Nature's Time Machine
How would you like to have a time machine that could take you back anywhere
over the past 300,000 years? You could see what the world was like when ice
sheets a thousand feet thick blanketed Canada and northern Europe, or when the
Indonesian volcano Toba blew its top in the largest volcanic eruption of the
last half million years.
Well, scientists have such a time machine. It's called an ice core. Scientists
collect ice cores by driving a hollow tube deep into the miles-thick ice sheets
of Antarctica and Greenland (and in glaciers elsewhere). The long cylinders of
ancient ice that they retrieve provide a dazzlingly detailed record of what was
happening in the world over the past several ice ages. That's because each
layer of ice in a core corresponds to a single year--or sometimes even a single
season--and most everything that fell in the snow that year remains behind,
including wind-blown dust, ash, atmospheric gases, even radioactivity.
Indeed, fallout from the Chernobyl nuclear accident has turned up in ice
cores, as has dust from violent desert storms countless millennia ago.
Collectively, these frozen archives give scientists unprecedented views of
global climate over the eons. More important, the records allow researchers to
predict the impact of significant events--from volcanic eruptions to global
warming--that could strike us today.
Special thanks to Mark Twickler, University of New Hampshire
In April 1986, Russia's nuclear power station at Chernobyl exploded, killing
250 people and sending radioactive fallout around the world. Less than two
years later, as the graph indicates, scientists detected Chernobyl
radioactivity in snow at the South Pole--a graphic reminder of how small our
planet is. In cores from Antarctica and Greenland, researchers have pinpointed
the beginning of atomic-bomb testing in the mid-1950s. They have also
identified a spike representing fallout from stepped-up atmospheric testing
that took place just prior to the 1963 Test Ban Treaty, which allowed for
underground tests only. In the years following 1965, by which time some 90
countries had signed the treaty, Antarctic snow revealed a sharp drop in
Graph modified from:
Dibb, J., Mayewski, P.A., Buck, C.F. and Drummey, S.M., 1990, Beta
radiation from snow, Nature, 344, 6270, 25
Gases trapped in ice cores show the dramatic impact that human activities have
had on the planet since the Industrial Revolution. The first graph reveals how
atmospheric carbon dioxide, methane, and nitrous oxides from coal- and
oil-burning power plants, cars, and other fossil-fuel-burning sources have
climbed along with the world population, with as yet unknown effects on the
The second graph displays similar results with sulfates and
nitrates. Sulfates, which originate primarily in coal-fired power plants,
started rising around 1900. (This rise is partially attributed to increased
volcanic activity in the Caribbean around the turn of the century; other
volcanic eruptions--represented by large spikes in the graph--can be seen at
numbers 1, 2, and 3.) Nitrates didn't begin to climb significantly until
after 1950, when cars and oil-powered plants appeared in a big way. Scientists
credit the leveling off in sulfates and nitrates at the graph's far right--that is, the
most recent period--to a less-polluted atmosphere after the 1972 U.S. Clean Air
Act went into effect.
Data in gases graph from:
Etheridge, D.M., Pearman, G.I., and Fraser, P.J., 1992, Changes in
tropospheric methane between 1841 and 1978 from a high accumulation rate
Antarctic ice core, Tellus, Ser. B, 44, 282-294. (CO2 and
Keeling, C.K., Adams, J.A., Ekdahl, C.A., and Guenther, P.R., 1976,
Atmospheric carbon dioxide variations at the South Pole, Tellus, 28,
552-564. (direct measurements)
Machida, T., Nakazawa, T., Fujii, Y., Aoke, S. and Watanabe, O., 1995,
Increase in atmospheric nitrous oxide concentrations during the last 250 years,
Geophys. Res. Lett., 22, 2921-2924. (N20)
McEvedy, C. and Jones, R., 1978, Atlas of world population history,
Penguin. (world population)
Data in particulates graph from:
Mayewski, P.A., Lyons, W.B., Spencer, M.J., Twickler, M.S., Buck, C.F. and
Whitlow, S., 1990, An ice core record of atmospheric response to anthropogenic
sulphate and nitrate, Nature 346(6284), 554-556.
