Cycles, Cycles Everywhere

Science -- Crowley 295 (5559): 1473
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Thomas J. Crowley [HN22]*
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At the recent American Geophysical Union meeting in San
Francisco, the 25th anniversary of one of the great papers
in paleoclimatology was celebrated [HN1] (1). The paper,
entitled "Variations in the Earth's orbit: Pacemaker of the
Ice Ages," [HN2] presented important new evidence
supporting the orbital theory of glaciation [HN3] (2).
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Crowley, T. J.
Orbital theory goes back over a century but is most closely
associated with Milankovitch [HN4] (3), who calculated
the effects of gravitational perturbations on the seasonal
cycle of Earth's insolation (the radiation incident at the top
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of the atmosphere). Insolation varies on several time scales,
including ~20,000 years (termed precession), ~40,000
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years (obliquity or tilt of Earth's axis), and ~100,000 and
~400,000 years (eccentricity of Earth's orbit around the
Sun) [HN5]. Together with the geographer Wladimir
Köppen [HN6], Milankovitch hypothesized that glaciations occurred when Northern
Hemisphere summer insolation was lowest. However, incomplete land records and poor
chronology prevented them from fully testing their hypothesis.
In their 1976 paper, Hays et al. reported new evidence for the Milankovitch imprint from a
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deep-sea core. Advances in the chronology of deep-sea cores [HN7] and the high resolution of
their data allowed them to detect the precession, obliquity, and ~100,000-year eccentricity
orbital periods in marine records. Although the precession and obliquity responses were linearly
related to orbital forcing, there was a much larger nonlinear response to the relatively weak
eccentricity forcing. The initial reaction to the paper was very positive, but some concerns were
raised about the possible circularity of tuning the time scale of the core to optimize the fit to
orbital forcing. This skepticism faded as evidence for a widespread imprint of orbital cycles in
the geologic record mounted (4).
In the 25 years since the publication of (2), the importance of Milankovitch cycles has
penetrated many areas of paleoclimatology. For example, John Kutzbach [HN8] (University of
Wisconsin) summarized evidence that orbital variations are now recognized to exert a strong
influence on tropical climates, especially the monsoon system. Jean Jouzel [HN9] (Laboratoire
des Sciences du Climat et de l'Environnement, Gif sur Yvette) highlighted the importance of ice
cores [HN10], which record the orbital imprint in the ice system and have provided exciting
insights into the effect of trace gas changes (such as carbon dioxide and methane) on ice volume.
Virtually all credible models of ice volume change in the Quaternary [which began 1.8 million
years ago (Ma)] require carbon dioxide changes to reproduce the observed record. Richard
Peltier [HN11] (University of Toronto) suggested that this link is so important that we may not
see future ice volume increases until the present anthropogenic perturbation has been neutralized
by the natural system (a process that is likely to take more than 10,000 years). According to
Peltier, the magnitude of the anthropogenic perturbation may mark the end of the Quaternary
(the perturbation is so large that the climate system may not settle back into its previous pattern
of oscillation even after the anthropogenic CO2 effect is reduced to its background state).
Over the past 25 years, a vast array of high-quality and high-resolution Quaternary and preQuaternary records has been collected by the Ocean Drilling Program [HN12]. These data, and
some classic land sections (see the figure), have helped scientists to stitch together a nearcontinuous orbital-scale chronology for the past ~40 million years, allowing much more precise
timing of important evolutionary and extinction events and better estimates of the timing and
rate of climate change. For example, Nicholas Shackleton [HN13] (University of Cambridge)
reconfirmed an abrupt transition in the Miocene [HN14] (~13.8 Ma) that was previously (5)
attributed to quasi-permanent expansion of the Antarctic Ice Sheet. The transition now appears
to be one of the best examples of abrupt climate change [HN15] in the pre-Quaternary and
presents an interesting target for modeling apparent instabilities in the climate system.
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Example of orbital forcing in a classic nonmarine sequence. These ~12-million-year-old
(Miocene) cyclic shallow lake sediments reflect astronomically controlled variations in lake
level. Individual cycles (alternations between dark mudstone and white carbonate) reflect the
precession cycle. Thick-thin alternations of carbonate beds in successive mudstone-carbonate
cycles in the central, regular part of the section reflect the obliquity cycle. The ~400,000-year
eccentricity cycle is visible in the lowermost part of the section (dark interval). Photograph
taken near the village Orera, northeastern Spain.
