Ice Cores
UCAR Office of Education and Outreach Dr. Randy M. Russell
Ice cores extracted from polar ice sheets and from glaciers worldwide are one type of paleoclimate proxy record
- a source of data that tells us about past climates.
Ice cores from glaciers and the polar ice caps are probably the most comprehensive type of proxy record of past
climates. Physical and chemical analysis of ice cores provides information on temperature, precipitation, atmospheric
aerosols (such as dust and volcanic ash), and even levels of solar activity. Ice cores can provide data with a resolution as
fine as yearly, and some records span periods of hundreds of thousands of years.
Continuous climate records embedded in ice form in areas where year-round cold temperatures prevent fresh
accumulations of snowfall from melting in the summertime. The North and South Polar Ice Caps are obviously good
locations for such accumulations, but high-altitude glaciers, even in low-latitude regions, can also be a source of ice
cores. As layer upon layer of new snow builds up, the older layers on the bottom gradually get compressed. This
compressed snow first transforms into a grainy material called "firn" that has a texture akin to granulated sugar. As the
firn is buried deeper beneath subsequent layers, pressure eventually condenses it into solid ice (at a depth of about 100
meters in Antarctic samples, for instance). This ice layer can become quite deep over time; the 25 million year old East
Antarctic Ice Sheet is more than 4.5 kilometers thick in places!
As the firn turns to ice, the air pockets between the grains become cut off from each other and produce bubbles
in the ice. The bubbles serve as miniscule air samples from past eras, which scientists can use to study the state of
Earth's atmosphere in ancient times. One of the most important paleoclimate indicators in such samples is the
concentration of greenhouse gases (such as carbon dioxide and methane).
Picture 1: Scientists examine an ice core sample; inset shows layering
Picture 2: Close-up view of
layers within a different
sample; (arrows indicate
lighter summer layers ).
Credits: Ken Abbott, Office of Public Relations, University of Colorado, Boulder (left).
Anthony Gow, United States Army Corps of Engineers, Cold Regions Research and Engineering Laboratory (right).
The uppermost portions of an ice core exhibit a layered structure that shows yearly variation, providing an
annual resolution like that in tree rings, corals, or varved lake sediments. Extreme pressures crush the deeper ice layers
together so tightly that annual variations cannot be distinguished, though climate variations with lower resolutions can
still be discerned. Typically, in ice samples from polar regions (primarily Greenland and Antarctica), the upper layers of
ice have alternating light and dark layers. Light bands correspond to the relatively fresh, clean snows that fall in the
summer when warmer conditions bring more moisture (and thus more precipitation) to these high-latitude locales. Dark
bands mark the polar winter season, when little new snow falls on these frigid deserts and blowing snow is mixed with
dust carried in from afar, discoloring the white snow. In many samples, this alternating light and dark layering is visible
to the naked eye; in others, the layers can only be found by looking through polarized filters or via chemical analysis.
Thick summer layers can indicate periods during which precipitation was heavier than usual, while especially dark winter
layers can tell the tale of dry spells with regional or global dust storms.
Most ice cores are collected from the vast ice sheets covering Greenland and Antarctica, which provide records
spanning the longest time periods. Scientists have drilled down through more than 3 kilometers of ice in Greenland to
the bedrock beneath; the samples thus extracted represent about 130,000 years of history dating back to the last
interglacial warm spell (during which Greenland was ice-free). Although deeper ice layers at a specific spot correspond
to older time periods, the depth of ice at different sites doesn't always directly relate to the age of a record; different
snowfall rates in different locales can lay down thicker or thinner layers over time. Although the ice in Antarctica is up to
4.5 km deep in spots, the oldest record recovered to date from Antarctica comes from a 3,270 meter deep drill hole that
provides a history of climate dating back about 740,000 years. Ice from the bottom of a drill hole is much, much more
compressed by pressure than is ice closer to the surface. At the South Pole firn turns to ice about 122 meters down and
represents the climate of 100 years ago, while the bottom 100 meters of ice from the oldest Antarctic site represents
the oldest 100,000 years of that record.
What type of information is archived in ice, and what does this data tell us about past climates?
As was mentioned above, gas bubbles in the ice indicate the relative abundances of various atmospheric gases.
Greenhouse gases like carbon dioxide and methane give us an insight into atmospheric heating. The thickness of an ice
layer provides clues about local precipitation; a thick layer corresponds to greater snowfall. Layer thickness also may tell
us something about global temperatures. Higher snowfall rates in polar regions generally correspond to more moisture
in the global atmosphere, which usually arises during periods when the overall global temperature is high, causing
increased evaporation from the oceans. Thus, counterintuitively, more snow tends to accumulate at the poles during
global warm spells.
