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?

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• Historical documents: paintings of glaciers, etc.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…”