Some intervals of the Mesozoic (245 to 65 Ma) and even Paleozoic (544 to 245 Ma) have also
yielded valuable information on the pervasiveness of orbital forcing and provided new insights
into climate dynamics and the pace of biotic recovery from the Cretaceous-Tertiary (K-T)
asteroid impact [HN16]. For example, orbital theory predicts million-year time scale oscillations
in addition to the ones already mentioned. But long geologic sequences have not been available
for testing this prediction, and there are a number of uncertainties with respect to the uniqueness
of the orbital predictions on this time scale (J. Laskar [HN17], Observatoire de Paris). Paul
Olsen [HN18] (Columbia University) and colleages have found such "grand cycles" in a 33million-year sequence (~200 to 233 Ma) of Triassic-Jurassic lake deposits from New Jersey.
Identification of these periods may not only help to constrain orbital calculations but could also
provide insight into observed climate transitions on a million-year time scale. Strong ~100,000and ~400,000-year eccentricity periods in the pre-Quaternary, before the time of large ice
volume changes, appear to require some tropical feedback.
Despite the great progress in Milankovitch studies, a number of problems remain. Several
models for the 100,000-year cycle have been proposed, which all seem to require a feedback
involving the adjustment of the lithosphere to ice sheet loading (the slow response time of
rebound essentially "traps" the ice sheet in the ablation zone once warming is initiated). There is
no consensus, however, as to which is the correct model.
The transition, about 1 million years ago, from dominant 41,000-year obliquity variations to the
100,000-year cycle also eludes a consensus explanation. The cause may be a threshold effect in
the response of the system to external forcing (such as CO2 changes) or to gradual changes in
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terrain elevation in areas of ice sheet inception. Because threshold changes can be triggered by
almost any small perturbation, it may be more productive to study the internal physics of the
system than to search for a trigger.
Similarly, there is no consensus on what causes ice-age CO2 changes [HN19]. The sheer
number of explanations for the 100,000-year cycle and for CO2 changes seem to have dulled the
scientific community into a semipermanent state of wariness about accepting any particular
explanation. This places a great burden of proof on proponents of any particular theory; any
explanation seeking consensus acceptance must at this stage be characterized by mathematic
completeness and predictability against a wide array of geological data. Most importantly, the
explanation should be falsifiable--a step sometimes neglected in this and other fields.
Over the years, doubts have been raised about the basic validity of the orbital forcing
mechanism and the role of eccentricity in the 100,000-year cycle. Many of these challenges are
related to the orbital tuning approach for establishing chronologies. An impressive test of the
tuning approach (6) was the prediction that the ages of the Brunhes-Matuyama and other
paleomagnetic boundaries [HN20] were incorrect by several tens of thousands of years. Argonargon dating methods [HN21] subsequently verified these predictions.
It is hard to believe that the standard model for Quaternary climate could be wrong and yet
allow such sterling predictions. Nevertheless, high-precision dating of coral reef terraces (and
other records) for the last interglacial (7) suggests that relatively high sea level occurred
~136,000 years ago--several thousand years earlier than predicted by orbital tuning. The results
suggest an unusual and almost uncomfortable level of complexity in the interactions between
orbital forcing and the climate system for at least some deglaciations. The tropical oceans may
well be involved in these interactions.
Other dynamical interactions between orbital forcing and climate, such as the connection
between large, orbital-scale ice volume fluctuations and smaller, rapid changes of climate on
millennial time scales, are also understood incompletely. A few explanations have been offered
for these rapid changes, but again there is no consensus.
On the tectonic time scale, the continuous marine chronology will likely be extended beyond the
K-T boundary into the Mesozoic and will increasing be linked to high-resolution records on
land. Among the many benefits to be gained by this effort will be an opportunity to determine
whether there is a connection between evolving tectonics and temporal variations in the climate
system response to orbital forcing. For example, Tibetan-Himalayan uplift may affect the
amplitude and phase of the monsoon system's response to orbital forcing, and the opening and
closing of ocean gateways (such as the central American isthmus and the Drake Passage) are
likely to affect ocean heat transport and high-latitude climate change.
These problems and new data should keep scientists busy for another 25 years. By that time,
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perihelion of the Earth's orbit will have crept another 8 hours closer to the Northern Hemisphere
summer solstice. The durability of the Milankovitch hypothesis will then be even easier to
measure on astronomical time scales.
