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Just as we have different time scales (seconds, days, weeks, years, centuries, forever) for different purposes, so weather and climate have different time scales. Indeed,

• weather means what's happening today (it's always changing), while
• climate change refers to the longer-term patterns of weather, such as changes in the annual high and low temperatures, the annual rainfall or cloud cover, etc.

• El Niño's time scale (within a decade),
• droughts can last for many decades.
• global warming's time scale of the next century or two, and
• the 100,000-year-cycle of the ice ages.

• the North Atlantic Oscillation, also on the time scale of El Niño but primarily affecting Europe and the Atlantic Coast of North America, and
• abrupt climate flips, which take just a decade to shift the whole earth's temperature to a very different level. Within a few centuries, it usually flips back. This is the equivalent of jacking the landscape up (or down) a few thousand feet; forests die and are slowly replaced by more suitable species -- but, in the meantime, life is very hard on the animals. Our ancestors endured dozens of these climate flips just during the last 120,000 years.

Abrupt climate changes have been studied since 1982, and have won major scientific prizes for the scientists studying them, but (at least in the U.S.) they have rarely been discussed outside the scientific community. The abrupt flips appear to share mechanisms with both the ice ages and the North Atlantic Oscillation, and it looks as if our gradual global warming is capable of triggering an abrupt, catastrophic cooling.

You can click to view my favorites for anthropology, biology, cognitive sciences, ethology, climate, evolution, brains, language, the future -- not to mention Patrick O'Brian novels and the Science Masters series.

Abrupt Climate Change We now know that global climate can undergo a very rapid change in just a decade. Abrupt coolings and warmings happened dozens of times in the last ice age, and a severe cooling even occurred during the last warm period like today's (another one ended it). It seems likely that CO2-induced greenhouse warming could trigger another such episode, which would be a far more serious problem than the greenhouse concerns thus far identified. From my "The Great Climate Flip-flop" (The Atlantic Monthly's cover story for January 1998). I've now posted my replies to the various letters to the editor.

Webbed Reprint Collection William H. Calvin University of Washington Seattle WA 98195-1800 USA Email || Home || publication list

I assume people are coming to this web page for several reasons, either

• to learn more about abrupt climate change, e.g.,

  W. S. Broecker, "What If the Conveyor Were to Shut Down? Reflections on a Possible Outcome of the Great Global Experiment," GSA Today 9(1):1-7 (January 1999). See also http://www.geosociety.org/pubs/gsatoday/gsat9901.htm

  Wallace S. Broecker (1997). "Thermohaline Circulation, the Achilles Heel of Our Climate System: Will Man-Made CO2 Upset the Current Balance?" Science 278(5343, Issue of 28 November 1997), pp. 1582 - 1588. And see "Warming's Unpleasant Surprise: Shivering in the Greenhouse?" Richard A. Kerr, Science news article (10 July 1998).

  Jonathan Adams, "Sudden (decade-timescale) transitions and short-lived cold and warm phases in the global climate record," at http://www.esd.ornl.gov/ern/qen/transit.html, is an excellent review, and there are many new references later on this web page.

• or to try and figure out where the author of "The Great Climate Flip-flop" is coming from. Ever since segments of the fossil fuels industry undertook a confuse-the-public campaign reminscent of the tobacco industry's thirty-year-long "It's not really proven" public relations gambit, anyone who writes about climate needs to supply some background information.

1) How is it that you came to have such an interest in climate change and global cooling? How does the study of these topics inform your other scientific research, and vice versa? By the 1980s, it was getting pretty obvious that a number of things started happening 2.5 million years ago: the ice ages began, our ancestors starting making lots of tools, and their brain size started to increase, leading to our brains that are four times larger than our closest cousins among the apes. I'm a neurophysiologist, mostly concerned with how brain circuits create our abilities to speak with syntax, to plan ahead, to make music. But I also ask how this circuitry come into being, what it was about the ice ages that started and sustained the brain's enlargement and reorganization.      And so, back about 1982 when the paleoclimate researchers started getting such fascinating results from the Greenland ice cores, I began paying close attention, regularly attending a seminar series [at the UW's Quaternary Research Center, organized by Stephen C. Porter] that brings many of the major researchers to Seattle to give talks. During the last fifteen years, I've been privileged to see a highly significant scientific story develop, its leaders receive major awards -- and I've puzzled over why the news media never had anything to say about the topic, even when well written news stories in Science and Nature should have helped to pave the way for science reporters to write about flip-flop climate tendencies. This isn't just another annoying detail of the greenhouse warming, but an abrupt cooling, one that has happened many times before, and potentially a catastrophic consequence of gradual warming. So while I'm puzzled about why it takes a neurophysiologist to finally write this story, I've also had ten years to think about our civilization's prospects, and a long association with the futurists has helped me to think in terms of alternative scenarios. PROGRESS TO REPORT: The New York Times has an excellent news article January 27, 1998 by William K. Stevens, "If the Climate Changes, It May Do So Fast, New Data Show." 2) I remember your Web site describing the fact that you wear many hats. How would you describe your professional self -- both in terms of the name of your profession and in terms of your many interests? I'm primarily a theoretical neurophysiologist (e.g., last year's research monograph The Cerebral Code from MIT Press). And I also write books about scientific topics for general readers (e.g., How Brains Think, which is getting more than a dozen translations). Evolution is heavily featured, both on the long time scale of the ice ages (how to evolve a brain capable of structuring sentences and plans) and on the short time scale of thought and action (how we quickly shape up sentences to speak that we've never spoken before).      But there are lots of related interests such as archaeology, apes, and linguistics [the evolution of syntax is the next book, written with the linguist Derek Bickerton, and entitled Lingua ex machina: Reconciling Darwin and Chomsky with the Human Brain]. I had a good liberal education forty years ago, while majoring in physics, and sometimes I think that I've subsequently made use of almost every single undergraduate course that I took at Northwestern University between 1957 and 1960.     [I'd add that, while I've wished since 1988 that someone would write an article on the climate flip-flop for general readers, I kept waiting for someone else to write it. When the editor of The Atlantic Monthly approached me in April 1997 and asked me to write it, I tried for some weeks to get him to find an insider to do it instead of me. Somehow, Bill Whitworth knew the story waiting to be told, and went looking for someone who could write it. Certainly, writing this story has given me some sympathy with the science reporters who didn't write it earlier; it's turned into one of the most ambitious writing jobs that I've ever undertaken, what with the need to invent general-reader-level analogies for both the climate change data and the oceanographic mechanisms, the need to explain how to stabilize nonlinear mechanisms, and then the need to sketch out three futuristic scenarios for climate stabilization. In particular, I felt the need to explain why I was reasonably optimistic -- given what, on the face of it, seems like a pretty depressing prospect. In my opinion, stabilizing the flip-flop aspect of climate is likely within the capabilities of our civilization's science and technology (and the economy that supports them); to think of "doing something" as surely involving the far-out scale of science-fiction terraforming is, I believe, a form of fatalism that might well be fatally foolish.] If you really need more author background, visit the web page that I created for lecture organizers. My 1997 talks on abrupt climate change are available as Powerpoint slides, for those with a fast network connection and sufficient patience. They're organized around the available graphics (many discovered via the web), unlike the magazine article which is in the tradition of unadorned prose. My hour-long radio interview on The Paula Gordon Show, "Civilization: Use It Or Lose It," on abrupt climate change can be heard on the web via RealAudio. "Climate instability and hominid brain evolution," is the abstract for my June 1998 talk to the American Geophysical Union's Chapman Conference, Mechanisms of Millennial-Scale Global Climate Change.