Many types of aerosols (extremely fine particles), often carried to the polar regions from afar by winds, settle
upon snow and become trapped within ice. These aerosols can include soot produced by burning (forest fires, slash-andburn agriculture, industrial output from smokestacks, etc.), ash from volcanic eruptions, and dust from large-scale dust
storms. Traces of unusual isotopes of certain elements also show up in ice core records. Chlorine-36 and iodine-129,
produced by early atmospheric testing of nuclear weapons, have been found in ice cores from the Fremont Glacier in
Wyoming. Beryllium-10 concentrations, linked to cosmic ray intensities, are an indirect indicator of solar activity.
The ratio of concentrations of two isotopes of oxygen in the water molecules in ice serves as a proxy indicator of global
temperatures. Oxygen has two commonly occurring natural isotopes, the usual 16O (which makes up more than 99% of
naturally occurring oxygen on Earth) and the less abundant 18O. The two extra neutrons in 18O cause water molecules
containing this isotope to be heavier than normal water molecules. These heavier water molecules cannot escape from
ocean water to become water vapor in the atmosphere via evaporation as readily as lighter water molecules. This
tendency for preferential evaporation of 18O varies, however, as a result of the ocean temperature. Through a rather
complex chain of events involving the global water cycle, this disparity between concentrations of oxygen-18 and
oxygen-16 shows up in the snow that falls in polar regions, and thus in the ice formed from this snow. The net result is
that the ratio of 18O to 16O in ice samples provides clues about global ocean temperatures and the extent of the polar ice
caps at a given time in Earth's history.
Scientists must be careful in their interpretations of ice core data. For example, the gases trapped in bubbles in
ice may do not have exactly the same age as the surrounding ice. For a few years after it falls, snow has air spaces
between the snowflakes that are still exposed to the atmosphere, so gases can move between the snow and the
atmosphere. As the deeper layers of snow are transformed into firn, the movement of air between these gaps and the
atmosphere diminishes. However, this gas exchange does not cease entirely until the firn turns to ice and the pores
become sealed bubbles. The gas in such bubbles therefore includes atmospheric "samples" from the years of the snowto-firn-to-ice transition period, somewhat blurring the resolution of the ice core records in terms of gas samples. The
pores tend to close off at depths of 50-150 meters in the Antarctic, but at much shallower depths in cores from
mountain glaciers. In polar ice samples, the difference between the "ice age" and the "gas age" can be as large as about
150 years.
Ice cores are extracted using drills with ring-shaped bits on the ends of hollow tubes. Cores are typically
removed in sections that are six meters long, so drilling to a depth of more than a kilometer requires many repetitions of
the drilling and core removal process. Upon removal from the drilling rig, the long cores are generally cut into shorter 2meter segments and their surfaces are carefully cleaned of fluids used in the drilling process. Deep ice is especially
brittle, so the cores must be handled carefully and must obviously be stored in cold conditions. The cores are kept in a
deep freeze as they are transported from their collection sites to one of a handful of archival storage and research
facilities around the world. One such facility, the U.S. National Ice Core Laboratory which is maintained by the USGS and
NSF near Denver, Colorado, stores over 14,000 meters of ice cores from 34 drill sites in Greenland, Antarctica, and high
mountain glaciers in the Western United States at a temperature of minus 35° C. The cores are split lengthwise for study
in a laboratory that is kept at a temperature of minus 22° C.
Data:
Ice core Data-Greenland Temperature vs. O18/O16 Ratio
Year
1970
1969
1968
1967
1966
1965
1964
1963
1962
1961
1960
1959
1958
1957
1956
1955
1954
1953
1952
1951
1950
1949
1948
1947
1946
1945
1944
1943
1942
1941
1940
1939
1938
1937
1936
1935
1934
1933
1932
1931
Avg. Temp
-6.383333333
-5
-6.866666667
-7.65
-5.533333333
-5.083333333
-5.616666667
-4.266666667
-4.9
-7.15
-6.733333333
-6.216666667
-6.416666667
-8.133333333
-5.7
