John Imbrie and Katherine Palmer Imbrie: Ice Ages, solving the mystery (Harvard University Press, Cambridge, MA, 1979, 1997), 224 pages, index, bibliography, chronology, suggested reading.
Ice Ages traces the history of the discovery of past ice ages from the 18th and 19th century and the evolution of the "astronomical theory" of their causation.... The focus is on the Milankovitch hypothesis which relates ice ages to the amount of sunlight received in northern regions of the globe as different "parameters" of the earth's orbit - like the tilt of the earth's axis of rotation - vary systematically over time.
This is one of the very best books of scientific popularization I have ever read. Earth science research tends to ressemble the plot of an Agatha Christie novel, a point the authors themselves make. Unlike the physical scientist who conducts tightly controlled experiments in a lab setting, the earth scientist is a sleuth, stalking a smoking gun admidst the chaos, confusion and accident of the "real world". The Imbries are masters at exploiting this narrative quality of earth science research: one keeps turning pages to find out the next twist of the plot. Remarkably readable and conscise, I finished the book in three days of leisurely evening reading. The textual figures, abundant and well explained, are admirably integrated into the text as illustrative material.
Here is an example of a "grassroots" scientific hypothesis rising from the observations of nature of peasants and hunters living off the land. These men observed that movements of Swiss glaciers transported material and conjectured that in the past glaciers were larger and had transported material over distances where today no glaciers exist. Academic scientists, on the other hand, tended to defend the Biblical story of the Deluge which they held responsable for the ancient transport of debris, even into the high reaches of mountain ranges. It was only after a few big intellectual guns like Swiss-American naturalist Louis Agassiz were won to the side of glacial expansion that the scientific establishment began to take the idea seriously.
The Imbries present an admirable synopsis of proposed causes for ice ages and the challenges these present to the researcher:
1- Variations in solar output. These are found to be too weak, accounting for a 1-2 C variations in temperature globally. Such variations do, in fact, correlate with the movement of glaciers over the past 1,000 years but are insufficient to explain full blown ice ages.
2- Passages of our solar system through clouds of "space dust". The hypothesis is internally inconsistent as it now stands. Some theorists argue that such a dust cloud would cool the earth by screening sunlight, others that the dust will stoke the fires of the sun and make it burn brighter!
3- Variations in atmospheric carbon dioxide (CO2) content. CO2 is a greenhouse gas, capable of warming the earth by trapping heat in the lower regions of the atmosphere. The difficulty is finding a mechanism that could reasonably account for the observed glaciation / deglaciation cycle. Observations of past CO2 levels even suggests that CO2 variation is a response to - not a cause of - ice age temperature variations.
4- Volcanic activity. Dust and sulfate droplet clouds reflect solar energy but evidence of sufficient volcanic activity is not found in the sedimentary record to confim the hypothesis. The problem is, in part, technical: lack of sufficiently accurate, standardized measurement procedures.
5- Rise in land elevation leading to cooling. There is no credible mechanism that could account for such elevations.
6- Antarctic ice sheet instability. Two variations of the hypothesis exist. Either the ice sheet periodically breaks down filling the sea with reflective ice, triggering an ice age, or periodic increases in moist air flow over the pole cause the sheet to grow, triggering an ice age. There is no evidence for either variant of the hypothesis and for the second, no plausible mechanism is known.
7- Stochastic hypothesis, based on sophisticated mathematical models proposed by Self-Organization Theory. Small random variations in the functioning of the climate-machine occur more often than large ones do. Thus annual variations in temperature are greater between decades than for the years within a decade. If one looks long enough - hundreds of thousands of years - we will find natural random variations accumulating into ice ages. Since no particular event actually causes ice ages, it is a hard hypothesis to test! Science requires testable hypotheses based on measurable properties which obey causal laws. Until theorists tell us how to get around these restrictions, we must remain unconvinced..
8- The astronomical hypothesis based on variations in the orbital alignments of the earth in relation to the sun. Since the 19th century various mechanisms have been proposed to explain the onset and termination of ice ages. At present, a version of this hypothesis, proposed by Milutin Milankovitch in the first half of the 2oth century, is the accepted explanation.