Recent Additions to the Reference List

There are two recent books which are exactly what you might want to give policymakers to help them sort through the more general issues of climate change.  Anyone who wishes to speak intelligently about ozone, greenhouse, and El Niño needs to read both of them. 

Floods, Famines, and Emperors : El Nino and the Fate of Civilizations
by Brian Fagan. Paperback (March 2000.)  Archaeologists have this wonderful perspective on what’s gone wrong in the past, both with climate and human institutions (http://www.perseusbooksgroup.com/perseus-cgi-bin/display/0-465-01121-7).

NOVA's Stories from the Ice.

"Warming's Unpleasant Surprise: Shivering in the Greenhouse?" Richard A. Kerr, Science news article (10 July 1998).

"Wallace Broecker is worried about the world's health. Not so much about the fever of global warming but about a sudden chill. For more than a decade, the marine geochemist has been fretting over the possibility that a world warming in a strengthening greenhouse might suffer a heart attack, of sorts: a sudden failure to pump vital heat-carrying fluids to remote corners of Earth. If greenhouse warming shut down the globe-girdling current that sweeps heat into the northern North Atlantic Ocean, he fears, much of Eurasia could within years be plunged into a deep chill."

Steven M. Stanley, Children of the Ice Age: How a Global Catashrophe Allowed Humans to Evolve (Crown 1996; Freeman pb 1998).

amazon.com

S. George Philander, Is the Temperature Rising? : The Uncertain Science of Global Warming (Princeton University Press, 1998).

A M McCabe & P U Clark, "Ice-sheet variability around the North Atlantic Ocean during the last deglaciation." Nature 392, 373 (1998). "Comparison with other terrestrial and marine ice-sheet records suggests that the dynamic collapse of the Laurentide Ice Sheet beginning at 14.6–15.014CkyrBP, (17.2–17.6 calendar kyrBP) initiated varied responses from other ice-sheet margins around the northern North Atlantic region. These observations support the argument that the release of icebergs and meltwater during Heinrich event 1 disrupted the North Atlantic thermohaline circulation, leading to a delay or reversal of deglaciation of the Northern Hemisphere and at least as far south as 40oS for two to three thousand years, suggesting a climate forcing and response similar to that of the ensuing Younger Dryas 'cold snap'."

Gideon Henderson, "Deep freeze," New Scientist pp.28ff (14 February 1998). Mistitled; it's really about what sets off the warm-up from a glacial period, updating Milankovitch with the issues of the role of the southern oceans. Nicely illustrates how the astronomical factors and the North Atlantic Current modes don't explain everything.

Richard B. Alley, Michael L. Bender (1998). "Greenland ice cores: Frozen in time," Scientific American 278(2):80-86 (February).

For tens of thousands of years, ice accumulating in Greenland has preserved details of the earth's climate and atmosphere. By extracting samples that run kilometers deep, researchers can peer directly into the past. Hidden in that ancient ice are subtle clues as to when the next ice age might begin.

Wallace S. Broecker (1997). "Thermohaline Circulation, the Achilles Heel of Our Climate System: Will Man-Made CO2 Upset the Current Balance?" Science 278(5343, Issue of 28 November 1997), pp. 1582 - 1588. http://www.sciencemag.org/cgi/content/full/278/5343/1582

During the last glacial period, Earth's climate underwent frequent large and abrupt global changes. This behavior appears to reflect the ability of the ocean's thermohaline circulation to assume more than one mode of operation. The record in ancient sedimentary rocks suggests that similar abrupt changes plagued the Earth at other times. The trigger mechanism for these reorganizations may have been the antiphasing of polar insolation associated with orbital cycles. Were the ongoing increase in atmospheric CO2 levels to trigger another such reorganization, it would be bad news for a world striving to feed 11 to 16 billion people.

Wallace S. Broecker, "Will Our Ride into the Greenhouse Future be a Smooth One?" GSA Today 7(5):11-7 (May 1997). http://www.geosociety.org/pubs/gsatoday/gsat9705.htm

J.-P. Severinghaus, T. Sowers, E. J. Brook, R. B. Alley, M. L. Bender (1998). "Timing of abrupt climate change at the end of the Younger Dryas interval from thermally fractionated gases in polar ice." Nature 391(6663, JAN 8, 1998):141-146.

Rapid temperature change fractionates gas Isotopes in unconsolidated snow, producing a signal that is preserved in trapped air bubbles as the snow forms ice, The fractionation of nitrogen and argon isotopes at the end of the Younger Dryas cold interval, recorded in Greenland ice, demonstrates that warming at this time was abrupt. This warming coincides with the onset of a prominent rise in atmospheric methane concentration, indicating that the climate change was synchronous (within a few decades) over a region of at least hemispheric extent, and providing constraints on previously proposed mechanisms of climate change at this time, The depth of the nitrogen-isotope signal relative to the depth of the climate change recorded in the Ice matrix indicates that, during the Younger Dryas, the summit of Greenland was 15 +/- 3 degrees C colder than today.

R G Curry, M S McCartney & T M Joyce (1998). "Oceanic transport of subpolar climate signals to mid-depth subtropical waters," Nature 391, 575 (5 February 1998).

The spatial distributions of certain sea-surface properties, such as temperature, fluctuate on timescales from months to decades and in synchrony with the main regional atmospheric patterns comprising the global climate system. Although it has long been assumed that the ocean is submissive to the dictates of the atmosphere, recent studies raise the possibility of an assertive, not merely passive, oceanic role in which water-mass circulation controls the timescales of climate fluctuations. Previously held notions of the immutability of the physical and chemical characteristics of deep water masses are changing as longer time series of ocean measurements indicate that the signatures of varying sea-surface conditions are translated to deep waters. Here the authors use such time-series measurements to track signals 'imprinted' at the sea surface in the North Atlantic Oceans subpolar Labrador Basin into the deep water of the subtropical basins near Bermuda, and infer an approximately 6-year transit time. They establish a geographic and temporal context for a portion of the long-term warming trend reported for mid-depth subtropical waters over the past 40 or so years, and we predict that waters at these depths will continue to cool well into the next decade.