References and Notes
1. Milankovitch and Climate: Twenty-five Years Later, American Geophysical Union Fall
Meeting, San Francisco, CA, 10 to 14 December 2001 [Meeting].
2. J. D. Hays et al., Science, 194, 1121 (1976) [GEOREF] [JSTOR].
3. M. Milankovitch, K. Serb. Acad. Beogr. Spec. Publ. 132, 1941 (English translation by
Israel program for Scientific Translations, 1969) [GEOREF].
4. A. Berger et al., Eds., Milankovitch and Climate (Reidel, Dordrecht, Netherlands, 1984).
5. N. J. Shackleton, J. P. Kennett, Initial Report Deep-Sea Drilling Project, vol. 29 (U.S.
Government Printing Office, Washington, DC, 1975), pp. 801-807 [GEOREF].
6. N. J. Shackleton et al., Trans. R. Soc. Edinburgh Earth Sci. 81, 251 (1990) [GEOREF].
7. C. Gallup et al., Science 295, 310 (2002).
The author is in the Division of Earth and Ocean Sciences, Nicholas School of the Environment
and Earth Sciences, Duke University, Durham, NC 27708, USA. E-mail: [email protected]
Related Resources on the World Wide Web
General Hypernotes
Dictionaries and Glossaries
The xrefer Web site provides dictionaries in scientific and other disciplines.
A glossary of oceanography and related geosciences is provided by S. Baum, Department
of Oceanography, Texas A&M University.
A climate glossary is provided by NASA's Goddard Institute for Space Studies (GISS).
A glossary of climatology and related sciences is provided by the Carbon Dioxide
Information Analysis Center (CDIAC).
Web Collections, References, and Resource Lists
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The Google Web Directory offers Internet links to resources on paleogeography and
The WWW Virtual Library of Paleoclimatology and Paleoceanography is maintained by
P. Farrar.
The Earth Science Resources Web page of R. H. Cummins, School of Interdisciplinary
Studies, Miami University, OH, provides links to Internet resources related to
paleoclimate, greenhouse warming, El NiÒo, and climate change.
A geologic time scale is provided by the Department of Geology and Geophysics,
University of Alaska, Fairbanks.
The University of California Museum of Paleontology (UCMP) offers a geologic time
scale with links to information about the time periods.
The NOAA Paleoclimatology Program at the National Geophysical Data Center is a
central resource for paleoclimate data, research, and education. An introduction to
paleoclimatology science, links to Internet resources, and a presentation titled "A paleo
perspective on global warming" are provided.
Online Texts and Lecture Notes
The Earth2Class educational Web site offers a presentation by J. Ortiz on
paleoclimatology titled "Perspectives on paleoclimate - The growth of a modern science."
Planet Earth and the New Geosciences is a hypermedia textbook by V. Schmidt and W.
Harbert, Department of Geology and Planetary Science, University of Pittsburgh. A
chapter on the climate puzzle is included.
Fundamentals of Physical Geography, an online text by M. Pidwirny, Department of
Geography, Okanagan University College, Kelowna, BC, Canada includes a section on
meteorology and climatology with a presentation on the causes of climate change.
The University of Michigan Global Change Program makes available lecture notes by P.
Samson on the paleoclimate record and climate models and paleoclimatology and climate
The Department of Earth and Environmental Sciences, Columbia University, makes
available lecture notes for a course on the climate system.
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S. Fitzsimons, Department of Geography, University of Otago, New Zealand, provides
lecture notes for a course on climate change of the past.
R. T. Patterson, Department of Earth Sciences, Carleton University, Ottawa, provides
lecture notes, readings, and other resources for a course titled "Climate change: A
geological perspective."
The School of the Environment, University of Leeds, UK, makes available lecture notes
for a course on the scientific issues of climate change. A presentation on time scales and
Milankovitch cycles is included.
R. Behl, Department of Geological Sciences, California State University, Long Beach,
offers lecture notes for a course on Earth systems and global change.
O. Davis, Department of Geosciences, University of Arizona, provides lecture notes for a
course on Quaternary ecology. A presentation on paleoclimatology is included.
General Reports and Articles
The Winter 1996 issue of Consequences had an article by T. Crowley titled
"Remembrance of things past: Greenhouse lessons from the geologic record."
R. Muller, Physics Department, University of California, Berkeley, makes available the
first chapter of his book Ice Ages and Astronomical Causes with introductions to the
history of climate, ice age theories, and spectra.