Viking colonies in Greenland abruptly vanished toward the end of the 14th
century. Why? One clue comes from ice cores. This graph, which combines results
from cores taken in both Antarctica and Greenland, tracks sodium levels over
the past 1,200 years. In colder periods, seas become stormier because of the
greater contrast in temperatures between the tropics and the poles, and so more
sodium--an indicator of seasalt--winds up on the ice caps. About 1400 AD, the
cores at both poles clearly show a sharp rise in sodium, which some scientists
say marks the onset of the Little Ice Age, a period of much cooler temperatures
that lasted into the 19th century. For the Vikings, a series of abnormally cold
winters in the late 1300s spelled doom.
Graph modified from:
Kreutz, K.J., Mayewski, P.A., Meeker, L.D., Twickler, M.S., Whitlow, S.I.
and Pittalwala, I.I., in press 1997, Bipolar changes in atmospheric circulation
during the Little Ice Age, Science.
Annual layers of snowfall in ice cores can be counted as easily as tree rings,
allowing precise dating of events such as volcanic eruptions. Distinct annual
layers stand out because, in snow that falls in summer, crystals are larger and
acidity higher than in winter snow. In some cases, scientists can even tell
seasons apart, by using a laser to measure the concentration of dust particles.
(Winds are generally stronger in springtime, meaning more dust gets blown into
the atmosphere.) In this photograph of an ice core drilled in the Kunlun
Mountains of western China, the thick, lighter bands indicate heavy snowfall
during the monsoon season in the year 1167 AD, while the thinner, darker strips
show layers of dust blown into the snowfield during the dry season.
Core photo courtesy of Lonnie G. Thompson, The Ohio University
(BP - Before Present)
Ice cores have revealed that global climate--long thought to change only very
gradually--can shift with frightening speed, in some cases in a matter of
years. As this graph shows, one such jump occurred about 12,000 years ago, as
the last glacial period (the Pleistocene) was giving way to our current warm
"interglacial" period (the Holocene). Suddenly, possibly in less than five
years, average temperatures, which were slightly cooler than today's, plunged
by about 27°F, returning the world to near-glacial conditions. (As the
graph indicates, calcium levels tend to go up and snow accumulation down with
temperature, which is estimated by comparing the ratio of oxygen isotopes in
water--see "Temperature" in core at left.) The Younger Dryas, as this freak period is
known, lasted about 1,300 years before it returned--just as abruptly--to the
temperatures typical of the period immediately preceding it.
Data in graph taken from:
Alley, R.B., Meese, D., Shuman, C.A., Gow, A.J., Taylor, K., Ram, M.,
Waddington, E.D. and Mayewski, P.A., 1993, Abrupt increase in Greenland snow
accumulation at the end of the Younger Dryas event, Nature 362,
Grootes, P.M., Stuiver, M., White, J.W.C., Johnsen, S. and Jouzel, J.,
1993, Comparison of oxygen isotope records from the GISP2 and GRIP Greenland
ice cores, Nature 336, 552-554.
Mayewski, P.A., Meeker, L.D., Whitlow,S., Twickler, M.S., Morrison, M.C.,
Grootes, P.M., Bond, G.C., Alley, R.B., Meese, D.A., Gow, A.J., Taylor, K.C.,
Ram, M. and Wumkes, M., 1994, Changes in atmospheric circulation and ocean ice
cover over the North Atlantic during the last 41,000 years, Science 263,
Mayewski, P.A., Meeker, L.D., Twickler, M.S., Whitlow, S.I., Yang, Q. and Prentice,
M., in press, 1997, Major Features and forcing of a high latitude Northern Hemisphere
atmospheric circulation over the last 110,000 years, Journal of Geophysical Research.
