Transcript Long-term Climate Variability

Last Time - Short Term Climate Change
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Methods to Document Climate Change
Sedimentation
Ice cores
Dendrochronolgy
Coral Reefs
Pollen
Direct land and ocean measurements
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Causes of Short-Term Change
Volcanoes (atmospheric aerosols) …ex. Mt. Pinatubo
El Nino Southern Oscillation & La Nina
Solar Variability (sunspot 11 year cycle)
Past periods of climate change
Insert
temperature
recoreds
The Medieval Warm Period
• 800 to 1300 AD
• regional warming (not necessarily global)
• Longer and warmer growing season
– grapes in England
• Higher treelines
• Warmer sea surface temperatures in North
Atlantic
• approx. 1o C warmer than present
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Recent Time Periods of Climate Change
Medieval Warm Period (800-1300 AD)
Little Ice Age (1450-1850 AD)
Dust Bowl (1940’s in North America)
Viking settlement on Iceland and
Greenland from 800 to 1200
The Little Ice Age
• Very cold climate between 1560 and 1850
• Greater frequency of storms
• Glacial advances 1560-1610,1816-1890
Iceland
• Population declines in
Iceland indicated by
tax records
• shift from grains to
barley (short growing
season) to no grains
• fishing failed as fish
migrated southward
due to water
temperatures.
• Height declines
– from 5’8” in 900s to
5’6” in 1700s in
Iceland
Iceland population
100000
barley no grain
0
1000
1200
1400
1600
1800
2000
Greenland
• 1300 highest
population (3000)
• Poor harvests,
fewer livestock
• Increase in sea ice
decreased trade
• Settlements
abandoned
• Height decrease
from 5’7” to < 5’
by 1400
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Recent Time Periods of Climate Change
Medieval Warm Period (800-1300 AD)
Little Ice Age (1450-1850 AD)
Dust Bowl (1930’s in North America)
Long Term Climate Change
Changes over hundreds of thousands to
millions of years
– Ice Ages
– Mesozoic Warm Period
Long Term Climate Change
Long Term Climate Change
• Five Major Glacial
Periods in Earth’s
History
Pleistocene - 15 myr to today
Permian - Carboniferous - 280 myr
Late Ordovician - 440 myr
Late Proterozoic - 800-600 myr
Huronian - 2200 -2500 myr
• How do we know this?
• What causes Ice Ages?
• Warm Mesozoic?
Evidence
• Geologic Record
– Unconformities
– Striations
– Tillites
• Oxygen Isotopes
for past 200 myrs
Pleistocene Ice Age
Cause of Ice Ages
• Many interacting factors including:
– Solar Radiation
– Greenhouse Effect - CO2 concentration
– Position of Land Masses/ Continents
Solar Radiation
• Milutin Milankovitch - Serb astronomer /
mathematician described:
• Orbital variations that affect climate
• Esp. Summer insolation in the high
latitudes …seasonal contrast
• Idea developed in 1920’s before the
oxygen isotope record
•Milankovitch Cycles
–Precession
–Tilt
–Eccentricity
Eccentricity
• Cycle - 100,000 yrs
• Elliptical - Circular
• Changes total solar
flux by 0.2%
• Does not affect
seasonal contrast
• Optimum for
glacial period?
Tilt
• Cycle - 41,000 yrs
• More tilt - more
sun in summer less in winter
• Changes seasonal
contrast
• Optimum for
glacial period?
Precession
• Cycle - 23,000 yrs
• Precession of the
equinoxes
• Due to wobble of the
axis
• Impact due to elliptical
nature of orbit
• Optimum for glacial
period?
Pleistocene Glaciations
• General cool down in Cenozoic
– More chemical weathering - lower CO2
• Variations during this period seem to have a
regular beat
– Milankovitch Cycles
• Problem is largest temp shift is 100 kyrs but
this is smallest orbital change
– Need a feedback to amplify this signal
– Look back at CO2 and the carbon cycle
The role of the earth’s surface:
The Ice - Albedo Feedback
Global Temp
Albedo
Summer
Insolation
Ice Sheets
• Change amount of summer sun - Grow ice
sheets - Change Albedo
• Change energy balance / temperature
• Positive Feedback
• Could explain amplification
Cause of Ice Ages
Solar Radiation
Position of Land Masses/ Continents/ Ice
Greenhouse Effect - CO2 concentration
CO2 Variation
Sharp Changes
• But from our ice
core records we
know that CO2
changes also
• Temperature
matches CO2
change closely
• Chicken or Egg?
• Change in
greenhouse effect
CO2 Variation
• Drop in CO2, where does it go?
• What can cause this variation of CO2 on this
time scale(100 kyrs)?
• Geologic reservoirs?
• Biomass? During a glacial period?
• Oceans? Biological Pump?
CO2 from atm.
Biological Pump
• What if we change
how efficient this is?
• Turn it off - CO2 will
rise in atmosphere to
720 ppm
• 100% efficient (i.e.
use all nutrients
• CO2 to 165 ppm
Biological Pump
• Change in marine productivity could explain
changes in CO2
• How do we make ocean during glacial times
more productive?
• Fertilize it. How?
– Shelf Nutrient Hypothesis
– Iron Fertilization
So…
Solar Radiation,
Ice albedo feedback and
Atmospheric CO2 changes
explain the cycle in ice ages
Mesozoic Warm Era
• Much warmer at high
latitudes
• Evidence from fossil
record
• Oxygen Isotopes - deep
ocean 15 C (today it is
2 C)
• Thermohaline
circulation ran
backwards?
20 degrees warmer at the poles
Mesozoic Earth
• W - Warm Water Fossils
• E - Evaporites
• C - Coal Deposits
• Ocean much higher
less land … why?
• Effect on albedo?
• Effect on carbon
cycle?
Stable Isotopes
• Isotopes - same number of protons
different number of neutrons
• Radioactive Isotopes - decay - age dating
• Stable Isotopes - do not decay but ….
• Due to difference in mass they react at
different rates in chemical processes such
as evaporation, photosynthesis, etc…
• Leads to variations in the ratio of these
stable isotopes
Stable Isotope Notation
• Express variation as per mil differences
from a standard
18O/ 16Osample  18O/ 16Os tan dard 
18
*1000
 O  
18
16


O
/
Os tan dard



• Called delta values
• Positive, relatively more of the heavy
isotope
• Negative more of the lighter isotope
Oxygen Isotopes
• Foraminifera Shells - CaCO3
• Look at oxygen isotopes in these shell
• 18O and 16O
• The colder the water the more oxygen-18
incorporated in the shell
• Use oxygen isotopes as a “paleo-thermometer
– idea developed by Urey in the 1950’s
Cenozoic Cool
Starting 30 myrs ago
Big cool down
• Why? Decrease CO2
– Slow down of midocean ridge spreading
– Uplift of Himalayas
– Start of Asian
Monsoon
– Increased weathering
• Look at this record...
Forams and Oxygen Isotopes
• Two types of
forams
• What does
each tell us?
Oxygen Isotope Record
Warmer
Colder
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Benthic Forams - deep water temp
Long term trend
Short term trend
Causes?