1. Resources http://www.giss.nasa.gov/research/briefs/schmidt_01/

2. How do we know how warm it was millions of years ago?
1. Ice cores: bubbles contain samples of the atmosphere that existed when the ice formed. (oxygen isotopes and pCO2)
2. Marine Sediments : oxygen isotopes in carbonate sediments from the deep ocean preserve a record of temperature.
The records indicate that glaciations advanced and retreated and that they did so frequently and in regular cycles.

3. ATOM:

4. More Neutrons=More MASS
Isotopes
Atoms of the same element can have different numbers of neutrons; the different possible versions of each element are called isotopes. For example, the most common isotope of hydrogen has no neutrons at all; there's also a hydrogen isotope called deuterium, with one neutron, and another, tritium, with two neutrons.
                                                                                                                  
Atoms of the same element with different numbers of neutrons are called isotopes.
More Neutrons=More MASS
HYDROGEN ISOTOPES
Hydrogen Deuterium

5. Oxygen isotopes and paleoclimate
Oxygen has three stable isotopes: 16O, 17O, and 18O. (We only care about 16O and 18O.)
18O is heavier than 16O.
The amount of 18O compared to 16O is expressed using delta notation:
Fractionation: Natural processes tend to preferentially take up the lighter isotope, and preferentially leave behind the heavier isotope.
d18O ‰ = 18O/16O of sample -18O/16O of standard
18O/16O of standard
 1000

6. Oxygen isotopes and paleoclimate
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.
18O is heavier than 16O
H218O is heavier
than H216O

7. What isotope of oxygen evaporates more readily? O18 or O16? Why?

8. Oxygen isotopes and paleoclimate
Precipitation favors
H218O
…so cloud water becomes progressively more depleted in H218O as it moves poleward…
… and snow and ice are depleted in H218O relative to H216O.
Evaporation favors
H216O
H218O
H218O
Ice
Land
H216O, H218O
Ocean
Carbonate sediments in equilibrium
with ocean water record a d18O signal which reflects the d18O of seawater and the reaction of marine CaCO3 producers to temperature.
CaCO3

10. Precipitation dO18
At the poles; what will the precipitation be? High in O18 or low in O18?

11. What isotope of oxygen will ocean water be enriched in if precipitation is stored in the ice sheets (during cold periods)? O18 or O16? Why?

12. If the temperatures are cooler, will more or less dO18 be evaporated
If the temperatures are cooler, will more or less dO18 be evaporated? Why?

13. What isotope of oxygen will precipitation be enriched in during cool periods then? O18 or O16? Why?

14. What will the ice be enriched in during cold periods? Why?

15. ICE BANK During a glacial period, where will the O16 be stored?
Then what will the ocean’s be enriched in?

16. HOW DO WE FIND Isotope ratios? Drilling Ocean Sediments ODP

17. Oceanic Sediments: Forams CaCO3

18. Oxygen isotopes and paleoclimate
As climate cools, marine carbonates record an increase in d18O.
Warming yeilds a decrease in d18O of marine carbonates.
JOIDES Resolution
Scientists examining core from the ocean floor.

19. Long-term MARINE oxygen isotope record
Ice cap begins to
form on Antarctica
around 35 Ma
This may be related
to the opening of
the Drake passage
between Antarctica
and S. America
From K. K. Turekian, Global Environmental
Change, 1996

20. Drake
passage
Once the Drake passage had formed, the
circum-Antarctic current prevented warm ocean
currents from reaching Antarctica

21. Marine O isotopes during the last 3 m.y.
Kump et al., The Earth System, Fig. 14-4
Climatic cooling accelerated during the last 3 m.y.
Note that the cyclicity changes around Ma
− 41,000 yrs prior to this time
− 100,000 yrs after this time

22. Do climate temperatures change?

23. MARINE O isotopes—the last 900 k.y.
Dominant period is ~100,000 yrs during this time
Note the “sawtooth” pattern..
after Bassinot et al. 1994

24. Explain the relationship between MARINE dO18 and temperature.

25. Global temperature- instrumental record (thermometers)
Global temperature- instrumental record (thermometers). Why are dO18 proxies are important?

26. Glaciers as records of climate
Ice cores:
Detailed records of temperature, precipitation, volcanic eruptions
Go back hundred of thousands years (400,000 YEARS)

27. Methods of Dating Ice Cores
Counting of Annual Layers
Temperature Dependent
Marker: ratio of 18O to 16O
find number of years that the ice-core accumulated over
Very time consuming; some errors
Using volcanic eruptions as Markers
Marker: volcanic ash and chemicals washed out of the atmosphere by precipitation
use recorded volcanic eruptions to calibrate age of the ice-core
must know date of the eruption

30. Delta O18 and temperature
Temperature affects 18O/16O ratio:
colder temperatures  more negative values for the delta 18O
warmer temperatures  delta 18O values that are less negative (closer to the standard ratio of ocean water)

31. ICE Delta 18O and temperature Explain the relashionship.

32. Temperature reconstructed from Greenland Ice core
Temperature reconstructed from Greenland Ice core. When did the last ice age end?

