New Research in Science Shows Highest CO2 Levels
In 650,000 Years
Thin cut of a polar ice sample illuminated through
two polarizing filters. Grain boundaries appear in rainbow colors,
the gas bubbles enclosed in the ice are dark. The analysis of the
gas composition in these bubbles permits the reconstruction of greenhouse
gas concentrations of the past 650,000 years.
[Image courtesy of W. Berner/University
With the first in-depth analysis of the air bubbles trapped in the “EPICA
Dome C” ice core from East Antarctica, European researchers have
extended the greenhouse gas record back to 650,000 years before the present.
This 210,000-year extension of atmospheric carbon dioxide and methane
records — encompassing two full glacial cycles — should help
scientists better understand climate change and the nature of the current
warm period on Earth.
The record may also aid researchers in reducing uncertainty in predictions
of future climate change and help to clarify when humans began significantly
changing the balance of greenhouse gasses in Earth’s atmosphere.
EPICA is the European Project for Ice Coring in Antarctica. The new ice
core, initially described in 2004, is from a site in East Antarctica known
as EPICA Dome C. This work represents a long-term European research collaboration
and appears in two studies and an accompanying “Perspective”
article in the 25 November 2005 issue of the journal Science,
published by AAAS.
One study chronicles the stable relationship between climate and the
carbon cycle during the Pleistocene (390,000 to 650,000 years before the
present). The second one documents atmospheric methane and nitrous oxide
levels over the same period.
The analysis highlights the fact that today’s rising atmospheric
carbon dioxide concentration, at 380 parts per million by volume, is already
27 percent higher than its highest recorded level during the last 650,000
years, said Science author Thomas Stocker of the Physics
Institute of the University of Bern, in Bern, Switzerland, who serves
as the corresponding author for both papers.
“We have added another piece of information showing that the timescales
on which humans have changed the composition of the atmosphere are extremely
short compared to the natural time cycles of the climate system,”
The new work confirms the stable relationship between Antarctic climate
and the greenhouse gasses carbon dioxide and methane during the last four
glacial cycles. The new ice core analysis also extends this relationship
back another two glacial cycles, to a time when the warm “interglacial”
periods were milder and longer than more recent warm periods, according
to the European researchers.
The fact that carbon dioxide and methane levels were lower during the
relatively mild warm periods of the two additional cycles, compared to
the warmer warm periods of the last 400,000 years, is especially interesting
for the study of climate sensitivity, which is a measure of how the climate
system reacts when atmospheric carbon dioxide concentrations double, explained
Science author Dominique Raynaud from LGGE in Grenoble, France.
Drill head and ice core from a depth of 2,874 meters
at Dome Concordia Station, drilled on 30 November 2002. The core
is about 491,000 years old and contains a continuous time series
of greenhouse gases over the last 650,000 years.
[Image courtesy of L. Augustin/LGGE]
The new atmospheric and climate records from the EPICA Dome C ice core
also indicate that the response of the natural carbon cycle to climate
warming remains the same over time — in terms of the mechanism involved
and the degree to which greenhouse gasses further amplify climate change,
explained Science author Jean Jouzel from LSCE and Institut
Pierre Simon Laplace in France.
The EPICA Dome C ice core contains hundreds of thousands of years’
worth of atmospheric air samples within tiny bubbles trapped in the ice.
The air bubbles form when snowflakes fall, and they contain a record of
global greenhouse gas concentrations.
The new ice core record described in the two Science papers
provides some overlap with a similar record from the Vostok ice core —
now, the second longest ice core record — and extends the Vostok
record by 210,000 years.
The nitrous oxide record in EPICA Dome C is more fragmented and less
clear than the carbon dioxide and methane records due to artifacts in
the ice that appear related to the dust levels.
The new ice core analysis provides insights on our present interglacial
warm period through a glimpse into Antarctic climate and greenhouse gas
concentrations during the most recent warm period that is relatively similar
to our current warm period. Known as Marine Isotope Stage 11 or MIS 11,
this analog warm period occurred between 420,000 and 400,000 years and
is not completely covered by the Vostok record.
The similarities between our current warm period and MIS 11 are primarily
due to a similar configuration of the orbits of the Earth around the Sun:
the relative positions of the Earth and Sun are thought to be the key
driver of ice age cycles.