The 27 April 2001 issue of Science had a special section on paleoclimate. Included was a
review by J. Zachos et al. titled "Trends, rhythms, and aberrations in global climate 65
Ma to present." The 23 July 1999 issue had a News of the Week article by R. Kerr titled
"Why the ice ages don't keep time" about the report by J. Rail titled "Pacemaking the ice
ages by frequency modulation of Earth's orbital eccentricity" and the report by K. J. Willis
et al. titled "The role of sub-Milankovitch climatic forcing in the initiation of the Northern
Hemisphere glaciation."
Global Environmental Change: Research Pathways for the Next Decade is a 1999
National Research Council report, available from the National Academy Press. A
paleoclimate overview and a chapter on modeling are included.
The December 2001 issue (a special issue on geosciences and climate change) of
Geotimes had an article by M. Huber titled "Global climate change: A glance in the rear
view mirror."
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Numbered Hypernotes
1. The Fall 2001 meeting Web page of the American Geophysical Union (AGU) provides
program information for the session (part one and part two) titled "Milankovitch and
climate: Twenty-five years later."
2. 1976 Science article. The article "Variations in the Earth's orbit: Pacemaker of the Ice
Ages" by J. D. Hays, J. Imbrie, and N. J. Shackleton appeared in the 10 December 1976
issue of Science; the full-text from JSTOR is accessible from the AAAS Members-Only
Web site. J. Hays is in the Department of Earth and Environmental Sciences and at the
Lamont-Doherty Earth Observatory, Columbia University; J. Imbrie is professor emeritus
at the Department of Geological Sciences, Brown University; and N. Shackleton is at the
Godwin Institute for Quaternary Research, Department of Earth Sciences, University of
3. The orbital theory of glaciation. The Milankovitch (or astronomical) theory is defined in
S. Baum's glossary and in the GISS climate glossary. The NOAA Paleoclimatology
Program provides an introduction to the astronomical theory of climate change. The
Glaciers and Glacial Geology Hypertext, prepared by students for a course on glacial
geology taught by W. Locke, Department of Earth Sciences, Montana State University,
Bozeman, includes a presentation on the Milankovitch cycles and glaciation. The Global
Climate Change Student Guide, provided by the ARIC Web site of the Department of
Environmental and Geographical Sciences, Manchester Metropolitan University, UK,
includes an introduction to Milankovitch cycles and ice ages; the paleoclimate chapter has
a section on orbital variations. O. Davis provides lecture notes on the astronomical theory
of climatic change for a course on Quaternary ecology. The report of the 1999
International Workshop for a Climatic, Biotic, and Tectonic, Pole-to-Pole Coring Transect
of Triassic-Jurassic Pangea includes an introduction by P. Olsen to climate, astronomical
forcing, and chaos.
4. A brief biographical entry for Milutin Milankovich is provided in xrefer's Dictionary of
Scientists. The European Geophysical Society (EGS) provides a biographical profile of
Milutin Milankovic. NASA's Earth Observatory offers a feature on Milankovitch and his
5. The effect of precision, obliquity, and eccentricity on insolation. Insolation is defined
in the CDIAC glossary. Milankovich cycles are defined in xrefer's Dictionary of Earth
Sciences. Precision, obliquity, and eccentricity are defined in S. Baum's glossary. M.
Pidwirny's Fundamentals of Physical Geography includes a section on Earth-Sun
relationships and insolation. The Astronomical Applications Department of the U.S.
Naval Observatory offers a fact sheet by G. Kaplan titled "The seasons and the Earth's
orbit -- Milankovitch cycles." S. Rutherford, Graduate School of Oceanography,
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University of Rhode Island, offers an illustrated presentation on Milankovitch cycles in
paleoclimate. The Illinois State Museum's Ice Ages exhibit offers a graph of insolation for
the past 750,000 years, as well as an introduction to the Milankovitch factors. This
Restless Globe, from the teachers' newsletter of the Astronomical Society of the Pacific, is
a presentation on obliquity, eccentricity, and precision. G. R. Heath, School of
Oceanography, University of Washington, provides lecture notes on orbital geometry for a
course on marine geological processes. J. Corripio, Department of Geography, University
of Edinburgh, presents two applets for calculating the orbital parameters and solar
irradiation at any time in the past according to the Milankovitch theory.
