Climate Archives: The Climate Record of the
Take away ideas and understandings
- Understand how oxygen isotopes in seawater are used to
reconstruct past changes in glacial ice volume.
- Understand how past changes in the Earth's orbit cause
changes in the seasonal distribution of solar radiation, and how they have
been the "pacemaker" for glacial-interglacial climate variations.
- Understand the kinds of paleoclimate data extracted from
the Vostok (Antarctic) and Greenland ice cores and how they can be used
to constrain past climate variations.
- Understand the difference between the longer-period "orbitally-driven"
climate variations and the shorter-period "millennial-scale"
I. Oxygen Isotopes in Seawater: How do we know how much
ice was in the large glaciers 18,000 years ago?
- In the Northern hemisphere, "permanent" ice
is located primarily on Greenland with a small amount of floating sea ice
located in the Arctic Ocean (Fig 1)
- Glacial ice on Greenland today is several thousand meters
thick (Fig 2)
- In the northern hemisphere, glacial maximum (18,000 years
ago) ice sheets covered a major fraction of North America with ice several
thousand meters thick (Fig 3)
- Water molecules: relative abundances of oxygen by stable
isotope composition (Fig 4)
- Stable isotope composition permille delta notation (Fig 5)
- The stable isotope composition of rain and snow today
shows large variability, especially with latitude (Fig
- Polar snow and ice water molecules are very depleted
in 18O (Fig 7)
- The 18O/16O ratio of global precipitation
shows a strong correlation with precipitation, resulting in very "light" water
molecules in polar ice sheets (See
- The mean delta 18O composition of "new" glacial
ice during the glacial period was probably about -25 permille.
- Sea level was lower by 130 meters 18,000 years B.P. (Fig 8)
- Sea water today has a delta 18O concentration
of 0 permille
From the above information, calculate the change in isotopic composition
you would expect to observe during the last glacial period.
- Would the glacial ocean have greater or lesser 18O/16O
ratios than today?
- Assuming sea level dropped by 130 meters, and the corresponding
volume of water was stored as ice in continental glaciers, what would the
magnitude of change in 18O be for average ocean water?
- Deep sea sediment fossil carbonates actually record these
past changes (Fig 9) in continental
ice volume. The calcite (CaCO3) shells have 18O/16O
in the mineral equilibrated isotopically with the sea water from which
- This record of the past volume of glacial ice is actually
recorded in sediments about 3 km below the sea surface, near the equator,
far away, both in space and time, from those ancient glaciers (Fig 10)
II. Why we study past climates (paleoclimatology)
A. We can measure many aspects of the climate system today
(Fig 11) and, from these measurements,
understand fundamental physical and chemical processes.
B. These measurements can be used as input to climate
models and the complex processes and their feedbacks can be simulated (Fig 12)
C. We cannot instrument the past; the complex atmospheric
processes can not be measured. The climate system must remain a black box
D. However, the mysterious climate system of the past
produces a record and from this record we learn about past climate systems.
III. The kinds of paleoclimate archives
A. Marine sediments (Fig 15)
accumulate slowly (typically 2-6 cm per 1000 years) but relatively continuously.
B. Lake sediments: Glacial-aged sediments from the Dead Sea
rift valley (Fig 16).
- Sediments were deposited in the Dead Sea rift valley basin
between 20-50 kyr. (Fig 17)
- The arrow of time points vertically up in sedimentary deposits
that are undisturbed. (Fig 18)
- Some sedimentary deposits have annual layers (seasonal
cycles). (Fig 19)
Annual layers can provide detailed histories of a basin
(a lake or region of the sea floor). (Fig
C. Tree rings:
annual growth layers in trees and shrubs.
- Can be used to reconstruct atmospheric temperature, precipitation,
sea surface temperature, or pressure indices (eg. ENSO SOI) depending on
the tree species used and its site ecology.
Can precisely date droughts, volcanic eruptions, and other
events in Earth history that might have been influenced by or caused changes
in climate. For more on tree-ring research, visit Columbia University's
Tree Ring Lab online.