A Ganopolski, S Rahmstorf, V Petoukhov & M Claussen (1998). "Simulation of modern and glacial climates with a coupled global model of intermediate complexity." Nature 391, 351 (22 January 1998)

A global coupled ocean--atmosphere model of intermediate complexity is used to simulate the equilibrium climate of both today and the Last Glacial Maximum, around 21,000 years ago. The model successfully predicts the atmospheric and oceanic circulations, temperature distribution, hydrological cycle and sea-ice cover of both periods without using 'flux adjustments'. Changes in oceanic circulation, particularly in the Atlantic Ocean, play an important role in glacial cooling.

D. Paillard (1998). "The timing of Pleistocene glaciations from a simple multiple-state climate model." Nature 391, 378 (22 January 1998).

The author presents two simple models that successfully simulate each glacial--interglacial cycle over the late Pleistocene epoch at the correct time and with approximately the correct amplitude. Moreover, in a simulation over the past 2 million years, the onset of the observed prominent 100-kyr cycles around 0.8 to 1 million years ago is correctly reproduced.

R. S. Pickart, M. A. Spall, J. R. N. Lazier (1997). " Mid-depth ventilation in the western boundary current system of the sub-polar gyre." DEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS. Vol. 44, no. 6, JUN 1997, p.1025- .

Two processes are investigated that result in the rapid (order of months) export of newly-ventilated water from the sub-polar north Atlantic. Both mechanisms involve mid-depth water mass formation within the western boundary current system, which leads to such rapid spreading. The first mechanism, which apparently occurs every winter, forms upper Labrador Sea water (LSW), which is a source of the high CFC layer of the upper deep western boundary current (DWBC). A mixed-layer model shows that this water mass can be formed by convection in the main branch of the Labrador Current. Strong heat loss near the boundary together with the existing potential vorticity structure of the current enables overturning to 1000 m. A regional numerical model of the circulation near Flemish Cap reveals how eddies of upper LSW are then shed by the baroclinically unstable Labrador Current. The eddies become detached from the boundary at the entrance to Flemish Cap and are entrained in to the offshore (barotropic) branch of the Labrador Current, which brings them seaward of Flemish Cap (where they have been previously observed). The second mechanism presented occurs only under extreme winter forcing, such as that experienced in the Labrador Sea in recent years. The enhanced heat loss forms classical LSW south of the cyclonic gyre, where the DWBC and North Atlantic Current can then quickly transport the water away from the Labrador Sea. It is shown that newly-ventilated lenses of classical LSW observed in the DWBC likely originate from this southern region, consistent with their sudden appearance downstream in the early 1990s.

Gerard Bond, William Showers, Maziet Cheseby, Rusty Lotti, Peter Almasi, Peter deMenocal, Paul Priore, Heidi Cullen, Irka Hajdas, Georges Bonani, "A Pervasive Millennial-Scale Cycle in North Atlantic Holocene and Glacial Climates," Science 278(5341, Issue of 14 November 1997), pp. 1257 - 1266. http://www.sciencemag.org/cgi/content/full/278/5341/1257. It seems that the North Atlantic has a major cooling cycle every 1400 years, both in ice ages and the Holocene -- and that sometimes this regional cooling has major worldwide consequences, such as the Little Ice Age.

Delia Oppo, "Paleoclimatology: Millennial Climate Oscillations," Science 278(5341, Issue of 14 November 1997), pp. 1244 - 1246. http://www.sciencemag.org/cgi/content/full/278/5341/1244.

K. C. Taylor, P. A. Mayewski, R. B. Alley, E. J. Brook, A. J. Gow, P. M. Grootes, D. A. Meese, E. S. Saltzman, J. P. Severinghaus, M. S. Twickler, J. W. C. White, S. Whitlow, G. A. Zielinski (1997). "The Holocene-Younger Dryas Transition Recorded at Summit, Greenland," Science 278(5339, Issue of 31 October 1997), pp. 825 - 827. http://www.sciencemag.org/cgi/content/full/278/5339/825.

"We speculate that the transition was characterized by a change in ocean circulation that significantly increased atmospheric water vapor. This added vapor could have increased retention of long-wave solar radiation and created a feedback that stabilized the new climate state. Regardless of the mechanism, the transition was a 50-year period during which wind speeds, precipitation, temperatures, and sea ice were changing throughout the Northern Hemisphere on subdecadal time scales."

R. B. Alley, P. A. Mayewski, T. Sowers, K. C. Taylor, and P. U. Clark (1997). "Holocene climatic instability: a prominent, widespread event 8200 yr ago." Geology 25:483-486.

Wallace S. Broecker, "Mountain glaciers: Recorders of atmospheric water vapor content?" Global Biogeochemical Cycles 11(4):589-597 (December 1997). Abstract:

A case is made that taken together, the 0.9 km lowering of tropical snow lines and the 8 parts per thousand reduction in the delta(18)O for tropical mountain Huascaran snowfall point to reductions in the water vapor inventory of the glacial atmosphere large enough to account for a several degree global cooling. The major weakness in this argument is that as no means exists to estimate the change in the magnitude of the radiative cooling term in the tropical atmosphere's heat budget, the assumption must be made that during glacial time the magnitude of this term was close to its present value. Changes in the inventory of atmospheric water vapor are tantalizing for they offer not only a means of cooling the glacial Earth but also a means of producing the abrupt changes in global climate, now well documented in climate record.

Robert Dickson, Harry Bryden, Jim Hurrel, John Marshall, Mike McCartney, Ray Schmitt, Ric Williams (1997). "THE NORTH ATLANTIC OSCILLATION (NAO)." Working paper. See Dickson's N&V in Nature, v386, pp649-650.

In the case of the third and deepest-reaching centre of Atlantic convection----the Greenland Sea -----the available instrumented record from the deepest layers (2000m) since the late 1950s suggests that trends in convective activity are the inverse of those in the Labrador Sea, driven by the same decadal changes in the NAO (Dickson et al 1996).

Dickson, R., J. Lazier, J. Meinke, P. Rhines and J. Swift, 1996: Long-term coordinated changes in the convective activity of the North Atlantic. Prog. Oceanogr., 38, 241-295

Rahmstorf, S., 1995: Bifurcations of the Atlantic thermohaline circulation in response to changes in the hydrological cycle. Nature, 378, 145-149.

Rahmstorf, S., 1996: On the freshwater forcing and transport of the Atlantic thermohaline circulation. Clim. Dyn., 12, 799-811.

Rahmstorf, S., 1997: An Atlantic Conveyor - refinement of a Good Idea. Ocean Challenge, 7 (3), 26-27.

Rahmstorf, S., 1998: Decadal variability of the thermohaline circulation. In Beyond ENSO: decadal variability in the climate system, edited by A. Navarra, Springer.

Rahmstorf, S., 1997: Ice-cold in Paris. New Scientist (8 February), 26-30.

Rahmstorf, S., 1997: Risk of sea-change in the Atlantic. Nature, 388, 825-826.

