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Natural Climate Forcing
Professor Menglin Jin
San Jose State University
Outline –
Paleoclimate – temperature and CO2
Natural forcing for temperature change
Features for Glacier and inter-glacier
Activity
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Paleoclimate
A lead to
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Earth geological time scale
Paleo : Greek root means “ancient”
Modern age, ice age, last 2 million years
Age of dinosaurs
Animal explosion of diversity
From the formation of earth to the evolution of macroscopic hard-shelled animals
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Climate record resolution
(years)
1 ,000, , , mon day
Satellite, in-situ observation
Historical data
Tree rings
Lake core, pollen
Ice core
Glacial features
Ocean sediment, isotopes
Fossils, sedimentary rocks
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1 ,000, , , mon day 4

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Climate record distribution from 1000 to 1750
AR4 6.11
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C14 and O18 proxy
C14 dating proxy
Cosmic rays produce C14
C14 has half-life of 5730 years and constitutes about one percent of the carbon in an organism.
When an organism dies, its C14 continues to decay.
The older the organism, the less C14
O18 temperature proxy
O18 is heavier, harder to evaporate. As temperature decreases (in an ice age), snow deposits contains lessO18 while ocean water and marine organisms (CaCO3) contain more O18
The O18/ O16 ratio or δO18 in ice and marine deposits constitutes a proxy thermometer that indicates ice ages and interglacials.
Low O18 in ice indicates it was deposited during cold conditions worldwide, while low O18 in marine deposits indicates warmth
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8. That there is a link between CO2 and global temperatures is irrefutable as this diagram shows, but exact details of the link and all the possible feedback mechanisms are not clear.
The interaction of positive and negative feedback effects associated with the greenhouse effect is currently a critical issue for the scientific community.

9. Natural Climate Change
External Forcing:
Internal Forcing:
The agent of change is outside of the Earth-atmosphere system
The agent of change is within the Earth-atmosphere system itself
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Faint-sun paradox
According to solar models, solar luminosity is 30% stronger nowadays than 4.5 billion years ago due to thermonuclear H to He makes the sun denser and hotter
There should be glaciations up to 2 billion years ago
Temperature should be 25K lower
However
According to record, glaciations are absent from 2 to 3 billion years ago
Possible reasons:
Higher CO2 concentration (Kuhn & Kasting 1983, Kasting 1993) or CH4 causing greenhouse gas effect
Less continent and faster rotation of earth increase temperature by 4k and 1.5K respectively (Jenckins 1993)
Stronger solar wind stopped cosmic rays reaching earth leading to heating (harrison & Aplin 2001, Eichkorn et al.2002, Shaviv 2003)
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Ice-covered earth
Ice-free earth
700 million years ago due to very low CO2 concentration
Hypothesis: plate tectonics and lack of weathering and photosynthesis left great amount of CO2 in the atmosphere (Kirshvink 1992)
Support: thick layer of carbonate and banded iron formation on top of tropic glaciations
Rapid transition from cold to warm climate would bring great changes in life on earth
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Continental drift
In 1915, German scientist Alfred Wengener first proposed continental drift theory and published book On the Origin of Continents and Oceans
Continental drift states:
In the beginning, a supercontinent called Pangaea. During Jurrasic, Pangaea breaks up into two smaller supercontinents, Laurasia and Gondwanaland,. By the end of the Cretaceous period, the continents were separating into land masses that look like our modern-day continents
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Consequences of continental drift on climate
Polarward drifting of continents provides land area for ice formation  cold climate
Antarctica separated from South America reduced oceanic heat transport  cold climate
Joint of North and South America strengthens Gulf Stream and increased oceanic heat transport  warm climate
Uplift of Tibetan Plateau  Indian monsoon
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Warm during Cretaceous
High CO2 may be responsible for the initiation of the warming
Higher water vapor concentration leads to increased latent heat transport to high latitudes
Decreased sensible heat transport to high latitudes results from decreased meridional temperature gradient
Thermal expansion of sea water increased oceanic heat transport to high latitudes
Psulsen 2004, nature
The Arctic SST was 15 oC or higher in mid and last Cretaceous. Global models can only represent this feature by restoring high level of CO2
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15. Cretaceous
being the last period of the Mesozoic era characterized by
continued dominance of reptiles,
emergent dominance of angiosperms, diversification
of mammals, and the extinction of many types of
organisms at the close of the period