“MIS 11 shows us that the climate system can indeed reside in a
warm period for 20,000 or 30,000 years, something that we can’t
say based on the last three warm phases which are no longer than about
10,000 years each,” said Stocker.
We are now about 10,000 years into our current warm period.
The new papers also document MIS 13 and 15 — two warm periods more
distant than MIS 11 that may have been about as long. The idea that MIS
13 and 15 were long warm periods contrasts the argument scientists have
made in the past suggesting that our current warm period is exceptionally
The authors note, however, that the records for MIS 13 and 15 are not
as clear as they are for MIS 11. One complicating factor is that the ice
core records do not exactly match records from marine sediments that are
used to help date the ice core data.
New insights important for understanding the impact early human activities
such as land clearing and rice culture had on atmospheric greenhouse gas
concentrations, the topic of several recent studies, are also now available,
thanks to the methane and carbon dioxide records from the EPICA Dome C
ice core. The new record shows that natural variability can result in
significant oscillations in greenhouse gasses during some interglacial
periods and raises the possibility that early human activities may not
be responsible for the greenhouse gas variability seen as early as 10,000
years ago, writes Ed Brook from Oregon State University in Corvallis,
Oregon in a related “Perspective” article.
The greenhouse gas record from EPICA Dome C during past ice ages also
provides indirect evidence for abrupt climate change in the past, the
authors found. This suggests that abrupt climatic events on time scales
relevant to societies may be common features of the last climatic cycles.
The stable relationship between carbon dioxide, methane and Antarctic
climate over the last 650,000 years highlights one of the major unsolved
mysteries of climate change — the origins of climate-greenhouse
gas relationships. Organic decomposition in subtropical wetlands remains
a strong candidate for explaining the climate-methane relationship. On
the other hand, oceans seem to play a critical role in the climate-carbon
dioxide relationship; and the new work strengthens the idea that high
latitude Southern Ocean processes are important for controlling glacial-interglacial
variations in carbon dioxide, according to the “Perspective”
author who says that retrieval and analysis of even older ice cores may
provide more definitive answers.
“Stable Carbon Cycle-Climate Relationship During the Late Pleistocene,”
by U. Siegenthaler, T.F. Stocker, E. Monnin, D. Lüthi, J. Schwander
and B. Stauffer at University of Bern in Switzerland; D. Raynaud and J.-M.
Barnola at Laboratoire de Glaciologie et de Géophysique de l'Environnement
(CNRS) St Martin d'Hères Cedex, France; H. Fischer at Alfred-Wegener-Institute
for Polar and Marine Research (AWI) in Bremerhaven, Germany; V. Masson-Delmotte
and J. Jouzel at LSCE and Institut Pierre Simon Laplace in France.
“Atmospheric Methane and Nitrous Oxide of the Late Pleistocene
from Antarctic Ice Cores,” by R. Spahni, T. Stocker, G. Hausammann,
K. Kawamura, J. Flückiger and Jakob Schwander at University of Bern,
in Bern, Switzerland; J. Chappellaz, L. Loulergue and D. Raynaud at Laboratoire
de Glaciologie et de Géophysique de l'Environnement (CNRS) in St
Martin d'Hères Cedex, France; V. Masson-Delmotte, J. Jouzel at
LSCE and Institut Pierre Simon Laplace in France. K. Kawamura is now at
Scripps Institution of Oceanography, University of California- San Diego
in La Jolla. J. Flückiger is now at Institute of Arctic and Alpine
Research, University of Colorado at Boulder.
The accompanying “Perspective” article “Tiny Bubbles
Tell All,” is by E. Brook from Oregon State University in Corvallis,
The work described in the Siegenthaler et al. and Spahni et al. Science
papers is a contribution to the “European Project for Ice Coring
in Antarctica” (EPICA), a joint ESF (European Science Foundation)/EC
scientific program, funded by the European Commission and by national
contributions from Belgium, Denmark, France, Germany, Italy, the Netherlands,
Norway, Sweden, Switzerland and the United Kingdom.
The researchers acknowledge long-term financial support by the Swiss
NSF, the University of Bern and the Swiss Federal Agency of Energy, and
EC Project EPICA-MIS. Support was also provided by the French program
This news release is available in German
and in French.
Daniel B. Kane
28 November 2005