As global temperatures increase, heat penetrates slowly into the arctic permafrost and the oceans. Both gradually warm up, and at some point, the hydrates will become unstable and begin to release their trapped methane, which could trigger abruptly increased warming. Even though methane has a short lifetime in the atmosphere, its greenhouse effect would produce a sharp upward temperature spike and could be prolonged if the very large amounts of existing hydrate were to decompose sporadically over a period of time.
There are indications in the geological record that sudden bursts of methane have been released into the atmosphere in the past. The physical evidence includes “pockmarked” sediments in the Arctic and sub- Arctic—areas where detailed mapping of the seafloor shows multiple craters up to a hundred meters across, interpreted to be the result of rapid release of large bubbles of methane gas, probably due to the decomposition of hydrate layers. Destruction of the hydrates most likely resulted from the gradual warming of seawater during the present interglacial period. The chemical composition of some ocean sediments also points to large-scale methane release. Carbon in the methane produced by bacteria has a very distinctive isotopic makeup, and when it is released into seawater that signature is transferred to organisms living in the water, and eventually gets preserved in the sediments. Along the central California coast and elsewhere, recent sediments show series of isotopic “spikes” that appear to be attributable to abrupt injection of methane into seawater—presumably from the decomposition of hydrates. And much farther back in the geologic record, about 55 million years ago, one of the largest recorded abrupt increases in ocean water temperature—7 to 8 degrees Celsius—is accompanied by similar isotopic evidence for methane release. Most scientists have concluded that huge volumes of methane hydrates must have suddenly decomposed, for reasons that are still unclear, and that the methane release was responsible for the sudden temperature increase that followed.
How effectively methane has contributed to the warming of the Earth during the present interglacial period is still a topic of debate. What its role will be in the future is also uncertain. But two things are clear: first, there are very large stocks of this gas stored both on land at high latitudes and along the continental shelves almost everywhere; and second, there is an undeniable correlation in the ice-core data between increased temperature and increased methane in the atmosphere. Even if methane is not the immediate cause, it follows temperature increases very closely and must amplify them.
In spite of the well-documented rise in atmospheric CO2 and the possibility that large amounts of methane gas will also be released, the consensus view until recently has been that the current warm interglacial period will end soon (in geological terms) and that the Earth is headed toward another glacial episode. This conclusion was based mainly on examination of the past climate record—the peak of the last glaciation was twenty thousand years ago, and over the past million years or so the warm interglacial periods that separate major ice advances have typically lasted only ten or twenty thousand years. Man’s additions of CO2 to the atmosphere may prolong the warm climate of the current interglacial period a bit, but at current rates of usage, our supply of fossil fuels will run out in a few centuries anyway. Elevated levels of carbon dioxide will linger in the atmosphere long after that, but will gradually decrease, reducing the greenhouse effect. Inexorably, the fluctuations in the Earth’s orbit will draw us into the next glacial episode, and ice sheets will once again build up from centers in Scandinavia, Canada, and Russia.
Or will they? It is possible that the consensus view is wrong. We have seen how the glacial-interglacial cycles of the past million years have closely followed the 100,000-year timescale of the eccentricity of the Earth’s orbit, its tendency to be more or less elliptical. Although exactly why climate tracks eccentricity is not known with certainty, the correlation is clear. And a close look at how the Earth’s orbit will change in the future shows that its eccentricity will decrease steadily to almost zero about 30,000 years from now. This is apparent even in James Croll’s original graph, reproduced in chapter 5 (figure 11). It is something that has not happened for hundreds of thousands of years. The practical effect is that the variability in the amount of solar radiation received by the Earth will be much less over the next 50,000 years or so than it has been through the past few glacial-interglacial cycles. Coupled with persisting high levels of CO2, this could push the next glacial advance far into the future. Some computer simulations suggest that under these conditions, significant glaciation will not occur before sixty or seventy thousand years from now, and even then the ice will not be as extensive as it was during the previous few glacial advances. And there is yet another possibility. If CO2 emissions are not curbed, global warming could completely melt the Greenland glaciers and a substantial part of the Antarctic ice sheet. This would not happen instantaneously; the melting would continue over many human generations. Nevertheless, the consequences for mankind would be serious: the sea level would rise by nearly sixty meters, flooding vast areas of the continents, including most parts of present-day cities like New York and London; weather patterns worldwide would be altered drastically, disrupting agriculture in unpredictable ways; the frequency and intensity of hurricanes would increase because they draw their energy from warm ocean water, which would be far more widespread than currently. Warming would be reinforced by the loss of highly reflective ice and snow, and possibly by the decomposition of unstable methane hydrates. The elevated temperatures coupled with complete loss of continental ice sheets might constitute a threshold-crossing event that would thrust the Earth into a regime from which the glaciers could not quickly recover, even with the return of greater eccentricity and lower CO2 levels. Only a few hundred years after Louis Agassiz announced his theory of a global ice age, mankind may inadvertently bring the Pleistocene Ice Age to a premature close, ushering in another long period of ice-free existence for our planet.