6. An entry for Wladimir Köppen is included in the Academic Press Dictionary of Science
and Technology. A brief biographical entry for Wladimir Peter Köppen is included in
xrefer's Dictionary of Scientists. A brief biography (in German) of Köppen is provided by
the Universitätbibliothek Graz, Austria. The Blue Planet Biomes Web site offers a
presentation on the K–ppen Climate Classification System.
7. Deep-sea cores. An introduction to deep-sea cores is presented by the British Ocean
Sediment Core Repository. The Deep-Sea Sample Repository at Lamont-Doherty Earth
Observatory provides an introduction to deep-sea cores; a virtual tour is provided by the
Earth2Class Web site. O. Davis provides lecture notes on ocean cores for a course on
Quaternary ecology.
8. J. Kutzbach is at the Center for Climatic Research, University of Wisconsin. EGS
awarded Kutzbach the 2001 Milutin Milankovic Medal. "Monsoons and Milankovitch"
was the title of Kutzbach's AGU presentation (abstract).
9. J. Jouzel is at the Laboratoire des Sciences du Climat et l'Environnement, Gif sur Yvette,
France. Jouzel's AGU paper (abstract) was titled "Milankovitch and ice core records."
EGS awarded Jouzel the 1997 Milutin Milankovic Medal.
10. Ice cores. The USGS National Ice Core Laboratory (NICL) provides introductions to the
why and the how of ice cores. Secrets of the Ice, a presentation of the Museum
of Science, Boston, includes a section on ice core research. A presentation by P. Tyson
titled "Stories in the ice" is provided by NOVA's What's Up with the Weather Web site.
The 1 April 1999 issue of Environmental Science and Technology had an article by D.
Schoen titled "Learning from polar ice core research." AGU makes available a 1995
article by the Vostok Project Members titled "Deciphering mysteries of past climate from
Antarctic ice cores." The 10 December 1999 issue of Science had a report by J. Jouzel et
al. titled "More Than 200 meters of lake ice above subglacial Lake Vostok, Antarctica."
The 3 June 1999 issue of Nature had an article by J. R. Petit et al. titled "Climate and
atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica." The
French Centre National de la Recherche Scientifique (CNRS) issued a 3 June 1999 press
release titled "420,000 years of atmospheric history revealed by the Vostok ice core,
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11. W. R. Peltier is in the Paleoclimate Research Group, Department of Physics, University
of Toronto. His AGU presentation (abstract) was titled "The 100 Kyr ice age cycle:
Inception and demise."
12. The Ocean Drilling Program. The Ocean Drilling Program Web site provides an
introduction to the program. The online Columbia Encyclopedia has an entry for the
earlier Deep Sea Drilling Project. The August 2001 issue of Geotimes had an article by K.
White titled "ODP: International Earth science." The Integrated Ocean Drilling Program
Web site provides an online brochure with a section on the importance of ocean drilling.
13. N. Shackleton (and also) is in the Palaeoceanography Group at the Godwin Laboratory,
Godwin Institute for Quaternary Research, Department of Earth Sciences, University of
Cambridge. Shackleton's AGU paper (abstract) was titled "Milankovitch in the Miocene."
14. The Miocene epoch. The online Columbia Encyclopedia includes an entry for the
Miocene epoch. UCMP provides information about the Miocene epoch and its climate. C.
Scotese's PALEOMAP Project Web site provides a map of the Middle Miocene, a map of
Miocene climate, and an introduction to the Miocene.
15. Abrupt climate change. The July-August 1999 issue of American Scientist had an
article by K. Taylor titled "Rapid climate change." The 15 February 2000 issue of the
Proceedings of National Academy of Sciences had an editorial, nine perspectives, and six
research articles related to rapid climate change. Nature had a 11 January 2001 Feature of
the Week on ice ages and instabilities; included is a News and Views article about rapid
climate change by R. Alley titled "Paleoclimatology: Icing the North Atlantic." Abrupt
Climate Change: Inevitable Surprises is a 2001 National Research Council report
available from the National Academy Press. The 21 December 2001 issue of Nature had a
Science Update about this report by T. Clarke titled "Abrupt climate change likely." The
February 2002 issue of Geotimes had a News Note by L. Pinsker about the report titled
"Cracking abrupt climate change."