Temperature has yo-yoed over the ages as wildly as it does through any single
year. Like natural thermometers, ice cores have recorded these fluctuations,
which scientists can "read" by examining isotopes of oxygen and hydrogen in
water trapped in the ice. These isotopes come in two forms--"light" and
"heavy." Light isotopes have regular hydrogen and oxygen, while heavy isotopes
have either hydrogen with an extra neutron or oxygen with one or two additional
neutrons. Since heavy isotopes precipitate out of the atmosphere more quickly
than light ones, scientists can measure the ratio between the two isotopes to
estimate the temperature at any given time. The data in this graph,
gleaned from a core drilled in central Greenland, shows how temperatures have
risen by more than 20°C (36°F) since the height of the Ice Age 25,000 years
Graph modified from:
Cuffey, K.M., Clow, G.D., Alley, R.B., Stuiver, M., Waddington, E.D. and
Saltus, R.W., 1995, Large Arctic-temperature change at the Wisconsin-Holocene
transition, Science 270, 455-458.
Approximately 73,000 years ago, an Indonesian volcano known as Toba erupted
with enough force to send more than 600 cubic miles of volcanic material into
the atmosphere. Detected on this graph, which displays volcanic sulfate
levels between 20,000 and 110,000 years ago, Toba was the largest eruption of the past
500,000 years. (The seemingly larger spike at about 53,000 years ago involved a series of
smaller eruptions on Iceland, which is far closer than Toba is to Greenland,
where this core was taken.) Such violent, so-called caldera eruptions can
drastically alter global climate, by spewing so much ash and sulfur compounds
into the atmosphere as to block sunlight and lower temperatures worldwide. Ice
cores offer scientists the best means available to learn how past eruptions
have affected climate--and thus to predict the impact that future ones might
have. If an eruption on the order of Toba, which climatologists believe may
have led to as much as several centuries of cold climatic conditions, were to
occur today, it could seriously disrupt life on Earth.
Graph modified from:
Zielinski, G.A., P.A. Mayewski, L.D. Meeker, S. Whitlow, and M. Twickler,
1996a, A 110,000-year record of explosive volcanism from the GISP2 (Greenland)
ice core, Quaternary Research, 45, 109-118.
Many scientists fear that rising levels of so-called "greenhouse gases" from
the burning of fossil fuels and other human activities will cause global
warming, with potentially grave consequences for human agriculture and society.
One of the clearest signs that elevated levels of greenhouse gases can result
in warming comes from an ice core taken near the Russian Vostok station in
Antarctica. This graph tracks temperature and atmospheric levels of carbon
dioxide (CO2) and methane (CH4) from the present back to
about 160,000 years ago. (This represents about 11,350 feet of ice
accumulation.) The graph clearly shows how a rise in gases will mean a rise in
global temperature (though whether rising gases trigger rising temperatures, or
vice versa, remains unknown). Also note that (though the graph, which has data up to two
decades old, does not show this), at about 360 parts per million,
the amount of CO2 in the atmosphere today far exceeds levels at any
time in the past 160,000 years--indeed, in the past few million years. For
those worried about global warming, this is a sobering statistic.
Graph data taken from:
> the debate
> carbon diet
> stories in ice
Barnola, J. M., D. Raynaud, Y. S. Korotkevich and C. Lorius, 1987, Vostok
ice core provides 160,000-year record of atmospheric CO2, Nature, 329,
Chappellaz, J., J.-M. Barnola, D. Raynaud, Y. S. Korotkevich and C.
Lorius, 1990, Atmospheric CH4 record over the last climatic cycle revealed by
the Vostok ice core, Nature, 345, 127-131.
Jouzel, J., C. Lorius, J. R. Petit, C. Genthon, N. I. Barkov, V. M.
Kotlyakov and V. M. Petrov, 1987, Vostok ice core: a continuous isotope
temperature record over the last climatic cycle (160,000 years), Nature,
Lorius, C., J. Jouzel, C. Ritz, L. Merlivat, N. E. Barkov and Y. S.
Korotkevich, A., 1985, 150,000-year climatic record from Antarctic ice,
Nature, 316, 591-595.
> beyond fossil fuels
> water world
> program excerpt
New Content Copyright ©2000 PBS Online and WGBH/NOVA/FRONTLINE