Rahmstorf, S., 1998: Ocean currents and climate change. Talk presented at the Symposium "Climate Impact Research: Why, How and When?" Berlin-Brandenburg Academy of Sciences and German Academy Leopoldina Berlin, 28 October 1997. Posted at http://www.pik-potsdam.de/~stefan/home_3.htm#HEADING3-0

Rahmstorf, S., J. Marotzke and J. Willebrand, 1996: Stability of the thermohaline circulation. In The warm water sphere of the North Atlantic ocean, edited by W. Krauss, Borntraeger, Stuttgart, pp. 129-158.

The American Geophysical Union's "Mechanisms of Millennial-Scale Global Climate Change" Chapman Conference has a useful list of references at http://www.agu.org/meetings/cc98dcall.html#Cus4

Yahoo's climate links.

CLIVAR's list of upcoming conferences on climate variability.

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Humanity's Descent : The Consequences of Ecological Instability ~ Usually ships in 2-3 days

Richard Potts, et al / Paperback / Published 1997

Magazine articles have no tradition of footnotes, so I have tried to augment the webbed version of the article. But since doing it right would be rather time-consuming, I have taken a shortcut. What follows are the end notes for a book manuscript of mine; they include references for a number of points not covered in The Atlantic article, but they have the virtue of including active web links. The links for Science won't work unless you have an on-line subscription to Science and are logged in; try following their PubMed links to discover related articles that have come out more recently.

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Further Reading

copyright ©1997 by William H. Calvin

Apropos abrupt climate change per se, let me start by recommending a good textbook and some authoritative web sites:

Brian J. Skinner, Stephen C. Porter, The Blue Planet: An Introduction to Earth Systems Science (Wiley, 1995).

Jonathan Adams, "Sudden (decade-timescale) transitions and short-lived cold and warm phases in the global climate record," at http://www.esd.ornl.gov/ern/qen/transit.html, is an excellent review. And his Global land environments since the last interglacial has many leads to follow.

Mark Maslin, "Sultry last interglacial gets a sudden chill," Earth in Space 9(7):12-14 (March 1997). http://www.agu.org/sci_soc/eismaslin.html

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A summary of ocean circulation can be found at http://www-ocean.tamu.edu/OOSDP/FinalRept/04h.html

The "Glossary of Oceanography and the Related Geosciences with References" is located at http://www-ocean.tamu.edu/~baum/paleo/paleogloss/paleogloss.html

The American Geophysical Union's web pages have many resources for the general reader, e.g., http://www.agu.org/sci_soc/everyone.html#climate

and the American Meteorological Society's page, "Weather and climate and the nation's well-being," is at http://atm.geo.nsf.gov/AMS/policy/nation.html

The best intro to climate-related oceanography is in Oceanus, Fall/winter 1996 issue, $7.95 from WHOI, MS-5, Woods Hole MA 02543 USA 1-508-289-3516, fax 508-457-2182

For the paleoanthropology, see the references in William H. Calvin, The Ascent of Mind (Bantam 1991), at http://WilliamCalvin.com/bk5/bk5notes.htm

William H. Calvin, "The emergence of intelligence," Scientific American 271(4):100-107 (October 1994; special issue Life in the Universe, out as a 1995 book of the same name). http://WilliamCalvin.com/1990s/1994SciAmer.htm

Paleoclimate and Evolution, with Emphasis on Human Origins, edited by Elisabeth S. Vrba, George H. Denton, Timothy C. Partridge, Lloyd H. Burckle (Yale University Press, 1995).

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Chapter Endnotes

1. The underreporting problem persisted, even in the face of substantial recognition of some of the major players. For example, Wallace S. Broecker - easily the most vigorous of the oceanographers in trying to alert the scientific community, and author of several Scientific American articles - was awarded the U.S. National Medal of Science by President Clinton in 1996 for "contributions to understanding chemical changes in the ocean and atmosphere." (http://imager.ldeo.columbia.edu/geol_sci/html/wallace_broecker.html).

The Danish ice-core expert Willi Dansgaard and the British oceanographer N. J. Shackleton received the Crafoord Prize from the Swedish Academy in 1995 (see http://www.kva.se/prizes.html).

Dansgaard, the Swiss climatologist Hans Oeschger, and the French climatologist Claude Lorius received the $150,000 Tyler Prize in 1996. The Tyler Prize news is in The Scientist (27 May 1996), at http://www.the-scientist.library.upenn.edu/yr1996/may/notebook_960527.html#note5.

Yet the bistable climate story itself was seldom reported.

2. Global Business Network, see http://www.gbn.org

3. Glacier facts, see http://www-nsidc.colorado.edu/NSIDC/EDUCATION/GLACIERS/glacier_facts.html

4. The beginning of the ice age at 2.5 million years is dated by N. J. Shackleton, J. Backman, H. Zimmerman, D. V. Kent, M. A. Hall, D. G. Roberts, D. Schnitker, J. G. Baldauf, A. Desprairies, R. Homrighausen, P. Huddlestun, J. B. Keene, A. J. Kaltenback, K. A. O. Krumsiek, A. C. Morton, J. W. Murray, and J. Westberg-Smith, "Oxygen isotope calibration of the onset of ice-rafting and history of glaciation in the North Atlantic region." Nature 307:620-623 (1984).

But, as would be expected from their origins in the earth's orbital cycles, the Milankovitch rhythms were present long before that, and can be seen as cycles of deep-sea anoxia: T. D. Herbert and A. G. Fischer, "Milankovitch climatic origin of mid-Cretaceous black shale rhythms in central Italy." Nature 321:739-743 (1986).

After a rearrangement of the ocean circulation was forced by North America connecting with South America about 3 million years ago, there was major climate change in Africa, as evidenced by the repeated speciation of antelopes between 2.9 and 2.5 million years ago.

Elisabeth S. Vrba, "The fossil record of African antelopes (Mammalia, Bovidae) in relation to human evolution and paleoclimate, " chapter 27 in Paleoclimate and Evolution, with Emphasis on Human Origins, edited by Elisabeth S. Vrba, George H. Denton, Timothy C. Partridge, Lloyd H. Burckle (Yale University Press, 1995), pp. 385-424.

P. B. deMenocal, "Plio-Pleistocene African climate," Science 270:53-59 (6 October 1995).

5. The ice sheets of Greenland and Antarctica cover close to 10 percent of the Earth's land surface area and contain over 75 percent of the world's fresh water. If all of this ice were returned to the oceans, global sea level would be raised by over 70 meters. At the maximum of an ice age, 32 percent of land mass is covered and sea level is about 125 meters lower than at present. See http://neptune.gsfc.nasa.gov/oceansice.html.

6. Little Ice Age, see http://gcrio.ciesin.org/CONSEQUENCES/winter96/geoclimate.html

7. The basic data on the cold spikes is in: W. Dansgaard, H. B. Clausen, N. Gundestrup, C. U. Hammer, S. F. Johnsen, P. M. Kristinsdottir, and N. Reeh, "A new Greenland deep ice core." Science 218:1273-1277 (1982).

