Transcript Paleoclimate Lecture - University of Michigan

Past Climate Change and
the Ice Ages
What we wish to learn Today:
1. What do we mean by "paleoclimate"?
2. What evidence exists for ice ages and
ancient climate change?
3. What causes the climate to change?
“Recent” climate change and variability…
N.H. Temperature
(°C)
2
1
0
-1
1000 1200
Mann et al. (1999) GRL 26:759-762
1400 1600 1800
Year
2000
…provides
perspective on where
we are headed
6
Global
Temperature
(°C)
4
5
3
N.H. Temperature
(°C)
2
1
1
0.5
0
-0.5
1000 1200 1400 1600 1800 2000
0
IPCC Projections
to 2100
We KNOW the climate has
changed recently
1856
1987
2000
The great Aletsch glacier, Switzerland
1970
2000
Area (km2)
Ice on Mt.
Kilimanjaro
15
10
5
0
1900 1920 1940 1960 1980 2000 2020
Year
L.Thompson, in prep.
Glacial Changes since last Ice Age
Ice and Trees
Percent of Spruce in Total Trees
Glacial Europe was treeless in the last ice age
Vegetation in present day Europe is
dominated by forest, with conifers in
the north and deciduous trees in the
south.
At the glacial maximum, 20 thousand
years ago, arctic tundra covered much
of Europe south of the ice sheet, and
only patches of forests remained near
the southern coasts.
PreCambrian
Paleozoic
Mesozoic Cenozoic
Ancient Climates
• Climate was warm during the Age
of the Dinosaurs (the Mesozoic)
– Alligators lived in Siberia!
– Dinosaurs lived north of the
Arctic Circle in Alaska!
Methods to establish past climate
•
Isotopic Geochemical Studies: the study of rock
isotopic ratios, ice core bubbles, etc.
•
Dendochronology: the study of tree rings
•
Pollen Distribution: the study of plant types and
prevalence (e.g., Europe’s vegetation in the last ice age)
•
Lake Varves: (like dendochronology, but with lake
•
Coral Bed Rings: (like dendochronology, but with corals)
•
Fossils: Studies of geological settings, etc.
•
Historical documents: paintings of glaciers, etc.
sediments)
Oxygen isotopes and paleoclimate
• Oxygen has three stable isotopes: 16O, 17O, and
(We only care about
•
18O
16O
and
18O)
18O.
is heavier than 16O (it has 2 extra neutrons).
• The amount of 18O compared to 16O is expressed using
“delta” notation - the unit is “per mil” (parts per thousand):
d18O ‰ =
18O/16O
of sample - 18O/16O of standard  1000
18O/16O
of standard
Fractionation:
Natural processes tend to preferentially take up the lighter
isotope, and preferentially leave behind the heavier isotope.
For most chemistry, the isotopes behave the same.
Isotope “fractionation”
• Oxygen isotopes are fractionated during evaporation
and precipitation of H2O
– H216O evaporates more readily than H218O
– H218O precipitates more readily than H216O
• Oxygen isotopes are also fractionated by marine
organisms that secrete CaCO3 shells. The organisms
preferentially take up more 16O as temperature
increases.
Fractionation effects
(1) Sea water is heavier
than water vapor
Precipitation favors
H218O
(2) cloud water
becomes more depleted
in H218O as it moves
inland or poleward…
16O 18O16O
18O
Evaporation favors
H216O
18O 16O
H218O
18O
H218O
16O
16O
16
16O 18O O
H216O,
18O
18O
H218O
18O
16O
18O
18O
18O 16
O
18O
Ocean
Ice
Land
(3) Snow and ice are
depleted in H218O
relative to sea water.
Fractionation effects
δ18O =
-15 o/oo
δ18O =
-10 o/oo
-11
= 0 o/oo
CaCO3
rain
o/oo
rain
-6
δ18O
δ18O =
-20 o/oo
Ocean
o/oo
Ice
Land
Carbonate sediments also record the signal
of the ocean, and the signal of temperature
We can also show
that the d18O of
precipitation is
well correlated
with temperature!