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Asteroid impact initializes chain of forcing on climate
Short-term forcing: The kinetic energy of thebollide is transferred to the atmosphere sufficient to warm the global mean temperature near the surface by 30 K over the first 30 days
The ejecta that are thrown up by the impact return to Earth over several days to weeks produce radiative heating.
Long-term forcing: Over several weeks to months, a global cloud of dust obscures the Sun, cooling the Earth’s surface, effectively eliminating photosynthesis and stabilizing the atmosphere to the degree that the hydrologic cycle is cut off.
The sum of these effects together could kill most flora. The latter results in a large increase in atmospheric CO2, enabling a large warming of the climate in the period after the dust cloud has settled back to Earth
This hypothesis is proposed to 65 Million years ago for one possible reason that kills the dinosaurs
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External Forcing
Variations in solar output
Orbital variations
Meteors
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A meteor is a bright streak of light that appears briefly in the sky. Observers often call meteors shooting stars or falling stars because they look like stars falling from the sky
Meteor showers
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Solar Variations
Sunspots correlate with solar activity
More sunspots, more solar energy
Sunspots are the most familiar type of solar activity.
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21. SOLAR ACTIVITY
Sunspots are the most familiar type of solar activity.

22. THE SOLAR CYCLE Sunspot numbers increase and decrease
over an 11-year cycle
Observed for centuries.
Individual spots last from a few hours to months.
Studies show the Sun is in fact about
0.1% brighter when solar activity is high.

23. SOLAR INFLUENCES ON CLIMATE
Solar activity appears to slightly change the Sun’s brightness and affect climate on the Earth...

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THE MAUNDER MINIMUM
An absence of sunspots was well observed
from 1645 to 1715.
The so-called “Maunder minimum” coincided with a cool climatic period in Europe and North America:
“Little Ice Age”
The Maunder Minimum was not unique.
Increased medieval activity
correlated with climate change.
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Orbital forcing on climate change
Coupled orbital variation and snow-albedo feedback to explain and predict ice age
He suggested that when orbital eccentricity is high, then winters will tend to be colder when earth is farther from the sun in that season. During the periods of high orbital eccentricity, ice ages occur on 22,000 year cycles in each hemisphere, and alternate between southern and northern hemispheres, lasting approximately 10,000 years each.
James Croll, 19th century Scottish scientist
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Further development of orbital forcing by Milutin Milankovitch
Mathematically calculated the timing and influence at different latitudes of changes in orbital eccentricity, precession of the equinoxes, and obliquity of the ecliptic.
Deep Sea sediments in late 1970’s strengthen Milankovitch cycles theory.
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Orbital changes
Milankovitch theory:
Serbian astrophysicist in 1920’s who studied effects of solar radiation on the irregularity of ice ages
Variations in the Earth’s orbit
Changes in shape of the earth’s orbit around sun:
Eccentricity (100,000 years)
Wobbling of the earth’s axis of rotation:
Precession (22,000 years)
Changes in the tilt of earth’s axis:
Obliquity (41,000 years)
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Earth’s orbit: an ellipse
Perihelion: place in the orbit closest to the Sun
Aphelion: place in the orbit farthest from the Sun
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: period ~
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Eccentricity: period ~ 100,000 years
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: period ~
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Precession: period ~ 22,000 years
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Axis tilt: period ~ 41,000 years
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Eccentricity affects seasons
Small eccentricity --> 7% energy difference between summer and winter
Large eccentricity --> 20% energy difference between summer and winter
Large eccentricity also changes the length of the seasons
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37. Obliquity explain seasonal variations
Ranges from 21.5 to 24.5 with current value of
Small tilt = less seasonal variation
cooler summers (less snow melt),
warmer winters -> more snowfall because air can hold more moisture
Source: 37

