Chapter Three: How do we know that humans are
the primary cause of global warming?
The average global
HNRS330
land temperature for
May 2012 was the
Dr. Hengchun Ye
warmest on record
and marks the 327th
consecutive month
with a global
temperature above
the 20th-century
average. The
average T across the
U.S. for the first six
months of 2012, and
the 12-month period
ending in June 2012,
were the warmest
on record.
Paleoclimate
Phanerozoic climate history comes from the interpretation of fossils and chemical clues in rocks and
sediments. (542 million years of the earth’s history: effects of long term processes)
1. Plate tectonics
2. Prolonged periods of volcanic outgassing
3. Chemical weathering of Earth’s crust that draws down CO2 in atmosphere
4. Changes in ocean circulation owing to shifting continental positions
5. Release of large quantities of frozen methane by warming ocean water
6. Impacts by large meteorites
7. Positive and negative feedbacks that amplify the effects
• Gradual cooling about 40-50 million years ago is
theorized due to the formation of the Himalayan
Mountain system during the Paleocene and Eocene
epochs caused an increase in chemical reactions
between newly exposed tock of the mountain system
and the atmosphere that reduced CO2 level (Uplift
Weathering Hypothesis)
• We currently live in the latest interglacial, known as the
Holocene Epoch, began about 10,000 years ago.
Climate Proxies
Climate data derived from
geologic indicators and
other sources rather than
instrumental records.
(Fossilized plankton, coral
from sediments on
seafloors, chemical telltales
from ice sheet, glaciers)
Ice drilling:
Ice core: air trapped in the
glacial can derive CO2 level
to approx temperature
Ocean floor drilling:
Fossil Plankton (from phylum
Foraminifera)provide
chemical clues to the
climate when they were
formed
O2 isotope: O-16, O-17, O-18
Heavy water (with O-18) does not evaporate as readily as the light one
(O-16), Also, it condenses and fall easily. Most water originate from
tropical oceans and by the time ravels to high latitudes and high
elevations where glaciers form, It is enriched in water with O-16
relative to sea water. Hence, snow and ice are also relatively enriched
in the O-16 water molecule.
During ice age, vast amount of O-16 water is locked up in glaciers, and
oceans are relatively enriched in O-18 water.
Both evaporation and precipitation of O isotopes occur in relation to
temperature. H2O(18) tends to left behind when water vapor is form
during evaporation of seawater. By the time water travels the long
distance to high latitudes and elevation where ice sheets and glaciers are
located, it enriched in H2O(16). The ratio of O-18/O-16 is the shells of
foraminifera mimics those in seawater, the oxygen isotope content of
these shells provides a record of global ice volume through time. Ice core
isotopes provide a record of changes in air temperature.
Paleoclimate Patterns
Ratio of O-18/O-16 in deep-sea ice cores of
fossil foraminifera provides a proxy for
global ice volume
The ratio of O-18/O-16 in cores of glacial
ice documents changes in atmospheric
temperature, confirming that decreases
ice volume in deep-sea cores correlates to
times of warmer atmosphere, whereas
increased ice volume recorded in deep-sea
cores correlates to times of cooler
atmosphere
Major glacial and interglacial periods are repeated approximately every 100,000 years
Numerous minor episodes of cooling (called stadials) and warming (called interstadials)
are spaced throughout the entire records
Global ice volume during the peak of the last interglacial (Eemian), approximately 125,000
years ago, was lower than at present and global climate was warmer
The history of retreating
ice in North America as
the last ice age ended
Role of astraumical factors to climate variations
Seasons
• Perihelion
(Jan 3)
• Aphelion
(July 4)
• Titled angle
of axis (23.5
degree)
• Solar Angle
• Vertical rays
Annual March of the Seasons
Milankovitch cycles
Fig 3.7
small variations in
Earth-Sun geometry
related to
eccentricity,
obliquity, and
precession changes
the amount of sun
light received by the
earth’s each
hemisphere
The Younger Dryas
12,800 years ago,
temperature in most in
most of the NH rapidly
returned to nearglacial conditions and
stayed there for 1300
years and warmed
again rapidly (within a
human life time).
Fig 3.9
The Conveyor belt hypothesis and Climate feedbacks
Fig 1.5
Thermohaline circulation
transports heat around the
planet. Influxes of
freshwater from melting ice
on the lands (Greenland) can
slow or shut sown the
circulation by preventing the
formation of deep waterleads to cooling in the
northern Hemispherenegative climate feedback
Evidence show that the
thermohaline circulation has
slowed 20% over the 20002009.
Dansgaard-Oeschger events Fig 3.10
climate follows semi-cyclic patterns of warming and cooling that are more frequent
than Milankovitch cycles. It is the product of complex interactions among solar and
terrestrial processes (feedback processes)
DO events in NH, rapid warming followed by gradual cooling; in SH, slow warming
and smaller temperature fluctuations.
Rapid climate changes in
the Greenland ice core
during the glacial age
(80,000-15,000 yrs ago
barely registered in the
corresponding Antarctic
record-DO event)
Un-testified theory: Periodic collapses of
thick glacier or changes in Atlantic
thermohaline circulation triggered by an
influx of freshwater (bipolar Seesawopposite in timing of DOs).
Bipolar seesaw hypothesis: DO slow
cooling may be associated with an influx
of fresh water to the North Atlantic-reduces the strength of the
thermohaline circulation---As a result,
there is excess heat in the tropics,
available for oceanic currents to transfer
toward the Southern Ocean. Thus
warming is recorded in Antarctic ice
core records.
Warming in Antarctic release fresh
water which weakens SH circulation–
allows the N. Atlantic thermohaline
circulation to sudden switch on—
producing a rapid DO warming event in
the North Atlantic.
Fig 3.12
If global warming were
caused by a more-active
sun, we would expect to
see warmer
temperature in all layers
of the atmosphere
In the past 30 years
global temperature and
solar radiation show little
correlation. The Sun’s
energy has been
measured by satellites
since 1978, and it has
followed its natural 11year cycle of small ups
and downs, but no net
increase. Over the same
period, global
temperature has strongly
increased.