33. Ice Age Cycles: *This was the dominant period prior to
100,000 years between ice ages
Smaller cycles also recorded every
41,000 years*,
19, ,000 years
*This was the dominant period prior to
900 Ma

34. Milutin Milankovitch, Serbian mathematician
                      
Milutin Milankovitch,
Serbian mathematician
1924--he suggested solar energy changes and seasonal contrasts varied with small variations in Earth’s orbit
He proposed these energy and seasonal changes led to climate variations
NOAA

35. Before studying Milankovitch cycles, we need
to become familiar with the basic characteristics
of planetary orbits
Much of this was worked out in the 17th century
by Johannes Kepler (who observed the planets
using telescopes) and Isaac Newton (who
invented calculas)

36. Kepler’s Laws First law:
Planets travel around the sun in elliptical orbits
with the Sun at one focus r’ r
r’ + r = 2a
a = semi-major axis
(= 1 AU for Earth) a
Major axis
Minor axis

37. Ellipse:
Combined distances to two fixed points (foci) is fixed r’ r
r’ + r = 2a a
The Sun is at one focus

38. Aphelion
Point in orbit furthest from the sun
Earth (not to scale!) ra
ra = aphelion distance

39. Aphelion
Point in orbit furthest from the sun
Perihelion
Point in orbit closest to the sun
Earth rp
rp = perihelion distance

40. Eccentricity
e = b/a so, b = ae
a = 1/2 major axis (semi-major axis)
b = 1/2 distance between foci b a

41. Kepler’s Second Law Corollary: Planets move fastest when
2nd law: A line joining the Earth to the Sun sweeps
out equal areas in equal times
Kump et al., The Earth System, Box Fig. 14-1
Corollary: Planets
move fastest when
they are closest to
the Sun

42. Kepler’s Third Law
3rd law: The square of a planet’s period, P, is proportional to the cube of its semi-major axis, a
Period—the time it takes for the planet to go around the Sun (i.e., the planet’s year)
If P is in Earth years and a is in A.U., then
P2 = a3

43. Other characteristics of Earth’s orbit vary as well.
The three factors that affect climate are 

44. Eccentricity (orbit shape) 100,000 yrs &400,000 yrs Obliquity
(tilt to 24.5o)
41,000 yrs
Precession
(wobble)
19,000 yrs
& 23,000 yrs

45. Q: What makes eccentricity vary
Q: What makes eccentricity vary? A: The gravitational pull of the other planets 
The pull of another
planet is strongest
when the planets
are close together
The net result of
all the mutual inter-
actions between
planets is to vary the
eccentricities of their
orbits

46. Eccentricity Variations
Current value: 0.017
Range:
Period(s): ~100,000 yrs
~400,000 yrs

47. 65o N solar insolation Unfiltered Orbital Element Variations Today
800 kA
Today
Unfiltered
Orbital
Element
Variations
0.06
65o N
solar
insolation
Imbrie et al., Milankovitch and Climate, Part 1, 1984

48. Q: What makes the obliquity and precession vary
Q: What makes the obliquity and precession vary? A: First, consider a better known example…
Example: a top
Gravity exerts a torque
--i.e., a force that acts
perpendicular to the spin
axis of the top
This causes the top to
precess and nutate g

49. Q: What makes the obliquity and precession vary
Q: What makes the obliquity and precession vary? A: i) The pull of the Sun and the Moon on Earth’s equatorial bulge N g g
Equator
The Moon’s torque on
the Earth is about twice
as strong as the Sun’s S

50. Q: What makes the obliquity and precession vary
Q: What makes the obliquity and precession vary? A: ii) Also, the tilting of Earth’s orbital plane N  N S 
Tilting of the orbital plane is like
a dinner plate rolling on a table
If the Earth was perfectly spherical,
its spin axis would always point in
the same direction but it would make
a different angle with its orbital plane
as the plane moved around S

51. Obliquity Variations Current value: 23.5o Range: 22o-24.5o
Period: 41,000 yrs

52. Precession Variations
Range: 0-360o
Current value: Perihelion occurs on Jan. 3
 North pole is pointed almost directly away from the Sun at perihelion
Periods*: ~19,000 yrs
~23,000 yrs
Today  N S
*Actual precession period
is 26,000 yrs, but the orienta-
tion of Earth’s orbit is varying,
too (precession of perihelion)

53. Which star is the North
Star today?
11,000 yrs ago
Today  N S

54.  Polaris N S Which star was the North Star at
the opposite side of the cycle?
Polaris
11,000 yrs ago
Today  N S

55.  *Actually, Vega would have been the North Star more Polaris Vega N S
11,000 yrs ago*
Today  N S
*Actually, Vega would have been the North Star more
like 13,000 years ago

56. 65o N solar insolation Unfiltered Orbital Element Variations Today
800 kA
Today
Unfiltered
Orbital
Element
Variations
0.06
65o N
solar
insolation
Imbrie et al., Milankovitch and Climate, Part 1, 1984

57. Ref: Imbrie et al., 1984
Eccentricity
Obliquity
Precession
Filtered
Orbital
Element
Variations
Today
800 kA

58. Optimal Conditions for Glaciation:
Low obliquity (low seasonal contrast)
High eccentricity and NH summers during aphelion (cold summers in the north)
Milankovitch’s key insight:
Ice and snow are not completely melted during very cold summers.
(Most land is in the Northern Hemisphere.)

59. Optimal Conditions for Deglaciation:
High obliquity (high seasonal contrast)
High eccentricity and NH summers during perihelion (hot summers in the north)
Today
11,000 yrs ago  N S N  S
Optimal for glaciation
Optimal for deglaciation

60. NH Insolation vs. Time

61. O isotopes—the last 900,000 yrs
Peak NH
summertime
insolation
after Bassinot et al. 1994

62. Big Mystery of the ice ages:
Why is the eccentricity cycle so prominent?
The change in annual average solar insolation is small (~0.5%), but this cycle records by far the largest climate change
Two possible explanations:
1) The eccentricity cycle modulates the effects of precession (no change in insolation when e = 0)
2) Some process or processes amplify the temperature change. This could take place by a positive feedback loop