SUGGESTIONS FOR FURTHER READING
ICE AGES AND GLACIATION, GENERAL
John C. Crowell, Pre-Mesozoic Ice Ages: Their Bearing on Understanding the Climate System (Boulder, CO: Geological Society of America, 1999). This is Memoir 192 of the Geological Society of America. Crowell has spent a distinguished career studying ice ages and here uses his immense expertise to sift through and summarize the disparate evidence for each of the Earth’s ancient ice ages and to search for their causes and connections to the climate system.
M.J. Hambrey, Glacial Environments (Vancouver: University of British Columbia Press, 1994). A well-illustrated treatment of the effects of glaciers on landscape.
J. Imbrie and K.P. Imbrie, Ice Ages: Solving the Mystery (Short Hills, NJ: Enslow, 1979). A well-written account of how ideas about ice ages developed, with insights (by one of the participants) into the work on sediment cores that confirmed the astronomical controls on Pleistocene glaciation.
R.A. Muller and Gordon J. Macdonald, Ice Ages and Astronomical Causes (New York: Springer, 2000). A technical and mathematical analysis of the evidence for astronomical control of ice ages.
LOUIS AGASSIZ
Louis Agassiz, Studies on Glaciers, Preceded by the Discourse of Neuchâtel, ed. and trans. by Albert V. Carozzi (New York: Hafner, 1967). This is an English translation of Agassiz’s famous Études sur les glaciers, originally published in 1840. It also includes the text of Agassiz’s address to the Natural History Society of Switzerland in Neuchâtel in 1837. The translation includes the magnificent drawings that accompanied the original book.
Edward Lurie, Louis Agassiz: A Life in Science (Chicago: University of Chicago Press, 1960). A comprehensive scholarly account of Louis Agassiz’s life. However, it focuses mainly on his contributions to biology, with very little discussion devoted to the theory of ice ages.
Jules Marcou, Life, Letters, and Works of Louis Agassiz (New York: Macmillan, 1896). A very detailed account of Agassiz’s life written by a colleague and personal friend who is perhaps a little biased in his treatmen
t. Although the book is written in English, Marcou reproduced many of Agassiz’s letters in their original French.
JAMES CROLL
James Croll, Climate and Time in Their Geological Relations: A Theory of Secular Changes of the Earth’s Climate (London: Daldy, Isbister, 1875). Croll’s masterpiece, in which he brings together his ideas about the Earth’s climate.
J.C. Irons, Autobiographical Sketch of James Croll, with Memoir of his Life and Work (London: Edward Stanford, 1896). A sympathetic account written by a friend who wished to make the remarkable details of Croll’s life known to a wider audience. It includes a listing of all of Croll’s publications. It is still the only biography available.
MILUTIN MILANKOVITCH
Milutin Milankovitch, Cannon of Insolation and the Ice-Age Problem (Jerusalem: Israel Program for Scientific Translations, 1969). Originally published in 1941 in Belgrade as Kanon der Erbestrahlund and seine Anwendung auf das Eiseitenproblem, this was Milankovitch’s culminating effort to bring together all of his calculations and ideas about the Earth’s climate. Much more mathematically based than James Croll’s Climate and Time, it is, like that earlier book, a masterpiece.
