Introduction

Report
MET 112 Global Climate Change -
Natural Climate Forcing
Professor Menglin Jin
San Jose State University
Outline –
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Paleoclimate – temperature and CO2
Natural forcing for temperature change
Features for Glacier and inter-glacier
Activity
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A lead to
Paleoclimate
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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
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Change
hard-shelled
animals
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Climate record
resolution
(years)
1 ,000,000
100,000
10,000
1000
100
10
1
1mon
1day
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 ,000,000
100,000
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10,000
1000
100
10
1
1mon
1day
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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Natural Climate Change
 External Forcing:
– The agent of change is outside of the
Earth-atmosphere system
 Internal Forcing:
– 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
Temperature should
be 25K lower
There should be
glaciations up to 2 billion
years ago
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
http://www.mun.ca/biology/scarr/Pangaea.html
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
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Cretaceous period, the continents
were
separating
into
land
masses
that
look
like
our
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modern-day continents
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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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
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.
This hypothesis is
proposed to 65
Million years ago for
one possible reason
that kills the
dinosaurs
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
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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
– http://www.nasa.gov/worldbook/meteor_worl
dbook.html
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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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SOLAR ACTIVITY
 Sunspots are the most
familiar type of solar
activity.
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.
SOLAR INFLUENCES ON CLIMATE
 Solar activity appears to
slightly change the Sun’s
brightness and affect
climate on the Earth...
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
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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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Eccentricity: period ~ 100,000 years
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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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Obliquity explain seasonal variations
Ranges from 21.5 to 24.5 with current value of 23.439281
Small tilt = less seasonal variation
cooler summers (less snow melt),
warmer winters -> more snowfall because air can hold more moisture
Source: http://www.solarviews.com/cap/misc/obliquity.htm
Precession of equinoxes
 Vernal equinox has 24 000 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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Temperature: the last 400,000 year
From the Vostok ice core (Antarctica)
Fig 4.5
High summer
sunshine,
lower ice
volume
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
Plate tectonics/mountain building
 ____________________________
Volcanoes
 ____________________________
 Ocean changes
 Chemical changes in the atmosphere (i.e. CO2)
– Natural variations
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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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If the earth’s tilt was to decrease, how
would the summer temperature change
at our latitude
1. Warmer summer
2. Cooler summer
3. Summer would stay the
same
4. Impossible to tell
79%
21%
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A: How would climate change
1. Warmer winters,
cooler summers
2. Warmer winters,
warmer summers
3. Cooler winters,
warmer summers
4. Cooler winter, cooler
summer
88%
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B: How would glaciers change?
1. Glaciers would grow
2. Glaciers would decay
3. Glaciers would stay
about constant
88%
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G
la
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48
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
http://www.ncdc.noaa.gov/paleo/abrupt/data_glacial3.html
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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
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proxy estimates of regional cooling
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Temperature changes in Last Interglacial
AR4 figure 6.5
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