Variations in the Earth’s Orbit: Pacemaker of the Ice Ages. Pre-publication typescript headed ‘Manuscript accepted for publication in Science (as submitted Oct. 12, 1976)’.
[Washington, D.C. American Association for the Advancement of Science, 1976]. Extremely rare pre-publication typescript of one of the most important papers in climate science, the definitive proof of Milanković’s theory of ice ages, that they result from variations in the precession, eccentricity, and obliquity of the Earth’s orbit around the Sun. This copy is heavily annotated on the title page by the celebrated American palaeontologist, evolutionary biologist, and science writer Steven Jay Gould. Milanković calculated the effects of the variation in the Earth’s orbit on the incoming solar radiation in the Northern Hemisphere. He concluded that Earth’s orbit changes in three cycles of different lengths and theorized that there were variations of more than twenty percent in the amount of sunshine reaching the northern latitudes. In his 1941 account, Canon of Insolation and the Ice Age Problem, he suggested that this caused the waxing and waning of the great continental ice sheets. Using ocean sediment cores, Shackleton, Hays and Imbrie demonstrated in the present paper that oscillations in climate over the past few million years could be correlated with variations in the orbital and positional relationship between the Earth and the Sun, as predicted by Milanković. “Shackleton’s work contributed to the first global compilation of climate data, CLIMAP1, in 1976 … He [of Cambridge University], John Imbrie of Brown University and Jim Hays of the Lamont Doherty Geophysics Observatory, showed that there was a strong signal in the ocean oxygen isotope record from the volume changes associated with ice ages. There were cyclic changes in the signals that took place over familiar periods of 23000 and 41000 years, timescales familiar from theoretical work in the 1930s by the Serbian climatologist Milutin Milankovitch. These and the 100,000-year cycle were identified with ice ages by Milankovitch, on the basis of on solar insolation theory, changes in the radiation reaching Earth from the Sun as a result of regular changes in the precession, obliquity and eccentricity of the Earth’s orbit. Shackleton, Imbrie and Hays’s confirmation of the Milankovitch cycles in the ice age record was a key finding: changes in our climate are induced by processes outside the Earth itself. It implied that these orbital cycles could be found in a range of palaeoclimate data. How and why the Earth’s orbital changes affect the climate remained – and remain – difficult questions, but the very existence of such cycles in the recent rock record sparked new interest. When researchers looked for these periodicities, in tree ring records or annual sediment layers in glacial lakes, they found them. Palaeoclimatology was born” (‘Ice Ages,’ Geoscientist 17, 1 January 2007). This article was subsequently published inScience, Vol. 194, No. 4270, 10 December 1976, pp. 1121-1132. No other copy of this pre-publication document located. Provenance: Stephen Jay Gould (1941-2002), American palaeontologist, evolutionary biologist, and science writer. Not signed by him but bearing his filing mark (which matches his usage elsewhere) in addition to his extensive annotations to the first page, under the strident heading ‘My approach’, and with four numbered sections, concluding ‘Funny / what looks good in direction, / a major problem / in magnitude / (my point).’ Gould’s interest stems from his attempt to develop a comprehensive theory of evolution over shorter ‘ecological’ time periods, ‘normal geological time’, and in periods of mass extinction (this is Gould’s ‘three tier’ system; see his classic ‘The paradox of the first tier’, 1985). Although he had previously acknowledged the role of Milanković Cycles in climate change, the evidence of his later work – and also his annotations here – is that he was unconvinced that the right level of precision had been reached, especially in measuring absolute temperatures. Gould apparently never published his response, so the annotations here are a unique record of his thinking. Earth’s history has been characterized by periods, called Ice Ages (with capital letters), when the climate was markedly colder than at other times. The most well known are the Pre-Cambrian, the late-Ordovician, the Permo-Carboniferous and the Pleistocene Ice Ages – the last of these, which began around 2-3 million years ago, is the one we are currently living through. Ice Ages are characterized by multiple switches of the global climate between cold periods (called ‘glacials’ and sometimes ice ages in lower case) when extensive ice sheets are present, and warm periods (‘interglacials’) when there is less ice over the Earth or when the climate is more or less similar to or warmer than today. In the second half of the 19th century several scientists, notably Joseph Alphonse Adhémar (1797-1862) and James Croll (1821-90), suggested that the origin and recurrence of these glaciations may be linked to changes in the Earth’s orbit. The most important features of the orbit as far as the effect on climate are concerned are its eccentricity (the degree to which the elliptical orbit differs from a circle); the tilt (or obliquity) of the Earth’s axis of rotation with respect to the plane of its orbit; the precession of the Earth’s axis of rotation (like the wobble of a spinning top); and the precession of the orbit (a slow rotation of the orbit itself around the Sun). Each of these quantities varies periodically, typically with periods of tens of thousands of years, due to the gravitational effects of other planets and the fact that the Earth is not a perfect sphere. And each can affect the amount and distribution of the solar radiation reaching the Earth’s surface. “By 1890, because of uncertainties in the timing of ice ages and deficiencies in the stratigraphic record, the astronomical theory was largely disregarded for at least three decades. Geologists and climatologists were trying to find the cause of the ice ages in Earth’s autonomous system (atmosphere–ocean–ice) as well as in the ‘solar theory,’ which postulated variations in the output of the Sun. None of these theories could be adequately tested. Milanković applied himself to reviving the astronomical theory when it was nearly entirely abandoned, having almost every geologist against it. He realized that the astronomical theory had fallen into disrepute not because of any intrinsic weakness, but because of