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?
What we wish to learn Today:
Past Climate Change and the Ice Ages
“Recent” climate change and variability…
N.H
. Tem
pera
ture
(°
C)
1000 14001200 1600 1800 2000
0
1
-1
2
YearMann et al. (1999) GRL 26:759-762
…provides perspective on where we are
headed
IPCC Projectionsto 2100
1000 1200 1400 1600 1800 2000
0
0.5
1
-0.5
2
4
3
5
6
1
0
N.H
. Tem
pera
ture
(°
C)
Global Temperatur
e (°C)
200019871856
The great Aletsch glacier, SwitzerlandThe great Aletsch glacier, Switzerland
We KNOW the climate has changed recently
L.Thompson, in prep.
Ice on Mt. Kilimanjaro
Are
a (
km
2)
0
5
10
15
1900 1920 1940 1960 1980 2000 2020Year
1970 2000
Glacial Changes since last Ice Age
Percent of Spruce in Total Trees
Ice and 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.
• Climate was warm during the Age of the Dinosaurs (the Mesozoic)– Alligators lived in Siberia!– Dinosaurs lived north of the
Arctic Circle in Alaska!
Ancient Climates
Mes
ozoi
cP
reC
ambr
ian
Pal
eozo
icC
enoz
oic
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 sediments)
• Coral Bed Rings: (like dendochronology, but with corals)
• Fossils: Studies of geological settings, etc.
• Historical documents: paintings of glaciers, etc.
• Oxygen has three stable isotopes: 16O, 17O, and 18O. (We only care about 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):
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.
Oxygen isotopes and paleoclimate
18O ‰ = 18O/16O of sample - 18O/16O of standard18O/16O of standard
1000
Isotope “fractionation”• Oxygen isotopes are fractionated during
evaporation and precipitation of H2O– H2
16O evaporates more readily than H218O
– H218O precipitates more readily than H2
16O
• Oxygen isotopes are also fractionated by marine organisms that secrete CaCO3 shells. The organisms preferentially take up more 16O as temperature increases.
OceanH216O, H2
18O
Evaporation favorsH2
16O H218O
Precipitation favorsH2
18O
H218O
(1) Sea water is heavier than water vapor
Land
Ice
(3) Snow and ice are depleted in H2
18O relative to sea water.
(2) cloud water becomes more depleted in H2
18O as it moves inland or poleward…
18O16O
16O18O
18O
18O
18O
18O
18O
16O
16O16O
18O
18O18O18O
16O
16O16O18O16O
Fractionation effects
Oceanδ18O = 0 o/ooLand
Ice
Carbonate sediments also record the signal of the ocean, and the signal of temperature
CaCO3
δ18O =-10 o/oo
Fractionation effects
δ18O =-15 o/oo
-6 o/oo
δ18O = -20 o/oo
-11 o/oo
rain
rain
Vostok RecordWe can also show that the
18O of precipitation is well correlated
with temperature!So, if we know the
18O of water or ice, we know what the air temperature was at
that time.
(Note that hydrogen isotopes work the same
way)
The Antarctic Ice Coring operation at Vostok
station
The Greenland Ice Coring operation at Summit
station
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
Ice Core Analyses
Ocean Sediment analysis Isotopes of organisms
The “Ice Volume” effect
• Light isotope removed from ocean, locked into large ice sheets
• Remaining ocean water was +1.5‰ heavier in 18O, as recorded in marine organism shells (CaCO3)
• Ocean level was ~120 m lower than today
Growing glaciersdeep-seaforaminifera
δ18O = - 35
δ18O = - 30
Ice
Ice
δ18O = 1.5
δ18O = 0.0
Glacial
Interglacial
Possible Causes of Climate ChangePossible Causes of Climate Change
Power: 4 x 1026 W 2 x 1017 W
Long-Term 1. Solar Luminosity2. Shifting Continents3. Greenhouse gases
Medium-Term 1. Orbital parameters2. Greenhouse gases
Short-Term 1. Greenhouse gases2. Sunspots3. Ocean currents
Evolution of our Sun‘s Evolution of our Sun‘s LuminosityLuminosity
.
16
.
0.6
1
1.2
1.4
0.8
0 2 4 6 8 10
Time (billions of years)
Lum
inosi
ty
SnowballEarth ?
Today
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
Ca2H2CO3
H2CO3 Ca2
① CO2 + H2O H2CO3 (carbonic acid)
② CaSiO3 + 2H2CO3 Ca2+ + 2HCO3- + SiO2 + H2O (silicate weathering)
③ Ca2+ + 2HCO3- CaCO3 + H2CO3 (carbonate precipitation & burial)
Ca3SiO3
Ca3SiO3
From C. Poulsen’s lecture, 24 Sep
CO2
Net: CaSiO3 + CO2 CaCO3 + SiO2 Conversion of CO2 gas to limestone!
CO2
Orbital forcing (Milankovitch)Orbital forcing (Milankovitch)
J.Beer-1996
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
~ 2323kyky
Tilt 41ky
0
-10
-20
Milankovitch Forcing Explains Ice Core DataMilankovitch Forcing Explains Ice Core Data
1000s of years Before Present ( kyr B.P. )
1818O in Chinese caves and insolationO in Chinese caves and insolation
D. Yuan et al., Science 304, 575
1000s of years Before Present ( kyr B.P. )
GISP2 Ice Core
Orbital forcing
What causes rapid and What causes rapid and unpredictable changes in unpredictable changes in
climate?climate?
-5
-20
-10
-15
-5
-10
orderorder
chaoschaos orderorder
Greenland
chaoschaos
Antarctica
1000s of years Before Present ( kyr B.P. )
Causes of Climate change
A. Tectonic C. ?? B. Orbital D. ??
Summary1. 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…”