The quality and richness of Perthshire's natural environment were formative influences on a young James Croll (1821–1890), which left him with a life-long appreciation of nature, landscape and natural meditation. Although Croll himself declares to have had little interest in geology in his earlier years, it became a central theme of his scientific understanding, which implies the clear influence of both his local environment and of his father David, a stonemason. His family and friends also shaped him in other ways, not least his love of reading, his unconstrained thinking and intellectual acuity. He inherited his father's moral character, amiability and an excitement about intellectual inquiry, which drew friends to him who made great efforts to assist him in his work, both personally and professionally, and played a role in his being offered a position by James Geikie with the Geological Survey of Scotland. Croll's financial position was often precarious; he spent a good deal of his life in relative poverty. Whilst this affected his opportunities for formal learning, it may well have led to his ability to think creatively and to seek answers more broadly than he might have if he had been able to engage in a more formal education. Ill health, which affected him throughout his life, could be seen to both hamper his work – but also through circumstance lead him to pursue a more academic path, as other routes of work were shut off to him. Ultimately Whitefield, Wolfhill and the wider Perthshire countryside in which he grew up can clearly be seen to have influenced his life in many ways, even, perhaps, to the extent of his chosen surname.

Popular and scholarly information concerning the life of James Croll has been accumulating slowly since the death in 1890 of the self-taught climate change pioneer. The papers in the current volume offer thorough assessments of topics associated with Croll's work, but this contribution seeks to provide a personal context for an understanding of James Croll the man, as well as James Croll the scholar of sciences and religion. Using archival as well as published sources, emphasis is placed upon selected components of his life and some of the less recognised features of his biography. These include his family history, his many homes, his health, participation in learned societies and attitudes to collegiality, financial problems including the failed efforts to secure a larger pension, and friendship. Life delivered a mixture of ‘trials and sorrows’, but it seems clear from the affection and respect accorded him that many looked upon James Croll as a ‘man greater far than his work’.

James Croll left school at the age of 13 years, yet while a janitor in Glasgow he published a landmark paper on astronomically-related climate change, claimed as ‘the most important discovery in paleoclimatology’, and which brought him to the attention of Charles Darwin, William Thomson and John Tyndall, amongst others. By 1867 he was persuaded to become Secretary and Accountant of the newly established Geological Survey of Scotland in Edinburgh, and a year after the appearance of his keynote volume Climate and time in 1875, he was lauded with an honorary doctorate from Scotland's oldest university, Fellowship of the Royal Society of London and Honorary Membership of the New York Academy of Sciences. Using a range of archival and published sources, this paper explores aspects of his ‘journey’ and the background to the award of these major accolades. It also discusses why he never became a Fellow of his national academy, the Royal Society of Edinburgh. In the world of 19th-Century science, Croll was not unusual in being both an autodidact and of humble origins, nor was he lacking in support for his endeavours. It is possible that a combination of Croll's modesty and innovative genius fostered advancement, though this did not hinder a willingness to engage in vigorous argument.

Scientific societies played a crucial role in the emergence of a professional culture of science in Britain in the mid- to late-19th Century. At first sight, James Croll's membership of a limited number of scientific associations may be assumed to be the result of his lack of social credit and scientific connections. In this article, by examining Croll's correspondence, I demonstrate that Croll's select participation in scientific clubs and associations reflected his strategic pursuit of a vision of science set apart from party or societal affiliation. I focus on the contrasting histories of the Royal Society of Edinburgh and the Geological Survey, as well as the institutional history of the Philosophical Magazine. Situating the institutions in their respective social and cultural contexts, I argue that the more meritocratic, inclusive social structure of the Survey and Magazine helps explain Croll's choice to avoid affiliation with the Royal Society of Edinburgh.

Science, for James Croll, began and ended in metaphysics. Metaphysics, in turn, provided proof of a First and Final Cause of all things. This proof rested on two metaphysical principles: that every event must have a cause, and that the determination of a cause is distinct from its production. This argument emerged from his deeply held religious commitments. As a 17-year-old, he converted to a Calvinist and evangelical form of Christianity. After a period of questioning the Calvinist system, he embraced it again through reading the famous treatise on the will by the New England theologian, Jonathan Edwards. This determinedly metaphysical work, which engaged as much with Enlightenment thought as with Calvinism, defended the view that the will was not a self-determining cause of human action. This ‘hard case’ provided the basis for a larger claim that every act whatever has a cause, and that the production of an act was different from its determination. In part through reading Edwards, Croll remained a devout and convinced ‘moderate’ Calvinist for the rest of his life. He also developed a deep love of metaphysics and became convinced that without it, everything, including science, remained confused and in darkness. For Croll, even the most basic science could not be properly conducted without prior metaphysical principles. But this was more than just an argument about the philosophical foundations of scientific inquiry. It was also based on Croll's conviction that the cosmos, earth history and life (including his own) was fully determined by a supreme and perfect intellect. This conviction entered into the marrow of Croll's scientific theories and shaped his interpretation of the twists and turns of his own life. In short, to take seriously Croll's own self-understanding, we need to allow him to ‘do God’.

