Glaciers
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 Earth’s yearly path of
revolution around the Sun
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
Lowest level of
submerged corals
is 120 meters
below present-day
sea level
Corals
formed at
15-20 meters
below
present-day
sea level
Corals found
about 6
meters above
present-day
sea level
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?
Carbon Dioxide

Measurements of CO2 concentration
 Core from rapidly accumulating ice
 Merge well with instrumental data
Methane

Measurements of CH4 concentration
 Core from rapidly accumulating ice
 Merge well with instrumental data
Orbital-Scale Changes in CH4


CH4 variability
 Interglacial maxima 550700 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
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

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
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
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?
Carbon Dioxide
Did climate really change in the
20th Century?
According to IPCC’s Third Assessment Report, Climate
Change 2001:
1. The global average surface temperature has
increased 0.6±0.2ºC over the 20th Century.
2. 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.
Variations of Earth’s surface temperature for
the past 140 years
From Climate Change 2001: SFP WG I
Variations of Earth’s surface temperature for
the past 1000 years
From Climate Change 2001: SFP WG I
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.”
How do we know??
Measure increases in atmospheric
concentrations of known greenhouse gases.
•
• Calculate the effects of these gases on the
Earth’s heat budget.
• Model the response of global climate to
calculated radiative forcing.
• Look at similar episodes in geologic
record
Indicators of human influence on the atmosphere:
d13C of CO2



Average annual decrease in d13C = ~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
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? ??
(Ruddiman 2005)
Ruddiman’s 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

Ruddiman’s Hypothesis



Challenges the conventional assumption that
greenhouse gases released by human activities have
perturbed the earth’s 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!
Agricultural terraces
have been constructed
for ~2000 years. The
photo on the right are
terraces in Guizhou
Province, China.
Minimum Ice
(Ferretti and others 2005)
What happened
in 1492?
Plows, Plagues & Petroleum
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.

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.

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.

Homework Question 3
How does carbon dioxide (CO2) in the
Earth’s 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.

Homework Question 4
When did humans 1st begin to affect Earth’s
climate?
 Subject covered in today’s lecture…