The rapidity of the warming and cooling is in W. Dansgaard, J. W. C. White, and S. J. Johnsen, "The abrupt termination of the Younger Dryas climate event." Nature 339:532-534 (15 June 1989); R. B. Alley , D. Meese, et al., "Abrupt increase in Greenland snow accumulation at the end of the Younger Dryas event," Nature 362: 527-529 (8 April 1993).

8. Wallace S. Broecker, Dorothy M. Peteet, and David Rind, "Does the ocean-atmosphere system have more than one stable mode of operation." Nature 315:21-26 (2 May 1985).

9. The end of the Younger Dryas is dated at 11,400 years ago by Svante Björck, Bernd Kromer, Sigfus Johnsen, Ole Bennike, Dan Hammarlund, Geoffrey Lemdahl, Göran Possnert, Tine Lander Rasmussen, Barbara Wohlfarth, Claus Uffe Hammer, Marco Spurk, "Synchronized Terrestrial Atmospheric Deglacial Records Around the North Atlantic," Science 274:1155-1160 (15 November 1996). http://www.sciencemag.org/cgi/content/full/274/5290/1155

10. Worldwide temperatures change very quickly, too, with New Zealand's glaciers advancing quite promptly in the Younger Dryas, causing some speculation that atmospheric circulation changes first, and ocean circulation secondarily. But this, at present, has no known mechanism while the ocean circulation has known vulnerabilities in the northern North Atlantic Ocean; also oceans and ice sheets have "memory" on the time scale of centuries, unlike the constituents of weather systems, and so they are better candidates for "chattering" between extremes. An ocean circulation mode change might, of course, trigger a rearrangement of the atmospheric circulation cells -- say, a two- or four-cell version of the usual three cells per hemisphere (air rising at the equator and 60, sinking at 30 and at the poles).

11. Wallace S. Broecker, "Chaotic climate," Scientific American 273(5):62-69 (November 1995). A hundred Amazons is Broecker's figure; if the Amazon flow is instead taken as 0.19 Sv, and the southbound NADW off Newfoundland as 13 Sv, then it is about 70 Amazons. But deep water production by convection may be less, depending on how much NADW is recirculated Antarctic deep water or Arctic in origin. GIN Sea production is said to be 5.6 Sv, about equally divided between the sills to the east and west of Iceland. Labrador Sea production is difficult to estimate because of the local gyre and the frequent annual failures.

12. G. H. Denton, C. H. Hendy, "Younger Dryas Age Advance of Franz Josef Glacier in the Southern Alps of New Zealand," Science 264:1434-1437 (3 June 1994; follow up at 271:668-669, 2 February 1996).

13. It isn't that the entire North Atlantic Current stops but that the downwelling forming it may shift to much lower latitudes. There are computer models which show total failure but, at least for the Younger Dryas, there is carbon circulation evidence (see Broecker 1994, 1995) that suggests the thermohaline circulation was maintained then.

14. Pieter M. Grootes, M. Stuiver, J. W. C. White, S. Johnsen, and J. Jouzel, "Comparison of oxygen isotope records from the GISP2 and GRIP Greenland ice cores," Nature 366:552-554 (1993).

Willi Dansgaard, S. J. Johnsen, H. B. Clausen, D. Dahl-Jensen, N. S. Gundestrup, C. U. Hammer, C. S. Hvidberg, J. P. Steffensen, A. E. Sveinbjörnsdottir, J. Jouzel, G. Bond, "Evidence for general instability of past climate from a 250 kyr ice core," Nature 364:218-219 (1993).

15. For an exposition of the ¹⁸O method of inferring temperature in ice cores, see http://www.agu.org/revgeophys/mayews01/node2.html

16. M. Milankovitch, Canon of Insolation and the Ice Age Problem (Königlich Serbische Akademie, 1941; English translation by the Israel Program for Scientific Translations, 1969).

Wallace S. Broecker and George H. Denton, "What drives glacial cycles?" Scientific American 262(1):48-56 (January 1990).

17. Intermediate meltoffs best correlated with June-July perihelion: from a lecture by the French glacial expert, Robert J. Delmas, "Climatic and environmental information from ice cores." Lecture at University of Washington (14 February 1989).

Note that I am sidestepping the 100,000 year problem; the astronomy doesn't predict a major meltoff at such intervals, and there is much speculation about exotic and terrestrial causes. See, for example, Richard A. Muller, "Glacial cycles and orbital inclination," Lawrence Berkeley Laboratory Report LBL-35665 (1994), available at http://www-physics.lbl.gov/www/astro/nemesis/LBL-35665.html.

18. I am using "flicker" to refer to the abrupt warming and cooling episodes. In the atmospheric sciences, the term "flicker" is often used for even briefer transients in the dust contained in the Greenland ice cores, e.g., K. C. Taylor, R. B. Alley, G. A. Doyle, P. M. Grootes, P. A. Mayewski, G. W. Lamorey, J. W.C. White, and L. K. Barlow, "The flickering switch of late Wisconsin climate change," Nature 361:432-436 (4 February 1993).

19. J.A. Eddy and Hans Oeschger, editors, Global Changes in the Perspective of the Past (Wiley, New York, 1993). I first heard of the abruptness in 1984 when Oeschger gave a talk in Seattle. He didn't mention the abruptness in his presentation but the time calibration on one of his slides prompted me to ask him afterward, about how quickly temperature had changed. Oh, he said, the big drop took just a few years. The enormity of such a whiplash caused me to assume that we were having some language difficulties and so I persisted, asking, "Just a few decades?" No, no, he replied, merely a few years.

Wally Broecker, too, heard Oeschger give a talk in 1984, and the quick flips gave Broecker his idea for failures of the salt conveyor as a cause. See Wallace S. Broecker, "Will Our Ride into the Greenhouse Future be a Smooth One?" GSA Today 7(5):11-7 (May 1997). http://www.geosociety.org/pubs/gsatoday/gsat9705.htm

20. Peter V. Foukal, "The variable sun," Scientific American 262(2):34-41 (February 1990). Reductions in solar output might, of course, be one of the things that set the stage for an abrupt cooling, as might the North Atlantic Oscillation and other atmospheric cycles like El Niño. Causes usually don't come one at a time, but combine in various ways.

21. Scott Lehman, "Flickers within cycles," Nature 361:404-405 (4 February 1993). And see the North Atlantic Oscillation discussion in Kerr (1997).

22. In glacial times, sea ice covered the Greenland-Iceland-Norwegian Seas and extended down to Ireland. There's still a lot of sea ice, an area equal to that of the North American continent. See http://neptune.gsfc.nasa.gov/oceansice.html

23. I am using the word "flush" in a more general way than some oceanographers. But then too, I don't use the word "water" in the restricted sense in which they do, to describe a sunken river or a drifting blob. For their use of "flush" for describing an overturning that reestablishes a stopped conveyor, see http://wikyonos.seaoar.uvic.ca/projects/CCC-Global-renewal.html

24. For another history of Nansen, see http://www.nrsc.no/nansen/fritjof_nansen.html

25. Wallace S. Broecker, "Will Our Ride into the Greenhouse Future be a Smooth One?" GSA Today 7(5):11-7 (May 1997). http://www.geosociety.org/pubs/gsatoday/gsat9705.htm

26. Count Rumford, see Warren M. Washington, "Where's the heat?" Natural History (3):66-72 (1990).

Andrew J. Weaver, Tertia M. C. Hughes, "Stability and variability of the thermohaline circulation and its link to climate," Trends in Physical Oceanography 1:15-70 (1992), at p. 16.