-7.116666667
-7.9
-5.15
-7.416666667
-7.2
-7.8
-10.01666667
-6.233333333
-2.766666667
-5.433333333
-8.233333333
-6.583333333
-5.9
-5.966666667
-3.633333333
-4.366666667
-7.866666667
-8.166666667
-7.733333333
-3.933333333
-5.766666667
-6.683333333
-7.683333333
-5.383333333
-7.333333333
PC1
2.277
2.928
-0.886
-0.271
1.715
3.630
2.682
3.092
0.618
2.096
4.183
1.336
-0.662
-2.041
0.643
-0.506
0.846
3.421
-1.570
0.568
-0.539
-1.425
2.647
5.455
5.387
-1.803
-0.959
-0.361
3.396
0.766
5.541
0.967
1.240
-1.732
0.788
2.675
3.906
0.482
2.819
-0.246
1930
1929
1928
1927
1926
1925
1924
1923
1922
1921
1920
1919
1918
1917
1916
1915
1914
1913
1912
1911
1910
1909
1908
1907
1906
1905
1904
1903
1902
1901
1900
1899
1898
1897
1896
1895
1894
1893
1892
1891
1890
1889
1888
1887
1886
1885
1884
1883
1882
1881
1880
1879
1878
1877
-6.433333333
-2.766666667
-5.633333333
-6.833333333
-4.9
-9.4
-5.466666667
-6.583333333
-9.216666667
-10.68333333
-8.666666667
-9.15
-10.16666667
-5.083333333
-5.3
-9.9
-10.88333333
-8.033333333
-7.116666667
-9.266666667
-10.8
-7.45
-8.566666667
-11.31666667
-10.58333333
-7.166666667
-9.216666667
-10.33333333
-7.366666667
-7.4
-7.533333333
-10.05
-11.08333333
-10.36666667
-10.91666667
-6.55
-12
-7.266666667
-8.116666667
-10.38333333
-10.26666667
-9.033333333
-6.833333333
-11.26666667
-10.1
-8.683333333
-12.76666667
-9.6
-11.6
-5.916666667
-9.8
-4.633333333
-7.7
-7.216666667
-0.388
3.827
-0.473
-2.741
1.875
-1.588
-2.249
1.420
-2.176
-2.401
-1.264
-3.102
-2.353
5.574
3.482
-3.439
-1.457
-1.973
1.850
-0.271
-2.106
2.325
-0.434
-5.378
-5.028
-2.887
-0.371
-1.585
-0.635
0.973
-4.145
-0.390
-2.485
-0.128
1.045
2.300
-1.262
1.184
0.381
-1.183
-3.772
-0.478
2.693
-0.261
-0.943
-1.642
-1.481
-2.253
-2.797
1.666
-0.357
3.038
-1.080
-0.371
1876
1875
1874
1873
1872
1871
1870
1869
1868
1867
1866
1865
1864
1863
1862
1861
1860
1859
1858
1857
1856
1855
1854
1853
1852
1851
1850
1849
1848
1847
1846
1845
1844
1843
1842
1841
1840
1839
1838
1837
1836
1835
1834
1833
1832
1831
1830
1829
-7.183333333
-6.566666667
-8.866666667
-7.883333333
-4.05
-6.733333333
-9.616666667
-9.583333333
-9.733333333
-9.166666667
-10.21666667
-7.55
-11.25
-14.13333333
-9.666666667
-9.516666667
-6.85
-9.8
-8.983333333
-7.583333333
-4.083333333
-8.233333333
-10.31666667
-6.716666667
-7.333333333
-7.15
-8.216666667
-9.966666667
-9.883333333
-2.7
-7.783333333
-8.4
-12.53333333
-7.516666667
-10.26666667
-8.85
-7.95
-10.26666667
-11.9
-11.76666667
-8.35
-11.65
-11.33333333
-11.08333333
-10.61666667
-6.9
-7.1
-5.7
Questions:
1. You will need to generate three graphs:
a. temperature (dependent variable) vs. time
b. PC1 (oxygen 18 isotope ratios) vs. time
2.939
1.464
-2.949
0.304
1.812
2.183
-1.419
-2.365
0.309
-0.436
-1.077
0.784
-5.969
-4.336
-0.671
-0.276
-1.009
-0.052
-0.767
-1.937
3.449
0.382
-0.554
3.191
3.038
1.697
0.373
-0.661
-1.731
3.710
-1.284
-0.707
-2.076
1.904
-1.201
1.669
-0.667
-1.963
1.857
-4.312
-1.907
-5.998
-2.981
-3.312
1.682
2.133
1.735
1.853
2.
3.
4.
5.
c. Scatter plot PC1 is dependent variable and temperature is the independent variable
*** be sure to include a trend line and r2 value for each graph***
Determine the relationship between temperature and PC1 (O18 ratio values).
What conclusions can be made about the climate through PC1 (O18 ratios) and temperature? Use 3 pieces of
evidences from your research to support your ideas.
How can present day data such as this be use by paleoclimatologists to reconstruct past climates?
What are the limitations of this source of information?
Resources:


2 minute video about ice cores - produced by NSF - features several prominent climate scientists
(Richard Alley, Michael Mann, Ellen Mosley-Thompson, and Jim White) - on Windows to the Universe
"Ice Core Secrets Could Reveal Answers to Global Warming" - 5 minute video from NSF's Science
Nation online magazine
Graphs of temperature variation from present-day values (blue), atmospheric carbon dioxide concentration (green), and dust
(red) based on data from ice cores retrieved at the Vostok drilling site in Antarctica.
Credits: Petit J.R., Jouzel J., Raynaud D., Barkov N.I.,Barnola J.M., Basile I., Bender M., Chappellaz J., Davis J., Delaygue G.,
Delmotte M., Kotlyakov V.M., Legrand M., Lipenkov V., Lorius C., Pépin L., Ritz C., Saltzman E., Stievenard M. (1999). Climate
and Atmospheric History of the Past 420,000 years from the Vostok Ice Core, Antarctica. Nature 399: 429-436.