James Croll was a pioneer in studies of the impact of the slowly changing orbital dynamics of the Earth on climate change. His book Climate and Time in their Geological Relations (1875) was far ahead of its time in seeking correlations between climate change, the occurrence of ice ages and perturbations to the Earth's orbit about the Sun. The astronomical cycles he discovered are now called ‘Milankovitch Cycles’ after the Serbian scientist whose research was first published in the Handbuch der Klimatologie in 1930. The celestial mechanical and astronomical background to Croll's research is the focus of this essay. The development of the understanding of the impact of perturbations of the elliptical planetary orbits by other bodies in the solar system paralleled new mathematical techniques, many of which were developed in association with celestial mechanical problems. The central contributions of many of the major mathematicians of the late 18th and 19th Centuries, including Euler, Lagrange, Laplace and Le Verrier, are highlighted. Although Croll's contributions faded from view for several generations, his pioneering insights have now been demonstrated to have been basically correct.

This paper examines the astronomical theory of ice ages of James Croll (1821–1890), its influence on contemporaries John Tyndall, Charles Lyell, and Charles Darwin, and the subsequent development of climate change science, giving special attention to the work of Svante Arrhenius, Nils Ekholm, and G. S. Callendar (for the carbon dioxide theory), and Milutin Milanković (for the astronomical theory). Croll's insight that the orbital elements triggered feedbacks leading to complex changes – in seasonality, ocean currents, ice sheets, radiative forcing, plant and animal life, and climate in general – placed his theory of the Glacial Epoch at the nexus of astronomy, terrestrial physics, and geology. He referred to climate change as the most important problem in terrestrial physics, and the one which will ultimately prove the most far reaching in its consequences. He was an autodidact deeply involved in philosophy and an early proponent of what came to be called ‘cosmic physics’ – later known as ‘Earth-system science.’ Croll opened up new dimensions of the ‘climate controversy’ that continue today in the interplay of geological and human influences on climate.

At a time when nobody has yet landed on the Antarctic continent (1879), this presentation and accompanying paper predicts the morphology, dynamics and thermal regime of the Antarctic ice sheet. Mathematical modelling of the ice sheet is based on the assumptions that the thickness of tabular icebergs reflects the average thickness of the ice at the margin and that the surface gradients are comparable to those of reconstructed former ice sheets in the Northern Hemisphere. The modelling shows that (a) ice is thickest near the centre at the South Pole and thins towards the margin; (b) the thickness at the pole is independent of the amount of snowfall at that place; and (c) the mean velocity at the margin, assuming a mean annual snowfall of two inches per year, is 400–500 feet per year. The thermal regime of the ice sheet is influenced by three heat sources – namely, the bed, the internal friction of ice flow and the atmosphere. The latter is the most significant and, since ice has a downwards as well as horizontal motion, this carries cold ice down into the ice sheet. Since the temperature at which ice melts is lowered by pressure at a rate of 0.0137 °F for every atmosphere of pressure (something known since 1784), much of the ice sheet and its base must be below the freezing point. Estimates of the thickness of ice at the centre depend closely on the surface gradients assumed and range between 3 and 24 miles. Such uncertainty is of concern since both the volume and gravitational attraction of the ice mass have an effect on global sea level. In order to improve our estimate of the volume of ice, we will have to wait 76 years for John Glen to develop a realistic flow law for ice.

The research of James Croll on the nature of Ice Ages led him into a detailed investigation of ocean currents. By the early 1870s he had calculated from first principles the quantities of heat delivered by ocean currents to high latitude areas and he understood how this heat supply may have altered drastically during ice ages. The publication of his many papers on ocean currents as well as his book, Climate and Time, coincided with Challenger expedition that, in 1872, embarked on a 4-year voyage of scientific exploration of the world's oceans. The expedition was crucially important for Croll since it enabled him to test his theories of ocean circulation using real data. His novel theories of ocean circulation based on this information conflicted with the established views popularly advocated by William Carpenter but they ultimately prevailed. In the many writings of Croll on ocean currents, we encounter, as with other areas of his research, numerous remarkable ideas many decades ahead their time.