27. For more on Count Rumford, see the biographical notes at http://www.english.upenn.edu/~jlynch/Frank/People/rumford.html

28. Henry Stommel, "Thermohaline convection with two stable regimes of flow," Tellus 13(2): 224--230 (1961).

Henry Stommel, "On the smallness of the sinking regions in the ocean," Proceedings of the National Academy of Science (U.S.A.) 48:766-772 (1962).

Raymond W. Schmitt, "The ocean component of the global water cycle," Rev. Geophys. Vol. 33 Suppl. (1995). http://earth.agu.org/revgeophys/schmit01/schmit01.html

29. Wallace S. Broecker, "Unpleasant surprises in the greenhouse?" Nature 328:123-126 (9 June 1987).

30. John Kenneth Galbraith, The New Industrial State (Hamish Hamilton, 1967).

31. Whirlpools associated with salt play an interesting role in Norse legends. See chapter six of Giorgio de Santillana and Hertha von Dechend, Hamlet's Mill: An Essay Investigating the Origins of Human Knowledge and its Transmission Through Myth (Godine, Boston, 1969).

32. Richard A. Kerr, news story "A New Driver for the Atlantic's Moods and Europe's Weather?" Science 275:754-755 (7 February 1997). http://www.sciencemag.org/cgi/content/full/275/5301/754a

33. Alexander Sy, Monika Rhein, John R. N. Lazier, Klaus Peter Koltermann, Jens Meincke, Alfred Putzka, and Manfred Bersch, "Surprisingly rapid spreading of newly formed intermediate waters across the North Atlantic Ocean," Nature 386:675-679 (17 April 1997).

And see Robert S. Pickart and Michael A. Spall, "Mid-Depth Ventilation Along the Western Boundary of the Sub-Polar Gyre," at http://www.aoml.noaa.gov/phod/accp/ap95/mid-d.html

34. Michael S. McCartney, Ruth G. Curry, Hugo F. Bezdek, "North Atlantic transformation pipeline chills and redistributes subtropical water," Oceanus 39(2):19-23 (Fall/winter 1996). This is the source of Jack Cook's excellent Labrador-to-Denmark diagram of the North Atlantic Current which I have modified for grayscale rendering. This article elaborates on the multi-year aspects of the formation of salt sinking in the sub-polar gyre, what I have too briefly characterized as the "down elevator."

Rana A. Fine, "Tracers, time scales, and the thermohaline circulation: The lower limb in the North Atlantic Ocean," Rev. Geophys. Vol. 33 Suppl. (1995). Available at http://www.agu.org/revgeophys/fine00/fine00.html

35. Wallace S. Broecker, "The great ocean conveyor," Oceanography 4:79-89 (1991).

36. Robert Ardrey, introduction to Eugène N. Marais, The Soul of the Ape (1969), p. 21 of the 1973 Penguin edition.

37. Failures of salt sinks and abrupt coolings may not always go together. One must distinguish between a number of separate -- though likely related -- things:

1. Abrupt warmings and coolings, regional

2. Abrupt warmings and coolings, worldwide

3. Shutdown of North Atlantic Deep Water production

4. Shutdown of the more superficial thermohaline circulation

5. Interglaciation versions of the above.

Some may occur without the others. For example, some of the Greenland coolings and warmings may prove to be local, a consequence of the North Atlantic Oscillation or some similar rearrangement of the air circulation patterns. But most are clearly worldwide, with sudden warmings in Greenland followed by increased atmospheric methane from tropical swamps within a decade.

The simplest hypothesis has been that all abrupt coolings were NADW shutdowns, both glacial and interglacial. The mid-Eemian cooling seems more complicated than that -- perhaps because of regional factors, perhaps because the Canadian and European ice sheets weren't around. But abrupt coolings per se are pretty secure; the problem is sorting out their various mechanisms, and the combinations thereof.

There was a minor abrupt cooling 8,000 years ago, during our present interglaciation. It lasted for about 200 years and was registered not only in Greenland but in Antarctica and Africa. See J. C. Stager, P. A. Mayewski, "Abrupt early to mid-Holocene climatic transition registered at the equator and the poles," Science 276:1834-1835 (20 June 1997). http://www.sciencemag.org/cgi/content/full/276/5320/1834

38. Neil D. Opdyke, "Mammalian migration and climate over the last seven million years," chapter 8 in Paleoclimate and Evolution, with Emphasis on Human Origins, edited by Elisabeth S. Vrba, George H. Denton, Timothy C. Partridge, Lloyd H. Burckle (Yale University Press, 1995), pp.109-114 at p.112.

39. USGS troubles, see http://baervan.nmt.edu/sci.geo.petroleum/archives/0612.html

40. R. J. Behl, J. P. Kennett, "Brief interstadial events in the Santa Barbara basin, NE Pacific, during the last 60 kyr," Nature 379:243-246 (18 January 1996).

41. Johann Wolfgang von Goethe, Maxims and Reflections I.

42. Peter Schlosser, Gerhard Bönisch, Monika Rhein, Reinhold Bayer, "Reduction of deepwater formation in the Greenland Sea during the 1980s: evidence from tracer data," Science 251:1054-1056 (1 March 1991).

Gerhard Bönisch, Johan Blindheim, John L. Bullister, Peter Schlosser, and Douglas W. R. Wallace, "Long Term Trends of Temperature, Salinity, Density, and Transient Tracers in the Central Greenland Sea," Journal of Geophysical Research 102:553 (1997). At http://www.ldeo.columbia.edu/~noblegas/gerhard/GIN/tspgs/tsp65.html.

The European ESOP-2 research project "Thermohaline circulation in the Greenland Sea" is at http://www.smr.uib.no. It is their excellent "whirlpool" illustration that I have modified and labeled.

43. See Kerr (1997) and Michael S. McCartney, "North Atlantic Oscillation," Oceanus 39(2):13 (Fall/winter 1996). The regional atmospheric circulation over the North Atlantic normally features a high over the Azores and a low near Greenland and Iceland -- the westerlies are intense but the cold air from Canada is warmed before reaching Europe. When the low shifts as far south as Newfoundland, a high develops over northern Greenland; this brings cold arctic air west from northern Europe to be warmed by the Norwegian Current and thus warm Greenland and North America rather than Europe. The Labrador Sea is much less stormy and this likely affects salt sinking. See http://www.ldeo.columbia.edu/NAO/.

