Earth

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Glaciers
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Astronomical Control of Solar Radiation

• Earth's present-day orbit around the Sun
• Not permanent
• Varies at cycles from 20,000-400,000 years
• Changes due to
• Tilt of Earth's axis
• Shape of Earths yearly path of revolution around
  the Sun

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d18O Record from Benthic Foraminifera

• Ice volume and T move d18O in same direction
• Two main trends
• Cyclic oscillations
• Orbital forcing
• Dominant cycles changed over last 2.75 my
• Long-term slow drift
• Change in CO2
• Constant slow cooling

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Orbital-Scale Change in CH4 CO2

• Important climate records from last 750 kya
• Direct sampling of greenhouse gases in ice
• Critical questions must be addressed
• Before scale of variability in records determined
• Reliability of age dating of ice core?
• Mechanisms and timing of gas trapping?
• Accuracy of the record?
• How well gases can be measured?
• How well do they represent atmospheric
  compositions and concentrations?

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Carbon Dioxide

• Measurements of CO2 concentration
• Core from rapidly accumulating ice
• Merge well with instrumental data

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Methane

• Measurements of CH4 concentration
• Core from rapidly accumulating ice
• Merge well with instrumental data

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Orbital-Scale Changes in CH4

• CH4 variability
• Interglacial maxima 550-700 ppb
• Glacial minima 350-450 ppb
• Five cycles apparent in record
• 23,000 precession period
• Dominates low-latitude insolation
• Resemble monsoon signal
• Magnitude of signals match

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Monsoon forcing of CH4

• Match of high CH4 with strong monsoon
• Strongly suggests connection
• Monsoon fluctuations in SE Asia
• Produce heavy rainfall, saturate ground
• Builds up bogs
• Organic matter deposition and anaerobic
  respiration likely
• Bogs expand during strong summer monsoon
• Shrink during weak summer monsoon

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Orbital-Scale Changes in CO2

• CO2 record from Vostok
• Interglacial maxima 280-300 ppm
• Glacial minima 180-190 ppm
• 100,000 year cycle dominant
• Match ice volume record
• Timing
• Asymmetry
• Abrupt increases in CO2 match rapid ice melting
• Slow decreases in CO2 match slow build-up of ice

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Orbital-Scale Changes in CO2

• Vostok 150,000 record
• 23,000 and 41,000 cycles
• Match similar cycles in ice volume
• Agreement suggests cause and effect relationship
• Relationship unknown
• e.g., does CO2 lead ice volume?
• Correlations not sufficient to provide definite
  evaluation

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Climate in the 20th Century

• Did climate really change in the 20th Century
  or perhaps earlier?
• How much of this change can be attributed to
  human impacts on the climate system?
• Do we have to do anything about it?

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Carbon Dioxide
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Did climate really change in the 20th Century?

• According to IPCCs Third Assessment Report,
  Climate Change 2001
• The global average surface temperature has
  increased 0.60.2ºC over the 20th Century.
• Globally, it is very likely that the 1990s were
  the warmest decade and 1998 the warmest year in
  the instrumental record, since 1861 (and likely
  the warmest in the last 1000 years).

2001 was second warmest according to NCDC.
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Variations of Earths surface temperature for
the past 140 years
From Climate Change 2001 SFP WG I
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Variations of Earths surface temperature for
the past 1000 years
From Climate Change 2001 SFP WG I
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How much of this change can be attributed to
human impacts on the climate system?
Climate Change 2001 There is new and stronger
evidence that most of the warming observed over
the last 50 years is attributable to human
activity. NRC 2001 The changes observed over
the last several decades are likely mostly due to
human activities.
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How do we know??

• Measure increases in atmospheric
  concentrations of known greenhouse gases.
• Calculate the effects of these gases on the
  Earths heat budget.
• Model the response of global climate to
  calculated radiative forcing.
• Look at similar episodes in geologic record

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Indicators of human influence on the atmosphere
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d13C of CO2

• Average annual decrease in ?13C 0.014 y-1
  over the last 10 years
• Reflects the influence of fossil-fuel carbon
  combustion in the atmosphere
• Additional influences of temporal variations in
  oceanic and terrestrial components of the global
  carbon cycle are also evident

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Climate in the 20th Century

• Did climate really change in the 20th Century?
  YES 0.6ºC
• How much of this change can be attributed to
  human impacts on the climate system? YES about
  0.4ºC
• Do we have to do anything about it? ??

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(Ruddiman 2005)
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Ruddimans Hypothesis

• Human activity influenced atmospheric gas
  concentrations
• CO2 increased about 8,000 years ago
• CH4 increased about 5,000 years ago
• Halted the development of another ice age
• Without increase in greenhouse gases, northern
  hemisphere would have cooled by 4ºC
• Cool enough to form glacial ice

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Ruddimans Hypothesis

• Challenges the conventional assumption that
  greenhouse gases released by human activities
  have perturbed the earths climate only with the
  last 200 y
• New evidence suggests instead that our human
  ancestors began contributing significant
  quantities of greenhouse gases to the atmosphere
  thousands of years earlier by clearing forests
  and irrigating fields to grow crops
• As a result, human beings kept the planet notably
  warmer than it would have been otherwise and
  possibly even averted the start of a new ice age!

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Agricultural terraces have been constructed for
2000 years. The photo on the right are terraces
in Guizhou Province, China.
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Minimum Ice
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(Ferretti and others 2005)
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Plows, Plagues Petroleum
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References Worth Reading

• Ruddiman W. F. (2005) How did humans first alter
  global climate? Scientific American 292 46-53.
• Ruddiman W. F. (2005) Plows, Plagues Petroleum,
  Princeton University Press, 202 p.

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Homework Question 1

• How does glacial growth or shrinkage result from
  the balance between ablation and accumulation?
• If ablation is equal to accumulation, the glacier
  is gaining as much ice as it is losing and will
  neither grow nor shrink. If accumulation is
  greater, the glacier will grow. If ablation is
  greater, the glacier will shrink.

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Homework Question 2

• How do glaciations affect sea level?
• The majority of the snow that forms glaciers
  originates as water vapor from the oceans. As ice
  accumulates on land, moisture is permanently
  removed from the oceans and sea level goes down.
  As glacial ice melts, sea level rises.

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Homework Question 3

• How does carbon dioxide (CO2) in the Earths
  atmosphere affect climate?
• Carbon dioxide is a "greenhouse" gas. This means
  that it absorbs infrared radiation and warms the
  atmosphere. Increases in the atmosphere's carbon
  dioxide content are expected to correspond to
  warmer climates.

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Homework Question 4

• When did humans 1st begin to affect Earths
  climate?
• Subject covered in todays lecture