Milutin Milankovitch, Milutin Milankovitch 1879–1958 (European Geophysical Society, 1995). This slim volume documenting Milankovitch’s life was put together by his son, Vasko, after his father’s death. It draws heavily on Milankovitch’s autobiography and is well-illustrated with photographs.
THE CHANNELED SCABLANDS
V. R. Baker, ed. Catastrophic Flooding: The Origin of the Channeled Scabland. Benchmark papers in Geology, 55 (Stroudsburg, PA: Dowden, Hutchinson & Ross, 1981). This compilation includes the important scientific papers (mostly authored by J. Harlan Bretz) that led to the acceptance of a catastrophic flood origin for the Channeled Scablands, each preceded by a commentary written by the editor.
SNOWBALL EARTH
Paul F. Hoffman and Daniel P. Schrag, “Snowball Earth” (Scientific American, January 2000). Hoffman has been the leading proponent of the Snowball Earth hypothesis, and here he and his Harvard colleague Daniel Schrag present their arguments in clear and convincing language.
Gabrielle Walker, Snowball Earth: The Story of the Great Global Catastrophe That Spawned Life As We Know It (New York: Crown Publishers, 2003). Walker accompanied Hoffman to some of the important geological localities that provide evidence for the Snowball Earth hypothesis. In this book she gives a lively and very readable account that focuses (positively) on Hofmann’s ideas but also touches on those of some of his opponents.
ICE AGES AND EVOLUTION
William H. Calvin, The Ascent of Mind: Ice Age Climates and the Evolution of Intelligence (New York: Bantam Books, 1991). Calvin argues that the evolution of human intelligence was stimulated by the ice age climate of Africa. He focuses especially on the implications of fluctuating climate (and the consequent effects on vegetation) for human behavior and brain size.
William H. Calvin, A Brain for All Seasons: Human Evolution and Abrupt Climate Change (Chicago: University of Chicago Press, 2002). Calvin updates the arguments of his previous book (above) with new evidence from ice cores for extremely rapid climate fluctuations.
Steven M. Stanley, Children of the Ice Age: How a Global Catastrophe Allowed Humans to Evolve (New York: Harmony Books, 1996). Stanley argues that the ice age climate in Africa was a key element in the evolution of humans.
CLIMATE AND HISTORY
Brian Fagan, The Little Ice Age: How Climate Made History 1300–1850 (New York: Basic Books, 2000). A delightful and fact-packed book that details the chronology of events during the period of the Little Ice Age.
P. D. Jones, A. E.J. Ogilvie, T. D. Davies, and K. R. Briffa, eds., History and Climate (New York: Kluwer Academic/Plenum Publishers, 2001). A series of academic papers on many aspects of climate and history, from effects on agriculture to the spread of disease. The contributions are based on papers given at the Second International Climate and History Conference, which took place at the University of East Anglia in the United Kingdom in September 1998.
H. H. Lamb, Climate, History and the Modern World (1982; 2d ed., New York: Routledge, 1995). A classic study of the effects of climate on world history.
ABRUPT CLIMATE CHANGE
Richard B. Alley, The Two-Mile Time Machine: Ice Cores, Abrupt Climate Change, and Our Future (Princeton: Princeton University Press, 2000). Alley has spent much of his career working on ice cores and in this book provides an enthusiastic insider’s view of the harsh working conditions at the Greenland Ice Cap and the excitement and scientific rewards that accrue from unraveling the story contained there. He also reflects on the implications of these records for our future.
Committee on Abrupt Climate Change, National Research Council, Abrupt Climate Change: Inevitable Surprises (Washington, D.C.: National Academy Press, 2002). This book, compiled by a committee of the National Research Council (U.S.A.), presents the evidence for abrupt climate change that was available at the time of its publication and makes recommendations about further research necessary to help deal with such changes should they occur in the future.