Milankovich, a mathematician by training, proposed that the critical trigger for glacial expansion is the amount of solar energy (insolation) received by the northern hemisphere in summer. Insolation varies because of periodic variation in the "parameters" of the earth's orbit: 1- eccentricity ("ovalness" of orbit), 2- tilt angle of the axis of rotation and 3- precession (direction of the earth's axis of rotation measured at Spring Equinox).
The twentieth century proved crucial to the verification of the essential correctness of Milankovitch's theory. Technological and scientific advances led to the discovery of several natural chronologies or calendars which permited accurate dating of glacial advances and retreats. Among these were the discovery of reversals of the earth's magnetic field which are recorded in the orientation of magnetic mineral grains in lava flows. The sequence of reversals allows dating of the original lava flow. Radio-isotopic measures permited dating of past events: since radio-isotopes decay at a know rate, they provide a clock measuring elapsed time. An example is radio-carbon chronology. Carbon 14 is formed in the atmosphere as a result of cosmic rays. Carbon is absorbed by living organisms (in photosynthesis) but when the organism dies it stops ingesting carbon and the radiocarbon decays into inert elements. The amount of C-14 left in the remains of the organism gives a measure of the time elapsed since its death. Other, non-radioactive, isotopic measures of oxygen indicate the amount of ice trapped in glacial sheets. These various measures now provide the earth scientist with an amazingly detailed picture of the evolution of past climate and have permitted the verefication of the (modified) Milankovitch hypothesis.
We now know, for example, that at this time of the earth's natural history, our planet is subject to periodic ice age / deglaciation cycles. These last about 100,000 years, corresponding to the major "driver" of the cycle, the eccentricity of earth's orbit around the sun: the orbit goes from circular to slightly oval and back again in about 100,000 years. Ice age cycles are characterized by slow cooling followed by a rapid warming (deglaciation or interglacial epoch). The latter phase is transient, lasting on average 10 to 12 thousand years. A spectral analysis of the frequency of ice age / deglaciation cycles preserved in oceanic sedimentary cores has permited the isolation of four major periodicities, each corresponding to dominant periods of earth's orbital mechanics. The 100,000 eccentricity cycle is the most powerful driver, followed by a strong "signal" of 43,000 years from the obliquity (axis tilt) parameter. And lastly, two signals of 24,000 and 19,000 from the two components of the precessional period (determined by the direction of the earth's axis on the Vernal Equinox). Milankovitch's work has survived the test of time and is today the dominant model of natural climate change for recent geological history (last few million years).
But what about the future? Today there is much talk and (pseudo-)debate about human induced climate change due to the combustion of fossil fuels and the cutting of forests. Orbital mechanics teaches that the earth is now in a cooling phase, heading into the next ice age. Since cooling, unlike interglacial warming, is a slow gradual process, it does not make much sense to ask when, exactly, the next ice age is expected to begin. For this we would need a universal, standard set of criterions which have not, at present, been formulated. The depth of the next scheduled ice age, though, should be in about 23,000 years. The Milankovitch theory is confirmed by the fact that the warmest period of our current interglacial occurred several thousand years ago. After which, the earth began to cool off - despite minor cyclic rises probably due to variations in solar output. When we reach the 20th century though, things begin to fall apart and the theory no longer works. CO2 emissions have risen dramatically since the industrial revolution and these, claim the Imbries, are creating a "super-interglacial" warming which overides the naturally occurring cooling of the planet.
The first edition of the Ice Ages was published in 1979. The Imbries predicted that human-induced warming would become noticeable around the year 2000. Even more interestingly, fig 47 (page 185) shows a version of the (in-)famous "Mann hockey stick" rise in temperture purportedly caused by human produced CO2 emissions - yet Ice Ages was published decades before the hockey stick controversy erupted!
It's hard to fault this book so I give it a 10 on 10 rating. Since perfection is not possible on earth, I feel I must hasten to pick out some fault, however triffling. The only thing I could find was a certain wordiness in the last pages. John Imbries, one of the authors, is also a climatological researcher and a participant in the later phases of the verification of the Milankovitch hypothesis. In an attempt not to grandstand his colleagues when discussing his own part, he gives exhausting - and boring! - lists of researchers, educational institutions, funding agencies, etc. But even there, it seems he was acting from a laudable motive of intellectual fairness and honesty. A really good book for anyone wanting to initiate themselves into the mysteries of climate change from a scientific perspective.