The relationships between the NAO and NADW are discussed by R. Dickson, "Observations of DecCen climate variability in convection and water mass formation in the northern hemisphere," in the CLIVAR Villefranche workshop summary at http://www.dkrz.de/clivar/villesum.html. More generally, see the Climate Research Committee, National Research Council, Natural Climate Variability on Decade-to-Century Time Scales (National Academy Press, 1995). Excerpts can be found at http://www.nap.edu/bookstore/isbn/0309054494.html. And, similarly, D. L. T. Anderson (Editor), J. Willebrand (Editor), Decadal Climate Variability: Dynamics and Predictability NATO Asi Series. Series I, Global Environmental Change, Vol 44 (Springer Verlag 1996).

44. W. D. Hibler III, "On the effect of sea-ice dynamics on oceanic thermohaline circulation," Annals of Glaciology 21:361-368 (1995).

45. Sinking to the very bottom may be important for the return leg of the trip, south past the coast of Newfoundland where it must stay beneath the Gulf Stream. There is some speculation (see Kerr 1997) that it sometimes doesn't, and is swept eastward, illustrating a mechanism for creating blobs that recirculate. It is known that semi-salty, or anomalously warm or cool, blobs even travel from the subtropics, up and around the GIN Seas and back around to the Labrador Sea, suggesting another failure mode for late winter downwelling; see Raymond W. Schmitt, "If rain falls on the ocean, does it make a sound?" Oceanus 39(2):4-8 (Fall/winter 1996).

46. As Andrew J. Weaver (personal communication 1997) notes, the ordinary worry is about how global warming will affect the Nordic heat pump via increased evaporation in the tropics and subtropics. That will create increased rainfall in the higher latitudes, where it will interfere with salt sinking. I tend to emphasize the extraordinary events such as fjord floods, but the more routine concern is with freshening the Nordic Seas via rainfall.

47. For the hinge from which icebergs break off, see E. J. Rignot, S. P. Gogineni, W. B. Krabill, S. Ekholm, "North and Northeast Greenland Ice Discharge from Satellite Radar Interferometry," Science 276:934-937 (9 May 1997). http://www.sciencemag.org/cgi/content/full/276/5314/934

48. G. C. Bond, R. Lotti, "Iceberg Discharges into the North Atlantic on Millennial Time Scales During the Last Glaciation," Science 267:1005 - (17 February 1995).

W. S. Broecker, "Massive iceberg discharges as triggers for global climate change," Nature 372:421-424 (1994).

49. William S. Reeburgh, D. L. Nebert, "The birth and death of Russell Lake," Alaska Science Forum 832 (3 August 1987). It is their photograph that is reproduced in the present text. http://www.gi.alaska.edu/ScienceForum/ASF8/832.html and see satellite photos via http://edcwww.cr.usgs.gov/earthshots/slow/Hubbard/Hubbard.

50. Robert R. Dickson, "The local, regional, and global significance of exchanges through the Denmark Strait and Irminger Sea," in Natural Climate Variability of Decade-to-Century Time Scales (National Academy Press 1995), pp. 305-317. Note that the Great Salinity Anomaly, equivalent to an extra 2,000 km³ of fresh water, is not usually treated as a fjord flood but as a variant on a semi-salty current out of the Arctic Sea. This volume is about 500 times the freshwater flux from Alaska's Russell Fjord over a five-month period -- but then the reservoir capacity of the east Greenland fjord system at 70N is also extraordinary and ice dams can span multiple years.

51. R. R. Dickson, J. Meincke, S-A. Malmberg, and A. J. Lee, "The 'great salinity anomaly'' in the northern North Atlantic 1968-1982," Progress in Oceanography 20:103-151 (1988). My illustration is adopted from the colored one in Oceanus (Fall/winter 1996).

52. Great Salinity Anomaly, see Schmitt (1996). The travel time from the Labrador Sea to Bermuda is about six years.

53. John McLeod and John Osborn, in Natural Automata and Useful Simulations, edited by H. H. Pattee et al (London: Macmillan, 1966).

54. A. J. Weaver and T. M. C. Hughes, "Rapid interglacial fluctuations driven by North Atlantic Ocean circulation," Nature 367:447- 450 (3 February 1994).

S. Rahmstorf, "Bifurcations of the Atlantic thermohaline circulation in response to changes in the hydrological cycle," Nature 378:145-149 (1995).

S. Manabe, R. J. Stouffer, "Simulation of abrupt climate change induced by freshwater input to the North Atlantic Ocean," Nature 378:165-167 (1995). And see S. Manabe and R. J. Stouffer, Paleoceanography 12, 321 (1997)

55. Werner Heisenberg, in Physics and Beyond, edited by R. N. Anshen (Harper & Row, 1971).

56. Thule history, see http://www.peregrinefund.org/ThulHist.html

57. See Grootes et al (1993) and Dansgaard et al (1993).

58. An even longer record comes from the high tropics, going back about 500,000 years and showing many abrupt temperature changes. L. G. Thompson, T. Yao, M. E. Davis, K. A. Henderson, E. Mosley-Thompson, P.-N. Lin, J. Beer, H.-A. Synal, J. Cole-Dai, J. F. Bolzan, "Tropical Climate Instability: The Last Glacial Cycle from a Qinghai-Tibetan Ice Core," Science 276:1821-1825 (20 June 1997). http://www.sciencemag.org/cgi/content/full/276/5320/1821

59. Mark Maslin, "Sultry last interglacial gets a sudden chill," Earth in Space 9(7):12-14 (March 1997). http://www.agu.org/sci_soc/eismaslin.html

60. Michael H. Field, Brian Huntley, Helmut Müller, "Eemian climate fluctuations in a European pollen record," Nature 371:779-783 (27 October 1994).

61. One of the current uncertainties about the mid-Eemian cooling event at 122,000 years is whether it involved the complete shutdown of the North Atlantic Deep Water production; various ocean-floor cores suggest it didn't.

L. D. Keigwin, W. B. Curry, S. J. Lehman, S. Johnsen, "The role of the deep ocean in North Atlantic climate change between 70 and 130 kyr ago," Nature 371:323-326 (22 September 1994).

J. F. McManus, G. C. Bond, W. S. Broecker, S. Johnsen, L. Labeyrie, S. Higgins, "High-resolution climate records from the North Atlantic during the last interglacial," Nature 371:326-329 (22 September 1994).

62. Broecker (1997, p.5)

63. Jared Diamond, The Third Chimpanzee (HarperCollins, 1992).

64. Syun-Ichi Akasofu, "The dynamic aurora." Scientific American 260(5):90-97 (May 1989).

65. Chinese treasure fleet story from Jared Diamond, Guns, Germs, and Steel: The Fates of Human Societies (W. W. Norton, 1997), p. 412.

66. Peter Schwartz, The Art of the Long View (Doubleday 1991).

67. Alison Jolly, "The evolution of purpose." In Machiavellian Intelligence, edited by Richard W. Byrne and Andrew Whiten (Clarendon Press, 1988), pp. 363-378 at p. 378.

68. K. E. Taylor, J. E. Penner, "Response of the climate system to atmospheric aerosols and greenhouse gases," Nature 369:734-737 (1994).