So, if we know the d18O
of water or ice, we
know what the air
temperature was at
that time.
(Note that hydrogen
isotopes work the same way)
Vostok Record
The Antarctic Ice Coring
operation at Vostok station
The Greenland Ice Coring
operation at Summit station
Ice Core Analyses
The ice can be analyzed
for its 18O content to
estimate temperature
The air bubbles trapped
in the ice can be analyzed
for their carbon dioxide
and methane content
Ocean Sediment analysis
Isotopes of organisms
deep-sea
foraminifera
Growing glaciers
Interglacial
Ice
δ18O = - 30
The “Ice Volume” effect
δ18O = 0.0
• Light isotope removed
from ocean, locked into
large ice sheets
Glacial
Ice
δ18O
δ18O = 1.5
= - 35
• Remaining ocean water was
+1.5‰ heavier in 18O, as
recorded in marine organism
shells (CaCO3)
• Ocean level was ~120 m
lower than today
Possible Causes of Climate Change
Long-Term
1. Solar Luminosity
2. Shifting Continents
3. Greenhouse gases
Medium-Term
1. Orbital parameters
2. Greenhouse gases
Short-Term
1. Greenhouse gases
2. Sunspots
3. Ocean currents
Power: 4 x 1026 W
2 x 1017 W
Evolution of our Sun‘s Luminosity
1.6
Today
1.4
Luminosity
.
1.2
1
0.8
0.6
Snowball
Earth ?
0
2
4
6
8
Time (billions of years)
10
Shifting land masses
(by plate tectonics)
may have changed
greenhouse gas
concentrations, thus
affecting climate
Today’s configuration
Past configurations
As the continents shift there is increased subduction and
volcanic activity which increases CO2 into the atmosphere
That atmospheric CO2 is then consumed in weathering
reactions on continents, and eventually returned to the ocean.
This is the long-term “weathering” control of climate.
Silicate weathering
From C. Poulsen’s
lecture, 24 Sep
①
CO2 + H2O  H2CO3 (carbonic acid)
②
CaSiO3 + 2H2CO3  Ca2+ + 2HCO3- + SiO2 + H2O (silicate weathering)
③
Ca2+ + 2HCO3-  CaCO3 + H2CO3 (carbonate precipitation & burial)
Net: CaSiO3 + CO2  CaCO3 + SiO2 Conversion of CO2 gas to limestone!
CO2
CO2
H2CO3 Ca2
Ca3SiO3
H2CO3
Ca2
Ca3SiO3
Orbital forcing (Milankovitch)
1879-1958
1. Shape (eccentricity, ~100K and 400K yrs)
2. Tilt (obliquity, ~41,000 yrs = 41K yrs)
3. Wobble (precession, ~23K yrs)
Interaction of orbital
periods give different
patterns of change.
The magnitude of
shifts in solar
insolation are large
enough to explain
changes in climate
Milankovitch Forcing Explains Ice Core Data
~ 23ky
0
-10
Tilt 41ky
-20
1000s of years Before Present ( kyr B.P. )
d18O in Chinese caves and insolation
Orbital forcing
GISP2 Ice Core
1000s of years Before Present ( kyr B.P. )
D. Yuan et al., Science 304, 575
What causes rapid and
unpredictable changes in climate?
Antarctica
-5
order
chaos
-10
chaos
order
-5
-10
-15
-20
1000s of years Before Present ( kyr B.P. )
Greenland
Causes of Climate change
A. Tectonic
B. Orbital
C.
??
D.
??
Summary
1. Past changes in climate have been dramatic on
Earth
2. The longest-term changes (100s Million years,
Ma) are driven by shifting continents and
interactions with greenhouse gases.
3. At medium time scales (1-10s Ma), changes are
triggered by variations in orbital
characteristics.
Take-home point:
“If you don’t like the climate,
hang around awhile…”