insufficient knowledge of celestial mechanics and Earth history. Determined to refine it, he built a mathematical apparatus for an exact survey of the insolation (the word is derived from incident solar radiation) of a planet, as well as the distribution and effects of heat in its atmosphere and created a method for calculating the consequent alterations of the climate” (DSB). Milanković suggested that insolation (the amount of incoming solar radiation) at a latitude of 65 °N, just south of the Arctic Circle, was critical. At this latitude, insolation can vary by 25% (from 430 to 560 watts per square metre). Milankovitch argued that when insolation was reduced during the summer, some of the ice in this region could survive. Each year, the ice would build up to eventually produce an ice sheet. “In 1976, three gifted scientists joined forces to test Milankovich’s theory, using long-term climate records that were obtained by analysing marine sediments. Jim Hays founded and led the international CLIMAP project, which used fossil assemblages to estimate past sea surface temperatures and brought the dream team together. Nick Shackleton was the master of stratigraphy and provided oxygen isotope records, which showed past global ice volumes. Finally, John Imbrie was the expert in time-series analysis, particularly spectral analysis, which showed that the climate records matched calculations of high-latitude insolation. Remarkably, the authors discovered that these records contained the same temporal cycles as three parameters that describe Earth’s orbit: eccentricity, obliquity and precession. “Eccentricity describes the shape of Earth’s orbit around the Sun, which varies from nearly a circle to an ellipse, in part because of Jupiter’s gravity. Obliquity is the tilt of Earth’s axis of rotation with respect to the plane of its orbit, which directly affects the intensity of the seasons. Finally, precession is the most complicated type of variation because it alters the distance between Earth and the Sun during each season, and has two components — Earth’s rotational axis precesses (rotates) due to tidal forces exerted by the Sun and the Moon on the solid Earth, and Earth’s orbital path itself precesses around the Sun. “By demonstrating the clear link between orbital forcing and Earth’s past climate, Hays, Imbrie and Shackleton legitimized what was to become one of the most powerful tools in stratigraphy. For example, reliable and comparable age models can be constructed for past climate records for at least the past 5 million years by tuning the orbital parameters to the ice age cycles. Such age models can be applied to any long palaeoclimate record, allowing marine and land records to be compared. “Additionally, the different effects of the three orbital parameters have been used to study orbital forcing at different latitudes. Obliquity has a strong influence at high latitudes, whereas precession has a significant impact on seasonality in the tropics — precession has been linked to the rise and fall of the African rift valley lakes, and even our own evolution. Evidence for the orbital forcing of climate has now been found as far back as 1.4 billion years ago, in the Proterozoic eon. “Hays, Imbrie and Shackleton clearly set out the limitations of their study and posed challenges to the scientific community, many of which still remain today. In particular, the authors recognized that variations in the orbital parameters did not cause the ice age cycles but rather paced them. Any given combination of parameters can be associated with many different climates — for example, we have a similar orbital configuration today as 18,000 years ago when there was a 3-kilometre-thick ice sheet sitting on North America. Feedback mechanisms take the small changes in insolation driven by the orbital parameters and push the Earth into or out of an ice age. Therefore, the next step was to understand the relative importance of ice sheet, ocean and atmospheric feedbacks, which led to the discovery that greenhouse gases had a pivotal role in controlling past climate. “The authors’ work also recognized the so-called ‘100,000-year’ problem. Before 1 million years ago, ice ages occurred roughly every 41,000 years due to variations in Earth’s obliquity. This makes climatological sense because the tilt of the Earth directly controls how warm or cold the summers are in the Northern Hemisphere. But the last eight ice age cycles had a longer period of. 100,000 years, which is similar to the period associated with eccentricity. In terms of forcing, eccentricity is by far the weakest of the three orbital parameters, which would suggest some complicated ‘nonlinear’ amplification affect by the Earth’s climate system. “However, the similarity between the two periods turned out to be an artefact of spectral analysis — although the last eight ice age cycles lasted for about 100,000 years on average, they ranged in length from 80,000 to 120,000 years. With the realisation that eccentricity is not the major driving force, a debate has emerged as to whether precession or obliquity controlled the timing of the most-recent ice age cycles. Some argue that deglaciations occurred every four or five precessional cycle, others suggest it is every second or third obliquity cycle9 and some argue it is a combination of both. The debate started 40 years ago and still rages today. “The authors’ work also provides us with a tool to investigate the future of Milankovitch cycles. It has been suggested that small increases in greenhouse gases due to the expansion of agriculture that started 8,000 years ago have in fact delayed the next ice age. Moreover, if our greenhouse gas emissions continue to grow, we might have put off the next ice age for at least half a million years. Understanding orbital forcing is therefore relevant to contemporary debates about the Anthropocene — a proposed geological Epoch that is shaped by human activity. If we have merely delayed the next ice age, we will still be in the Quaternary Period and the Anthropocene can be defined as an Epoch. But if we have stopped the ice ages, we will have entered the Anthropocene Period, marking a larger change in the Earth System. Hence understanding the causes of the great ice ages is central to both understanding the past and our future” (Maslin). Maslin, ‘Forty years of linking orbits to ice ages,’ Nature 540 (2016), pp. 208-209.
Reproduced typescript (280 x 217 mm), 56 leaves, printed on rectos only, with tables and diagrams in text. Stapled in upper left corner.
Item #6419
Price: $4,500.00