Within the context of the work and achievements of James Croll, this paper reviews the records of direct observations of glacial landforms and sediments made by Charles Lyell, Archibald and James Geikie and James Croll himself, in order to evaluate their contributions to the sciences of glacial geology and Quaternary environmental change. The paper outlines the social and physical environment of Croll's youth and contrasts this with the status and experiences of Lyell and the Geikies. It also outlines the character and role of the ‘Glasgow School’ of geologists, who stimulated Croll's interest into the causes of climate change and directed his focus to the glacial and ‘interglacial’ deposits of central Scotland. Contributions are outlined in chronological order, drawing attention to: (i) Lyell's high-quality observations and interpretations of glacial features in Glen Clova and Strathmore and his subsequent rejection of the glacial theory in favour of processes attributed to floating icebergs; (ii) the significant impact of Archibald Geikie's 1863 paper on the ‘glacial drift of Scotland’, which firmly established the land-ice theory; (iii) the fact that, despite James Croll's inherent dislike of geology and fieldwork, he provided high-quality descriptions and interpretations of the landforms and sediments of central Scotland in order to test his theory of climate change; and (iv) the great communication skills of James Geikie, enhanced by contacts and evidence from around the world. It is concluded that whilst direct observations of glacial landforms and sediments were critical to the long-term development of the study of glaciation, the acceptance of this theory was dependent also upon the skills, personality and status of the Geikies and Croll, who developed and promoted the concepts. Sadly, the subsequent rejection of the land-ice concept by Lyell resulted in the same factors challenging the acceptance of the glacial theory.

James Croll's Physical Theory of Secular Changes of Climate emerged during an age of revolution in geology that included the rise of the glacial theory and the search for its underlying causes. According to Croll, periods of high eccentricity are associated with the persistence of long glacial epochs, within which glaciations occur in alternate hemispheres when winter is at aphelion every ~11,000 years; however, astronomical forcing is only able to produce glaciation by means of physical agencies (climate feedbacks) that amplify the small effects of varying seasonal irradiation. Croll understood the importance of interglacial deposits because they provided evidence for the occurrence of multiple glaciations within his long glacial epochs. He was aware of the limitations of the terrestrial record and suggested that deep-sea sediments would contain a continuous succession of glacial-interglacial cycles. Contrary to a widespread view, however, Croll was not envisaging the advent of palaeoceanographic exploration avant la lettre, but instead was drawing attention to the inadequacy of the land record as a testbed of his astronomical theory. Yet, the marine record did eventually deliver a test of astronomical theories almost exactly 100 years after the publication of his 1875 book Climate and Time in their Geological Relations. Here, we provide an historical account of the technological and scientific developments that led to this and a summary of insights on astronomically paced climate changes from marine, terrestrial and ice core records. We finally assess Croll's ideas in the context of our current understanding of the theory of ice ages.

Our climate future depends on the delicate, fine balance of earth processes first elaborated on by James Croll, born 200 years ago in 1821. A childhood victim of the Scottish clearances, Croll, after following various indifferent occupations, managed to remove to the then rapidly industrialising city of Glasgow and eventually to Scotland's capital, Edinburgh. He blossomed as a most original, outside-the-box, thinker of great intellectual strength and modesty. He carried out scores of studies across a broad range of research topics, many related to the physical causes of climate change. He is well known for his astronomical theory of the ice ages, but should be much better regarded for his incisive physical insights into the central importance of feedbacks in the Earth system. Although humble, Croll was an ardent controversialist who strongly, perhaps over-strongly, always defended his corner. As well as his many accomplishments as a man of science, Croll was committed to exploring philosophical questions of theism and determinism, topics which occupied his earliest and last publications. A ‘top ten’ selection out of the varied subject areas that Croll tackled are explored herein, along with a brisk survey of their legacy to contemporary modelling studies and to Earth's climate future: (1) causes of climate change (1864); (2) ice-cap melt and sea-level rise (1865); (3) predicting future climates using eccentricity (1866); (4) combining orbital precession, eccentricity and obliquity (1867); (5) geological time and the date of the glacial epochs (1868); (6) geological time and denudation rates (1868); (7) ocean currents and the hemispherical temperature difference (1869); (8) feedbacks – a remarkable circumstance which led to changes of climate (1875); (9) temperature of space and its bearing on terrestrial physics (1880); (10) the causes of mild polar climates (1884).

James Croll (1821–1890) was a gifted scientist whose revolutionary theories had a profound impact on our understanding of the Earth's climate, ice ages and glaciation. While his contributions are recognised by an increasing number of modern-day scientists, in the public domain his legacy has been all but forgotten. Popularising Croll's story brings its own challenges: we know more about the science than we do about the man, and his theories do not lend themselves to quick and easy explanation. While Croll's scientific theories, presented alone, risk being viewed as complex and difficult to digest, his lifelong struggle against adversity is a compelling story with the potential for widespread public appeal. In recent years, interested individuals and institutions have begun to increase public awareness of Croll through talks and lectures, exhibitions, theatrical events and articles in print and online. There are many more possibilities that are worth investigating, in order to inspire and engage people locally, nationally and even internationally. Croll's story is fascinating from many points of view, and is open to interpretation by people of different ages and backgrounds. Perhaps even the gaps in our knowledge can be turned to advantage, allowing for imagination, creativity and expression.

Source materials for investigating the life of James Croll are examined and collated. This is organised around the topics of: Croll's Autobiographical sketch and the Memoir of his life and work, both contained within the volume produced by James Campbell Irons; publications by Croll; aspects of his genealogy; manuscript sources in publicly accessible archives and in private ownership; and other published sources.