2.
16O
forms 99.8 % of all the oxygen present
on Earth.
3. Measuring the ratios in marine carbonates
and ice-cores provides information on
fractionation,
4. The fractionation values reflect past
temperatures AND ice volumes.
“Light” oxygen-16, with 8 protons
and 8 neutrons, is the most
common isotope found in nature,
followed by much lesser amounts
of “heavy” oxygen-18, with 8
protons and 10 neutrons.
Oxygen isotope stratigraphy
1. The average ratio of
18O
to
16O
is about 1/500 (0.002)
2. Small variations around this average are measured and
reported as departures (in ‰) from a standard:
δ18O
= 1000 x
18O/16O
sample
–
18O/16O
18O/16O
standard
standard
Why?
Present day 18O values:
Oxygen isotope stratigraphy
4. During glacials, large amounts
of 16O were locked in ice
sheets, so that the oceans
were relatively enriched in 18O.
3. Thus, polar ice sheets have
more negative, isotopically
lighter 18O values than
equatorial waters.
“lighter” values
“heavier” values
2. Successive condensation during
transport to higher latitudes
(and into continental interiors).
Preferential loss of H218O.
1. Evaporation of water at the
sea surface. The lighter H216O
molecules are drawn into the
atmosphere in preference to
the heavier H218O molecules.
The principle
Kurt Hollocher, 2002
Oxygen isotope stratigraphy
The 18O values of carbonate in marine
organisms, such as shells of planktonic and
benthic Foraminifera are dependent on two
main factors.
1. Temperature at the time of secretion, c.
0.230/00 per 1oC.
2. Isotopic composition of seawater at the time
of secretion.
C. Emeliani (1955) measured stratigraphic
changes in the isotopic composition of
planktonic Foraminifera. He estimated
glacial/interglacial changes of 6 oC.
BUT, he underestimated the effect of global
ice volume on the isotopic composition.
Oxygen isotope stratigraphy
Using more detailed and revised estimates of the isotopic composition of ice
volumes, Shackleton (1967) measured benthic Foraminifera and showed that the
18O values were providing a record of ice-volume, NOT temperature.
This work gave birth to Marine Isotope Stages – MIS, or Oxygen Isotope – OI stages
where c. 104 Quaternary stages have been identified (52 glacial/interglacial cycles).
OI stage 1 = the warm Holocene, OI stage 2 = cold Last Glacial Maximum etc.
Marine isotope stratigraphy
Glacial stage
= isotopically heavy ocean
+ 1-1.5 per mil
Interglacial stage
= isotopically light ocean
~0 per mil
Oxygen isotope stratigraphy
The SPECMAP timescale: Imbrie et al. 1984.
Orbital tuning of the oxygen isotope stages, via independently K/Ar dated palaeomagnetic
tie points lead to the construction of a globally applicable oxygen isotope stratigraphy.
I
II
III
IV
V
VI
VII
“Saw-tooth” pattern of slow
growth of ice sheets (and
lowering of sea-level)
followed by rapid decreases,
so-called “Terminations”.
What mechanism paces the
ice ages?
A marine oxygen isotope record
So, can you spot what is wrong with this figure…..?
MARGO Project Members*. Constraints on the
magnitude and patterns of ocean cooling at the
Last Glacial Maximum. Nature Geoscience 2,
127 - 132 (2009).
NH
summer
NH
winter
Annual
What theory of climate change does
geological data test?
A valid, or proven, theory must explain:
1.
2.
3.
repeated glaciations in both hemispheres
global synchronicity
rhythmic behaviour
The Astronomical Theory of
the Ice-Ages
The Astronomical Theory of the Ice-Ages
James Croll:
argued that the total amount of
insolation received at a given latitude
in a given season could vary
significantly from year to year
because of changes in the Earth's
orbit.
He also understood that “feedback”
mechanisms were necessary to amplify
the insolation changes within the
climate system.
Photograph of James Croll from J. C.
Irons (1896).
Three basic orbital parameters
What are they?
1. Eccentricity
2. Tilt (Obliquity)
3. Precession of the equinoxes
The Astronomical Theory of the Ice-Ages
1. Eccentricity - 100,000 & 413,000 years
Earth’s orbit around the Sun is
elliptical, not circular.