INDEX
abrupt climate change. See climate, rapid changes in
Acheulean hand axe, 202–3
Adhémar, Joseph, 77, 80–81; Révolutions de la mer, 77
Agassiz, Lake, 6, 108–10, 229
Agassiz, Louis, 4–5, 15–16, 25–44, 43, 55, 58, 65, 95, 141, 244; Brazilian Fishes, 27–28; departure for United States, 40; early life, 25–27; ice age theory of, 5, 7, 15, 21, 24, 34–40; holiday in Bex, 32–34; interest in glaciers, 28; opposition to Darwinian evolution, 28, 38, 42–44; professor in Neuchâtel, 31–41; Studies on Glaciers, 34, 36, 39, 52; work with Baron Georges Cuvier, 28–30; work on fossil fish, 32
Age of Reason, 18–19
air bubbles, trapped in ice, 174, 179, 182–83
Akkadian civilization, 213–14
Albatross (ship), 168
Alley, Richard, 175; The Two Mile Time Machine, 175
Alps, 141–42, 208, 215, 221; glaciation of, 124, 127–28, 216; gravel river terraces in, 127–28, 132
Altay Mountains, 108
Amazon Basin, 42
American Association for the Advancement of Science, 99–100
Anderson College, 72–73
Antarctic ice sheet, 3, 8, 149, 176–77, 243
antelope, 194
Archean eon, 161
Archean Ice Age, 143, 161–63
Arrhenius, Gustaf, 168–69
Arrhenius, Svante, 168–69, 235
astronomical variations. See orbit, of Earth around sun
Australopithecus, 190–91, 193, 195–96, 201, 206
axis of rotation (Earth), 76, 78; tilt of, 76, 78, 79, 120–21, 124; wobble of, 78, 79, 80–81, 120–21
Bader, Henri, 177
banded iron formations, 154
Belgrade, 118–19, 130
Bengal, Bay of, 240
bipedalism, hominid, 189–91, 195, 206
Black death. See plague, bubonic
Black Sea, 203
Bonneville, Lake, 101, 105–7
boom and bust, 195, 197, 207
brain size, human, 188–90, 196, 203
Brazil, 42
Brazilian Fishes (Agassiz), 27–28
Bretz, J. Harlan, 90–99, 102–5, 104, 111
Broecker, Wally, 137–38, 182
Brückner, Eduard, 127–28, 132
Buckland, William, 57, 58
Burckle, Lloyd, 226
Burnet, Thomas, 19–21; The Sacred Theory of the Earth, 20
Byrd Polar Research Center, 184
Byrd Station, 176
Cabot, John. See Caboto, Giovanni
Caboto, Giovanni, 223
calcium carbonate: in deep sea sediments, 169; shells of, 136, 237. see also cap carbonates; limestone
Calvin, William, 190, 195, 199, 205–7
Cambrian explosion, 210
Canon of Insolation and the Ice Age Problem (Mil
ankovitch), 130
canyons, dry, of the Channeled Scablands, 93, 96
cap carbonates, 155, 157, 159, 161
carbon dioxide: changes in atmospheric, 169, 240; effect on climate, 169, 235–36; in ice core air bubbles, 180, 181–82; in Snowball Earth atmosphere, 155–57, 165; sources and sinks of, 235–38. See also greenhouse gases
carbon, isotopes of, 157, 159, 160; fractionation of, 157; in methane, 160, 242; in seawater, 159
Carbon-14, 220; dating, 132–34
Carboniferous period, 149–50
catastrophism, 7, 94
cave painting, 204
Challenger, H.M.S., 166–67
Channeled Scablands, 92–99, 100, 101, 102; multiple floods in, 103, 111
Charpentier, Jean de, 32–35, 37
chemical weathering, and CO2, 236–40
chimpanzees, 188, 195
Clarke River, 100
Climate and Time in Their Geological Relations (Croll), 85
climate: cycles of, 175; effect of volcanic activity on, 230; external forcing of, 227–28, 231; future, 6; interconnections in, 84; past, 5, 7, 139, 171–72; mathematical and computer simulations of, 60–61, 186, 228, 240, 243; mathematical theory of Earth’s, 120–22; of Pleistocene Ice Age, 164–65; positive feedback, 228; rapid changes in, 165, 197–201, 205, 213, 230; role of ocean currents in, 228; threshold, 227, 244; today’s warming, 10. See also ice age climate
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