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Precession of equinoxes
Vernal equinox has period around the orbit.
Moon’s gravitational pull on Earth’s equatorial bulge causes wobling
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Milankovitch cycles suggest changes in the mean temperatures of earth
Source: Whyte (1995)
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41. Temperature: the last 400,000 years
From the Vostok ice core (Antarctica)
That there is a link between CO2 and global temperatures is irrefutable as this diagram shows, but exact details of the link and all the possible feedback mechanisms are not clear.
The interaction of positive and negative feedback effects associated with the greenhouse effect is currently a critical issue for the scientific community.

42. Fig 4.5
High summer sunshine, lower ice volume

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Formation of Glaciers
Glaciers - composed of fallen snow that is compressed into a large, thickened mass of ice over many years
Glacier Growth: When over a year snowfall (winter) is larger than snowmelt (summer)
Glacier Decay: When over a year snowfall (winter) is less than snowmelt (summer)
Glacier growth and decay largely influenced by summer temperatures.
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Internal Forcing
____________________________
Ocean changes
Chemical changes in the atmosphere (i.e. CO2)
Natural variations
Plate tectonics/mountain building
Volcanoes
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Activity
Consider the fact that today, the perihelion of the Earth’s orbit around the sun occurs in the Northern Hemisphere winter. In 11,000 years, the perihelion will occur during Northern Hemisphere summer. A) Explain how the climate (i.e. temperature of summer compared to temperature of winter) of the Northern Hemisphere would change in 11,000 years just due to the precession.
B) How would this affect the presence of Northern Hemisphere glaciers (growing or decaying)? Assume growth is largely controlled by summer temperature.
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46. If the earth’s tilt was to decrease, how would the summer temperature change at our latitude
Warmer summer
Cooler summer
Summer would stay the same
Impossible to tell
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47. A: How would climate change
Warmer winters, cooler summers
Warmer winters, warmer summers
Cooler winters, warmer summers
Cooler winter, cooler summer
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48. B: How would glaciers change?
Glaciers would grow
Glaciers would decay
Glaciers would stay about constant
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Major features of ice age
Minimum insolation could be explained by
Milankovitch cycle followed by advancement
of glaciers
Polar front moves south
Salinity increases
Thermohaline circulation increases
Lower sea surface temperatures and sea
levels followed by reduced evaporation and
precipitation
Nutrients and biological productivity increase
Deep water sequesters CO2 from atmosphere
Cooling due to expanding ice caps and
decreased CO2
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Last Glacial Maximum (LGM) 22 ~ 14 K year
3.5 –4 km thick
50-60 x 106 km3 water
120 m sea level reduction
700 –800 m geosyncline depression (still rebounding)
Large changes in flora and fauna
Most of planet equatorward of ice sheets:
→colder and drier
→wind speed 20 –50% higher
→higher dust levels
→lower CO2 concentration (~200ppm) and CH4 concentration feedback
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Major features of interglacial (Honocene)
Glaciers retreat
shows maximum insolation Milankovitch cycle
Higher sea levels
Higher sea surface temperatures
Enhanced evaporation and precipitation
Salinity decreases
Polar front moves north
Thermohaline circulation decreases
Nutrients and biological productivity decrease
Warming due to shrinking ice caps and increased CO2
Abrupt warming: one of most rapid transitions
Interrupted by brief period of cold –YoungerDryas (~11 KABP)
Continuation of warming beginning in ~10 KABP
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Glacial to interglacial cycle
Vostok data
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Temperature changes in LGM
AR4 figure 6.5
Left: Multi-model average SST change for LGM PMIP-2 simulations by five AOGCMs. North America and east europe were largely covered by ice sheet
Right: LGM regional cooling compared to LGM global cooling as simulatedin PMIP-2, with AOGCM results shown as red circles and EMIC (ECBilt-CLIO) results shown as blue circles. Grey shading indicates the range of observed proxy estimates of regional cooling
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Temperature changes in Last Interglacial
AR4 figure 6.5
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