The “degree” of eccentricity
varies between є = 0.005 0.0607.
It is only the eccentricity of
Earth’s orbit that can change the
total amount of solar radiation
reaching the surface.
(apart from changes in the solar
”constant”)
The Astronomical Theory of the Ice-Ages
2. Tilt, or Obliquity - 41,000 years
Earth’s rotational (spin) axis is currently tilted 23.5o
away from a line perpendicular to the plane of its
orbit around the Sun.
Changes in tilt affect the degree of seasonality.
The Astronomical Theory of the Ice-Ages
3. Precession of the equinoxes – 23,000 years
E.g. Now Perihelion occurs just
after the N. Hemisphere winter
solstice (when the N. Pole points
away from the Sun)
Sometimes in the past Perihelion
occurred during the N.
Hemisphere summer solstice,
thereby increasing seasonality.
Perihelion = when the Earth is
closest to the Sun.
Aphelion = when the Earth is
furthest from the Sun.
The Astronomical Theory of the Ice-Ages
Milutin Milankovitch
This Serbian geophysicist finally was
able to include the effect of changes in
obliquity in the calculations.
He calculated summer radiation curves
for the key latitudes of 55o, 60o, and 65o
N that seemed to correlate well with
evidence then available from the
geologic record.
Milutin Milankovitch by Paja Jovanovic (1943).
The Astronomical Theory of the Ice-Ages
The combination of cycles
Back to carbon and CO2
Note the dust record
Revealing changes in the carbon cycle: the
geological perspective
1. Long-term evolution of atmospheric CO2 concentration,
2. Changes in the size of carbon reservoirs.
A long term record of
atmospheric CO2 concentration
An estimate of the changes of
CO2 concentration during the
Phanerozoic (the last 600 million
years).
Devonian and Carboniferous reduction
caused by significantly increased
weathering rates.
Forests adapted to inhabit upland
environments.
What is this long
term trend due to?
Q. Where did this carbon go?
A. It formed the fossil fuels that
humans are burning today.
The main “modern” carbon reservoirs
The largest reservoir of carbon is that composed of sediments and rocks
This carbon is not necessarily “buried forever”!
“Modern” fluxes of carbon
On the other hand, the fastest rates of carbon cycling occur
between vegetation/soil and the atmosphere. These rates vary over
geological time. Which rates have human activity accelerated?
Quaternary variations in the carbon cycle
The changing atmospheric concentration
of CO2 shows that large changes in
carbon storage took place during the
glacial cycles. ~ 90 ppm drop in
atmospheric CO2 during the glacials.
What explanations are there?
1. Colder sea surface water absorbs CO2
more effectively than warmer water.
2. But, saltier sea surface water absorbs
CO2 less effectively than less salty
water.
The net effect can only account for 11
ppm of the ~ 90 ppm difference!
Where does the carbon go?
Carbon cycle
Interglacial to glacial values of
carbon stored in the mobile
reservoirs (in gigatons).
Quaternary variations in the carbon cycle
Terrestrial data show a tremendous reduction (25 %) of C in vegetation
and soil during glacial stages. But, atmospheric CO2 concentrations went
down: this carbon did not go into the atmosphere. It must have gone
into the ocean.
1. The surface ocean is almost in equilibrium with the atmosphere on glacial
timescales.
2. The deep ocean most likely stored an extra 1010 gigatons of carbon during glacial
stages.
3. This value represents only a 2.7 % increase in the deep ocean, compared to
interglacials.
Carbon cycle
The ocean carbon pump hypothesis
(Wally Broecker).
Glacial stages are characterised by
higher ocean productivity due to:
1. Nutrients supplied from the land in
the form of dust (iron fertilization).
2. Increased winds cause the upwelling
of nutrient-rich waters,
3. Changes in deep water circulation.
One way to test the sensitivity of the
climate system to different CO2 and CH4
concentration scenarios is to let them
happen. But, there is only one Earth, so
only one scenario can be tested.
We can try to constrain the predictions
of climate models by using ’palaeodata’.
What are the problems in this approach?
End