69. Robert J. Charlson, James E. Lovelock, Meinrat O. Andreae, and Stephen G. Warren, "Oceanic phytoplankton, atmospheric sulphur, cloud albedo, and climate." Nature 326:655-661 (1987).

70. B. D. Santer, et al., "Towards the detection and attribution of an anthropogenic effect on climate," Climate Dynamics 12:77-100 (1995).

71. Leonard A. Sagan, "Family ties." The Sciences 28(2):20-29 (March 1988).

72. Vito Volterra, Nature 118:558-560 (1926). See http://marine.geol.sc.edu/BIOL/Courses/BIOL301/Wethey/pred.html and http://www.math.usouthal.edu/~hitt/courses/590/population/population09.html.

73. Alfred W. Crosby, Ecological Imperialism: The Biological Expansion of Europe, 900-1900 (Cambridge University Press, 1986).

74. In a drought, we can switch how we use our grain harvests. Chickens, for example, are more efficient at converting grain into meat than are pigs. We can even minimize the meat we eat, consuming the grain directly. Such measures can buy some time, in the face of harvests that are only half the expected, but they are incapable of adjusting to the 25-fold differences that one might expect in European harvests, were Europe to get a Canadian climate.

75. The antelopes developed a number of new species in Africa, between 2.9 and 2.6 million years ago. This is thought to be diversification into ecological niches, prompted by climate changes. See Vrba (1995).

76. For an excellent example of how predation changes the rates of somatic growth and reproductive maturity (and makes bodies much larger and longer-lasting), see Todd A. Crowl and Alan P. Covich, "Predator-induced life-history shifts in a freshwater snail," Science 247:949-951 (23 February 1990). For a survey of dwarf species on the Mediterranean islands, see Paul Sondaar, "The island sweepstakes," Natural History 95(9):50-57 (September 1986).

77. During the last interglacial about 128,000 years ago, a range of hills in western Normandy was isolated by rising sea level, becoming the island of Jersey. And within a time span of only 6,000 years (during which mainland deer didn't change -- and hadn't for the previous 400,000 years, either), the body size of the deer inhabiting the island dropped to about one-sixth of their original size. A. M. Lister, "Rapid dwarfing of red deer on Jersey in the last interglacial," Nature 342:539-542 (30 November 1989).

78. Frans de Waal, Good Natured: The Origins of Right and Wrong (Harvard University Press 1996).

79. Loren Eiseley, The Immense Journey (Doubleday, 1957), p. 125, pp. 129-131.

80. Lionel E. Jackson, Jr. and Alejandra Duk-Rodkin, "Geology of the ice-free corridor." University of Washington lecture (19 January 1988).

The present calibration for the radiocarbon dates puts the Clovis horizon at 13,000 years ago (11,000 b.c.), not the 11,000 years ago usually quoted (which is the uncalibrated date). See p.35 of Jared Diamond, Guns, Germs, and Steel: The Fates of Human Societies (Norton, 1997).

81. Larry D. Agenbroad, "New World Mammoth Distribution," chapter 3 of Paul S. Martin, Richard G. Klein, editors, Quaternary Extinctions (University of Arizona Press 1984), pp. 90-108.

82. For a useful account of some of the controversies in the archaeology of the Americas, see Douglas Preston, "The lost man," The New Yorker, pp.70-81 (16 June 1997).

83. Michael Wendorf, "Diabetes, the ice free corridor, and the paleoindian settlement of North America." American Journal of Physical Anthropology 79(4):503-520 (1989).

Joseph H. Greenberg, Christy G. Turner II, and Stephen L. Zegura, "The settlement of the Americas: A Comparison of the linguistic, dental, and genetic evidence." Current Anthropology 27(5):477-497 (1986).

84. For group selection, see the special issue of American Naturalist edited by David Sloan Wilson or his short piece, "Human groups as units of selection," Science 276:1816-1817 (20 June 1997). http://www.sciencemag.org/cgi/content/full/276/5320/1816

85. John Gribbin, Mary Gribbin, Children of the Ice (Basil Blackwell 1980).

86. William H. Calvin, "The emergence of intelligence," Scientific American 271(4):100-107 (October 1994; also appears in the Scientific American book Life in the Universe, 1995). http://WilliamCalvin.com/1990s/1994SciAmer.htm

87. The modern version of the "throwing theory" is in William H. Calvin, "The unitary hypothesis: A common neural circuitry for novel manipulations, language, plan-ahead, and throwing?" pp. 230-250 in Tools, Language, and Cognition in Human Evolution, edited by Kathleen R. Gibson and Tim Ingold (Cambridge University Press 1993). http://WilliamCalvin.com/1990s/1993Unitary.htm

88. Derek Bickerton, lecture in Budapest at the evolutionary biology congress (1996).

89. Lewis Thomas, Late Night Thoughts on Listening to Mahler's Ninth Symphony, (Viking, 1983), p. 15.

90. When the astronomical factors in sunshine ("insolation") seemed to be the main drivers of ice age fluctuations, one could engage in some speculation about when it dipped below some particular value. This had the virtue of allowing you to say how much longer our present warm period had to go, before insolation reached that value.

But, since it became apparent that great short-term bistable fluctuations are superimposed on the longer-term trends, one tends to define the beginning not by ice itself but by abrupt rewarmings, e.g., the one at 130,000 years ago. And the end of a warm period by an abrupt cooling that doesn't recover for any length of time, e.g., the sharp temperature drop at117,000 years ago. Defining in terms of ice sheets, as in the term "interglaciation," gives the appearance of longer durations without the warm reality (to say that ice sheets returned at 70,000 years ago is to ignore how cold it was in the intervening millennia). Unlike insolation, abrupt coolings don't have a schedule to follow. Warm interludes in the ice ages are simply the times, during the upper parts of the insolation cycles, between an abrupt warming that lasts for a while, and an abrupt cooling that also lasts for a while.

The abrupt warming that began our present warm period was the Bølling at about 15,000 years ago. The last warm period wasn't as stable as ours (and it was also warmer at times, with higher sea levels), but it might be said to have had a 13,000-year run. Earlier warm periods varied, and we lack the good ice-core records that tell us which warmings and coolings were abrupt.

But I don't think anyone would argue that, at 15,000 years and running, our present warm period was only two-thirds over. It's getting late, and the real issue isn't some average time but the present stability of the Nordic heat pump. Scientifically, you can't argue that we've got a while longer, by rights. You can only argue that, from here on out, it's pretty chancy - and that the abrupt coolings which have happened so often in the past are likely to happen again.

91. Robert Burns, The Shape and Form of Puget Sound (University of Washington Press 1985), p. 43.

92. Michael Parfit, "Before Noah, there were the Lake Missoula Floods," Smithsonian (April 1995).

93. John Eliot Allen and Marjorie Burns, Cataclysms on the Columbia (Portland: Timber Press, 1986).

94. W. H. Auden, from "Shorts" ....

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