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An understanding of long-term climate changes, with some
surprises, is emerging from analysis of ice core samples.
DEBORAH SCHOEN
In 1998, the
Russian-French-American ice-coring team at the Vostok station in
eastern Antarctica reached a depth of 3623 m in the ice. The
technical feat of extracting ancient ice samples from the
coldest spot in the world has been, in itself, an impressive
accomplishment, providing a continuous ice core record spanning
420,000 years. More extraordinary, however, are the histories
preserved in the ice cores from Vostok, other Antarctic
stations, and Greenland. These records have contributed
surprising information on long-term climatic phenomena,
underlining the importance of looking beyond the snapshot
provided by recent instrument-based monitoring.
Physical and chemical--particularly
isotopic--analyses of ice cores have provided information on a
wide array of parameters, including age, temperature at the time
of ice formation, aerosol deposition, and composition of the
atmosphere. In the case of the Vostok ice core samples, such
measurements enabled glaciologist Claude Lorius and his
colleagues at the French laboratories Laboratoire de Glaciologie
et Géophysique de l'Environnement, Grenoble, France (LGGE) and
Laboratoire des Sciences de l'Environnement, Saclay, France (LSCE)
to estimate the atmospheric temperature at the time that the ice
formed and, through analysis of air bubbles trapped in the ice,
to describe the composition of the ancient atmospheres. They
found a strong positive correlation between temperature changes
and changes in carbon dioxide and methane; this has contributed
empirical evidence of the magnitude of climatic feedback between
increasing levels of greenhouse gases and temperature (1).
Greenland ice cores are not as deep as
those in Antarctica, but they provide greater resolution with
respect to time. The Greenland cores best reflect the rapid
temperature fluctuations of the last ice age, characterized by
increases of up to 6 oC in a few years or decades.
The discovery of these fluctuations, known as Dansgaard-Oeschger
oscillations after the Danish and Swiss investigators who first
documented them, has given rise to intense interest in their
causes and speculation that current increasing levels of
greenhouse gases in the atmosphere could trigger such rapid
change in the coming decades.
The importance of the ice core record was
recognized by the North American environmental science community
in 1996, when Claude Lorius, Willi Dansgaard, and Hans Oeschger
received the Tyler Prize for Environmental Achievement. Their
work has stimulated efforts for continued polar ice core
research and more investigation into other paleoclimate records,
such as marine sediments, and provided essential input for the
modeling of past climates. Researchers argue that paleoclimate
models, by addressing the full range of variability documented
by ice core records, can advance our understanding of climate
mechanisms--mechanisms that may produce only gradual changes
over the coming millennia or, on the other hand, bring about an
abrupt and fundamental disruption of the relatively benign
climate that we know today.
Antarctic ice cores
Ancient ice has been extracted and analyzed at research stations
throughout Antarctica for more than three decades. In the
coastal areas, however, lateral ice flow makes it difficult to
obtain undisturbed records. To obtain continuous, long-term data
from the last glaciation (ice age) and beyond, researchers have
gone inland to the high plateau, to Byrd Station in western
Antarctica, and to Dome C and Vostok stations in eastern
Antarctica.
In the 1970s, the Soviet Mining Institute
set up the drilling station at Vostok. Because of a personal
friendship with V. M. Kotlyakov, the lead scientist at Vostok
and the director of the Institute of Geography of the Russian
Academy of Sciences, Claude Lorius was able to arrange French
participation in the drilling project. "The mining institute had
experience drilling in ice and measuring ice deformation and
temperature in situ. The French and the Americans had the
laboratory technology for analyzing the ice," he explained.
"When we first arrived at Vostok in the early 1980s, there were
two kilometers of ice already extracted, and we took samples
back to France for analysis."