D. Ice cores.
Boreholes drilled into polar ice sheets (Fig 21) have recovered long cores of
ice which contain information about the temperature and chemistry of the
E. Peat deposits.
- Contain pollen grains which record changes in vegetation
and climate over time (eg. Malaspina
- Contain plant macrofossils which define the changes in
vegetation and climate for the region (eg. Siberian
- A. The Milankovitch or astronomical theory of climate change
is an explanation for the changes in the seasons which result from changes
in the earth's orbit around the sun. The theory is named for Serbian astronomer
Milutin Milankovitch, who calculated the slow changes in the earth's orbit
by careful measurements of the position of the stars, and through equations
using the gravitational pull of other planets and stars. (See this excellent
summary of the Milankovitch Mechanism by the NOAA Paleoclimatology
- B. There are three main components to Earth orbital variability
(Fig. 22): Eccentricity
(100,000 year period), Tilt (41,000 year period), and precession
(23,000 and 19,000 year periods). Only variations in orbital tilt and
precession significantly affect the amount of radiation received during
a given season.
- C. These orbital changes cause large changes (up to ±15%)
in the amount of sunlight received during a given season. Here is how
the northern hemisphere cooled and warmed over the 25,000 to 10,000
years ago due to these variations (Fig. 23).
- D. We can reconstruct past variations in ice sheet size
using measurements of oxygen isotopes in the calcite (the "O" in CaCO3)
of shells (called foraminifera (Fig 24)) found in
deep-sea sediments. As you learned above, seawater 18O
variations over time can be linked to the growth and decay of ice
sheets. During the last ice age sea level was ~130m lower than today
reflecting about 3% of the ocean's volume. Consequently the
18O of the ocean was ~1.2 permille heavier (higher) than it
is today. Measuring the 18O in foraminifera allows to
reconstruct past variations in the size of the ice sheets over millions
- E. Here is a summary of the relationship between changes
in polar ice sheet size (ice growth and decay) and orbital
eccentricity, tilt, and precession (Fig. 25). The key variable is
the amount of summer radiation at high northern latitudes. Most of the
18O variability can be related to direct radiation forcing
by orbital variations. Periodic variations in the Earth's orbit have
been the pacemaker for the ice ages - there have been many (~50-60) ice
ages over the last several million years, not just one.
V. The climate record of ice cores
- Antarctica (Fig 26)
- Greenland (See Fig 2)
B. Latitudinal balance of incoming versus outgoing radiation
C. Ice cores
- Result of snow accumulation
- Snow contains air (Fig
- Some air gets trapped in bubbles
- Bubbles then contain "fossil air."
- Ice contains water and water contains isotopes of both
hydrogen and oxygen
- Factors controlling the behavior of hydrogen and oxygen
- Geographical distribution of dO18
(See Fig 6)
- Relation of dO18 values
to temperature. (See Fig 7)
D. The glacial ice record
- Behavior of snow as it turns to ice (see again Fig 28). Pockets of ancient air are trapped by
closing pores in ice.
- The flow of glacial ice (Fig
- The relation of age to depth in the ice core (Fig 30)
- Records of past temperature and atmosphere CO2 AND CH4 variations
preserved in glacial ice (Fig 31
and Fig 32). Note that variations in
temperature and greenhouse gases (CO2 and CH4) appear to covary with temperature
over both glacial and interglacial periods.
VI. Anthropogenic Greenhouse Gas increases and future climate change
A. The anthropogenic rise (Fig 33 in CO2 is ~1.5% each year due
to fossil fuel combustion and deforestation.
B. The preindustrial CO2 level was ~280 ppm, it is now (1998)
at 375 ppm By the time most of you are 30 years old it will be close to 460-470
PPM Doubling of atmospheric CO2 is expected by the year 2050, perhaps sooner
depending on the emission scenario which plays out.
C. Here is a comparison of the CO2 variations over the full
glacial to interglacial cycle of the last 140,000 years compared to the anthropogenic
rise in CO2 just over the past 300 years (Fig. 34).
D. Based on what you learned about the past links between
temperature and CO2 from the ice cores, would you predict that the world
will become warmer with rising CO2 levels?
Lecture text by James D. Hays and Peter B. deMenocal, 1998.