16. Recovery from the Cretaceous-Tertiary (K-T) impact. The
Department of Paleobiology, Smithsonian National Museum of Natural History, offers a
presentation on the K-T boundary. The KT Event is a student project made available by
the Palaeontology Research Group at the Department of Earth Sciences, University of
Bristol, UK. UCMP makes available an essay by R. Cowen on the K-T extinction; the
Web page for the third edition of R. Cowen's History of Life includes a resource page on
the K-T extinction. The January-February 1997 issue of American Scientist had a Science
Observer article by L. Penvenne titled "The bolide and the biosphere." The 28 February
1997 issue of Science had a Research News article by R. Kerr titled "Cores document
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ancient catastrophe." The 23 November 2001 issue had a Perspective by T. Flannery titled
"North American devastation or global cataclysm?" and a report by V. Vajda, J. I. Raine,
and C. Hollis titled "Indication of global deforestation at the Cretaceous-Tertiary
boundary by New Zealand fern spike." P. Olsen provides lecture notes on the K-T
extinction for a course on dinosaurs and the history of life. R. Taggart, Department of
Geological Sciences and Department of Plant Biology, Michigan State University, offers
lecture notes on the K/T boundary impact hypothesis for a course on the history of life. D.
Russell, Department of Marine, Earth and Atmospheric Sciences, North Carolina State
University, includes a chapter on the extinction of dinosaurs and other ecological effects
of the K-T impact in the online text for a course on dinosaurs. The August 2000 issue of
GSA Today had an article by D. Kring titled "Impact events and their effect on the origin,
evolution, and distribution of life." BBC News offers a 22 November 2001 article by H.
Briggs titled "Dino asteroid led to 'global devastation'."
17. J. Laskar is at the Institut de MÈcanique CÈleste, Observatoire de Paris. His AGU paper
(abstract) was titled "Astronomical solutions for paleoclimates studies."
18. P. Olsen (and also) is at the Lamont-Doherty Earth Observatory and in the Department of
Earth and Environmental Sciences, Columbia University. Olsen's
AGU presentation (abstract) was titled "Grand cycles of the Milankovitch band." The
Newark Basin Coring Project documents the record of astronomically driven climate
cycles preserved in 200- to 230-million-year-old lake sediments by drilling a series of
core holes in the Newark rift basin.
19. Ice age CO2 changes. The Global Climate Change Student Guide has a section on CO2
feedback in the chapter on paleoclimatic changes. Carbon Dioxide and Climate Change,
made available by the National Academy Press, is the proceedings of a National Academy
of Sciences colloquium (originally published in the 5 August 1997 issue of the
Proceedings of the National Academy of Sciences). The 15 September
2000 issue of Science had a News Focus article by R. Kerr titled "Ice, mud point to CO2
role in glacial cycle" about a Research Article in that issue by N. Shackleton titled "The
100,000-year ice age cycle identified and found to lag temperature, carbon dioxide, and
orbital eccentricity." The 13 October 2000 issue had a Review by P. Falkowski et al.
titled "The global carbon cycle: A test of our knowledge of Earth as a system." The 4 May
2001 issue had an Enhanced Perspective by T. Crowley and R. Berner titled "CO2 and
climate change."
20. Brunhes-Matuyama and other paleomagnetic boundaries. The Brunhes and
Matuyama polarity chrons are defined in xrefer's Dictionary of Earth Sciences; a
biographical entry for Motonori Matuyama is provided by xrefer's Dictionary of Scientists.
An entry for magnetic field reversals is included in S. Baum's glossary. Magnetic Fields
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of the Planets, a student Web project prepared for the Department of Geological Sciences,
University College London, includes a brief history of observations of Earth's magnetic
field. A section on the geomagnetic reversal time scale is provided in a presentation on
paleomagnetism by L. Tauxe, Paleomagnetic Laboratory, Scripps Institution of
Oceanography. O. Davis provides lecture notes on the paleomagnetic time scale for a
course on Quaternary ecology. J. B. Bennington, Department of Geology, Hofstra
University, Hempstead, NY, offers lecture notes on paleomagnetism for a historical
geology course. The U.S. National Report to IUGG, 1991-1994 includes an article by J.
Channell titled "Recalibration of the geomagnetic polarity timescale." AGU makes
available a 1995 article by K. Hoffman titled "How are geomagnetic reversals related to
field intensity?"
21. Argon-argon dating is defined in S. Baum's glossary. A section on Ar isotope geology is
provided in M. Palmer's course on isotope geochemistry, made available by the
Department of Earth Sciences, University of Bristol, UK.
22. T. J. Crowley is in the Division of Earth and Ocean Sciences, Duke University.
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