At the Vostok station in Antarctica, scientists are obtaining
ice core samples down to ice depths exceeding 3600 m. (Courtesy
Claude Lorius, LLGE)
Lorius and his colleagues analyzed the
chemical and isotopic composition of the ice and its trapped air
bubbles. Proxy data for temperature were obtained from the
deuterium (D)-to-hydrogen ratio. This was based on a previously
observed linear relationship between deuterium concentration in
Antarctic ice and local temperature, an observation attributed
to kinetic and vapor pressure differences between DHO and H2O.
They determined the composition of the ancient atmospheres
through direct analysis of trapped air bubbles; an age
correction factor was estimated to account for the time lapse
between the formation of the precipitation and the isolation of
air bubbles from the atmosphere (1).
The scientists dated the ice core based on ice flow modeling
using electrical conductivity measurements, ice accumulation
changes, and correlation with other paleoclimatic records (2).
The pattern of temperature climate change
recorded in the Vostok ice core supports the orbital theory of
ice ages, in which the timing of glaciation cycles is attributed
to the periodicity of changes in the shape of the Earth's orbit
(eccentricity), the tilt of the Earth's axis (obliquity) and the
timing of its closest approach to the Sun (precession). The
astronomical effect is evident in the Vostok record, with a
strong eccentric signal, noted Lorius. "The amount of energy
coming to the Earth doesn't change much but the [latitudinal]
distribution of the energy does, and this can affect the
building or decay of northern hemisphere polar ice sheets. A
decrease in solar input at high latitudes, for example, can lead
to building ice sheets. This, in turn, reinforces the orbital
effect, as more radiation is reflected away."
It is the ice core
data on greenhouse gases, however, that led Lorius and his
colleagues to conclude that these gases, as well as the ice
sheets, "played a significant part in the glacial-interglacial
climate changes, by amplifying the relatively weak orbital
forcing and by constituting a link between the Northern and
Southern Hemisphere climates." The magnitude of the change in
greenhouse gas concentrations was in itself unexpected (see
figure on next page). At the last glacial maximum (21,000 years
before present), for example, carbon dioxide is estimated to be
190-200 ppm, compared with the 270- to 280-ppm preindustrial
average for the Holocene (last 10,000 years). The warming that
occurred between the glaciation and the Holocene was
approximately 10 oC over Antarctica or 4-5 oC
when averaged globally (1).
The changeable climate of Earth
Where did the carbon dioxide come from?
"This is one of the grand unsolved puzzles in climate research,"
said Thomas Stocker, a climate modeler at the Physics Institute
of the University of Bern. "About 50% of the 80-ppm
glacial-to-interglacial increase can be explained by a change in
the solubility of carbon dioxide. Warmer ocean water carries
less carbon dioxide than colder water. However, there are
complicated biochemical processes in the ocean, such as pH, the
depth of the dissolution level for calcium carbonate, and the
net primary productivity of the marine carbon cycle that are
also playing a role."
Another puzzle concerns the sequence of
forcing events--what climate researchers refer to as the
"chicken and egg" problem. Lorius argues, however, that our lack
of knowledge of the mechanisms and timing of climate change does
not preclude an assessment of the role of greenhouse forcing in
past climatic shifts. Through a multivariate analysis in which
the temperature variance was decomposed into forcing
factors--greenhouse gases, dust and non-sea-salt sulfate, ice
volume, and local insolation--Lorius and his colleagues
estimated that greenhouse gases were responsible for at least
40% of the glacial-interglacial temperature change, or roughly 2
oC of the global average. This estimate implies a
considerable climate feedback resulting from increased
greenhouse gas concentrations, in that direct greenhouse
radiative forcing with no climate feedback was estimated to have
an amplitude of only 0.7 oC (1).
"This was a first attempt at evaluating the magnitude of the
feedback from the ice core data," said Lorius. "But estimates
derived independently from general circulation models give
rather close results."
The changeable climate of Earth
Greenland ice cores
Ice coring in Greenland dates back to the 1950s, when scientists
with the U.S. Cold Regions Research and Engineering Laboratory
in Hanover, N.H., set up drilling stations. Many of the
investigations at this time were in conjunction with the
International Geophysical Year, an international research effort
that focused on the Arctic and Antarctica. In the late 1960s and
early 1970s, glaciologists Willi Dansgaard, of the University of
Copenhagen, and Hans Oeschger, of the University of Bern,
pioneered developing techniques for dating the ice, measuring
temperature, and analyzing the chemical composition of the ice
and trapped air bubbles.
Greenland cores do not date back as far as
those from the Antarctic high plateau, although recent drillings
date to the last interglacial, the Eemian (135,000-115,000 years
ago). However, because of greater snow accumulation and less
compression of the ice, the climatic history of the Northern
hemisphere ice sheets is preserved with a years-to-decades
resolution in contrast to the century-to-millennial scale
resolution of the Antarctic cores. Scientists can directly date
the Greenland ice for the past 14,500 years by counting annual
layers. Beyond that time, the ice is dated through ice flow
modeling; proxy temperature data are derived from the ratio of
18O/16O (3).
Greenland also receives considerably more dust than Antarctica.
Dust particles, through chemical reactions with the trapped air,
can interfere with the atmospheric carbon dioxide record. They
provide unique information, however, on the atmospheric
circulation at the time of deposition (4).
The discovery of abrupt climatic shifts, or
Dansgaard-Oeschger (D-O) oscillations, was the most surprising
feature of the Greenland ice core data. The 24 D-O oscillations
of the last glaciation are marked by temperature increases over
Greenland of up to 6 oC, in a time span of less than
a decade, followed by a more gradual return to the cooler
glacial climate (3). These events are
not clearly manifested in the Vostok record. However, diverse
paleoclimatic evidence, including marine sediment data, pollen
profiles, and glacial snow line data, indicates that the effects
of at least some events were felt on a global scale (5).
The thermohaline circulation
The reports of the Dansgaard-Oeschger fluctuations have since
generated tremendous excitement in the climate sciences
community. "They indicate a climate capable of violent change,"
said Lawrence Mysak, a climate dynamicist at McGill University's
Center for Climate and Global Change Research in Montreal. One
area of Mysak's research focuses on understanding the mechanisms
for such abrupt climate shifts through coupled modeling of
atmospheric and ocean processes (6).
"Today, we think the only way such events could have happened is
through major changes in the ocean's thermohaline circulation."
The thermohaline circulation is a
pole-to-pole overturning circulation in the Atlantic, which
drives warm water north from the tropics, raising Europe's
temperature an estimated 5 o to 10 oC. At
high latitudes the water is cooled by the winds off of Canada
and Greenland and, because cool water is denser than warm water,
it sinks to the bottom. While storms add freshwater, making the
cooled water lighter, the cooling effect nonetheless overcomes
the buoyancy effect of the freshwater input. The deep-bottom
flow then turns south, eventually reaching the Antarctic
circumpolar current. The North Atlantic downwelling draws more
warm replacement water up from the tropics. Thus, a cooler
Europe would be the most direct manifestation of a weakening or
shutdown of the thermohaline circulation.
What could have caused a major shift in the
operation of the thermohaline circulation? Wallace Broecker of
Columbia University's Lamont-Doherty Earth Observatory has
hypothesized that six Heinrich events--the "melting of huge
armadas of icebergs"--that curred during the last glaciation
could have provided the freshwater to shut down the thermohaline
circulation (7). Syukuro Manabe and
Ronald Stouffer, climate modelers with the Geophysical Fluid
Dynamics Laboratory/NOAA at Princeton University, investigated
the effects of such a massive flux of freshwater to the North
Atlantic using a coupled ocean-atmosphere model (8).
They concluded that in response to the flux of freshwater, "the
thermohaline circulation weakens abruptly, intensifies, and
weakens again, followed by a gradual recovery, generating
episodes that resemble the abrupt changes [D-O oscillations] of
the ocean-atmosphere system recorded in ice and deep-sea cores."
"This is why it's so important to monitor
the Arctic sea ice cover," says Mysak. "As the ice shrinks due
to global warming, the exposed ocean warms the atmosphere,
leading to more breakup of the ice and a greater flux of
freshwater to the North Atlantic." Scientists monitoring the
Arctic have indeed observed a net decrease of 0.6% per year in
the summer sea ice cover for the period between 1978 and 1995.
The change, however, has not been uniform across the Arctic
Ocean, as most of the decrease occurred in the Siberian sector.
The trend is compatible with observed increases in temperature,
but because of the high interannual climatic variability of the
region, researchers have not yet determined if the decrease in
sea ice cover is part of an Arctic-wide response to global
change (9).
Predictive modeling
Understanding how the ocean's thermohaline circulation and its
interaction with other climate processes could produce the
extreme variability that has been recorded in ice cores is part
of an increasingly influential area in climate change research
known as paleoclimatic modeling. Models used to simulate past
climates are generally simpler than three-dimensional general
circulation models, allowing finer tuning through a greater
number of computer runs. The simpler models can also be run over
thousands of years to track the climatic response to an
experimental condition, such as freshwater input.
Modeling the past, in effect, provides the
link between the ice core data and models currently used to
forecast climate change. As Anthony Broccoli, a climate modeler
with the Geophysical Fluid Dynamics Laboratory explained, "The
ice core records contain information about past concentrations
of greenhouse gases, volcanic aerosols, and atmospheric dust,
all of which influence the radiation balance of the earth. This
information is essential as input to model simulations of
ancient climates. Ice cores also tell us about past temperature,
precipitation, atmospheric circulation, and other climatic
variables. We can use such information to determine if our
simulations can reproduce important climatic features of the
past. This is an important way in which we evaluate the
performance of climate models."
How well do current models recreate past
climates? Stocker believes the major processes are well
described but that there is considerable uncertainty regarding
the effects of cloud cover and height and condensation
processes, as well as regarding the response of small-scale
processes, such as heat and freshwater fluxes across the air-sea
interface, the behavior of sea ice, and deep water formation. In
addition, he argues for greater attention to the role of the
tropics and low latitudes, particularly the hydrological cycle (10).
An unprecedented rate of
increase
The Vostok core showed that atmospheric carbon dioxide increased
by approximately 80 ppm over a span of 10,000 years, between the
Last Glacial Maximum and the pre-industrial times of the
Holocene. In the past 200 years the atmospheric carbon dioxide
has again increased by 80 ppm to the present 360 ppm, most of
the increase in the past 50 years. "What is new is the rate of
increase," Mysak emphasized. Adding that the scientific
uncertainties regarding climate change are not limited to the
physical processes, Mysak commented, "How robust are ecosystems
in handling this carbon dioxide? We don't know."
Stocker, as well,
underlines the significance of the rate of increase in
greenhouse gas concentrations. He and colleague Andreas
Schmittner looked at the problem through experiments with a
simple, coupled atmosphere-ocean climate model in which a final
carbon dioxide concentration of 750 ppm was attained over
different time spans. They found that the thermohaline
circulation weakens when the increase in carbon dioxide to 750
ppm is relatively slow, spanning several centuries or more.
However, when the rate of increase in atmospheric greenhouse
gases (expressed as CO2) is similar to today's rate
of growth (1% per year)--or the concentration of 750 ppm is
reached in 100 years--the thermohaline circulation permanently
shuts down (11). The
quantitative results are dependent on model parameters and their
associated uncertainties. Nonetheless, the lesson from the
modeling experiments is that the climate is sensitive not only
to the level of carbon dioxide in the atmosphere, but also to
the rate at which this level is attained. This is relevant to
policy decisions concerning the timing of reductions in
greenhouse gas emissions, noted Stocker. "It demonstrates that
early reductions of emissions make sense."
References
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