Six-minute summary: James Croll

Hello readers! This week, I’m summarising the life of a lesser-known scientist. James Croll was once a big name in the British scientific community, exchanging ideas with contemporaries such as Charles Lyell and Charles Darwin. He was a physicist, geologist, and mathematician – but when he published his first paper, he was employed only as a janitor at Anderson’s College in Glasgow. He was the first to propose that Earth’s climate could be influenced by planetary motions (quite literally a revolutionary idea), yet he was quickly forgotten. It’s time he got a bit more recognition.

A love of science

James Croll was born in Perthshire, Scotland, in 1821. He was the son of a stonemason, and was raised on a farm with no formal education. At the age of eleven, he got a copy of the Penny Magazine, a weekly publication that aimed to educate working class people on scientific discoveries (the Victorian equivalent of New Scientist). This was when James Croll discovered his love of science.

He wasn’t particularly interested in nitty-gritty “facts or details”, such as the meticulous recording of plants, shells, or mineral properties. Instead, he saw facts and details only as a means to illustrate larger underlying laws and principles. He wasn’t fond of geology, because most geologists at the time were making detailed observations of rocks without pondering how they got there (God only made unconformities to test us).

An intriguing LinkedIn profile

The young James Croll never stayed in one job for long. He started off as a millwright, then became a house joiner, but had to give up manual labour at the age of 25 due to an inflamed elbow. He then managed a temperance inn, and when it went under (nothing to do with it being in Scotland), he became an insurance agent, then a newspaper man. Finally, he got a job as a janitor in Anderson’s College in Glasgow, a technical school that much later became part of the University of Strathclyde. This role allowed him access to the library, where he continued to educate himself.

Starting a scientific career

Throughout his various jobs, James Croll kept reading. In fact, he soon started writing, and in 1857, when he was 36 and working as an insurance salesman, he had his first book published: The Philosophy of Theism (undoubtedly a page-turner). By the time he was working as a janitor, he had published other papers on electricity and heat. However, it was his paper on climate fluctuations, published in the Philosophical Magazine in 1864, that launched him to scientific stardom.

The success of his climate paper helped him get a job at the Geological Survey of Scotland in 1867. However, his role wasn’t scientific: he was in charge of ordering, printing, colouring and selling geological maps, and for keeping the stores in order (clearly a role better suited to a geographer). He worked from ten until four, then spent the evening writing, churning out a number of books and papers, including the book Climate and Time.

Scientific renown

In 1876, James Croll was elected Fellow of the Royal Society. He was also given an honorary degree by the University of St Andrews, although he had to pay 10 pounds and 10 shillings for it – equivalent to more than £1000 today. Despite having never received a formal education, James Croll had become part of the scientific community, and had attained widespread recognition.

However, it appears that he never felt at home with other scientists. He had issues with the back-stabbing, name-dropping and showmanship of science, and turned down an offer to lecture at the Royal Institution. He kept himself to himself, and was seemingly too reserved to throw himself into public, attention-grabbing scientific arguments.

Later life and eventual obscurity

In 1880, James Croll retired from the Geological Survey early due to heart trouble. Having attained this job late in life, his pension was terrible, and his income dropped from £350 to £75. The Crolls were forced to sell their house and start renting again, struggling to make ends meet. James Croll still managed to write three more books in his retirement, but he died in 1890, at the age of 69 – and by this point, his theory had been widely disregarded.

James Croll’s climate theory

James Croll knew that the eccentricity of Earth’s orbit around the sun varied over thousands of years, slowly oscillating between being circular and elliptical. When the orbit is very eccentric (oval), the Earth is much further from the sun at certain times of year. He predicted that if winter occurred when the Earth was furthest from the sun, then this winter would be particularly cold, triggering a glacial period.

Crucially, he predicted that the minor cooling caused by orbital eccentricity would then be amplified by other environmental processes, creating positive feedback. The harsh winters would increase snow and ice, which would reflect more sunlight, making the planet colder, and leading to more snow and ice (what we now call the albedo effect). He also proposed that a slight cooling of the North Atlantic could modify ocean currents, making Europe colder. Today, these feedback mechanisms are being researched as potential tipping points for catastrophic climate change. However, when James Croll presented them, their significance was largely overlooked.

A revolutionary theory in more ways than one

In the mid nineteenth century, there was a growing pile of evidence for multiple ice ages, but geologists had yet to come up with an explanation. Forty years before James Croll presented his theory, John Herschel (son of William Herschel, of Uranus fame) ruled out orbital eccentricity as a means of altering climate. He argued that the total energy received from the sun was constant each year, because when the Earth is closer to the sun, it moves faster, and vice versa. Laplace had previously ruled out the obliquity of the ecliptic as a means for changing the climate, and with such respected mathematicians having discredited these mechanisms, it was hardly surprising that people abandoned them.

Charles Lyell, one of the most famous geologists at the time, believed that the climate was influenced by the distribution of land. If land existed only at the poles, it would collect ice, and the climate would be colder than if all the land existed at the equator (although he couldn’t explain how or why the land would move so quickly). However, geologists soon ascertained that landmasses in the last glacial period were in pretty much the same place as they are now.

Many geologists at the time had no mathematical background. This led to them concocting some wild ideas; for example, that huge mountains had suddenly arisen and impacted the Earth’s rotation, causing an ice age (Lord Kelvin stepped in to dismiss this one). The craziest hypothesis of them all was that the Earth may have passed through hotter and colder regions of space. How? Why? Who knows.

It is therefore unsurprising that James Croll’s theory gathered so much attention. It was the first time someone had proposed something feasible, backed up with maths. Charles Lyell revised his Principles of Geology to include Croll’s theory, having written to John Herschel and Sir George Biddell Airy (the Astronomer Royal), to ascertain whether Croll could be right. Both of them thought he probably was – and although Lyell initially wrote a 22-page rebuttal, he eventually accepted that orbital eccentricity might have an impact. Still, he told Darwin that he considered it “quite subordinate” to his own theory. Darwin, by contrast, was very positive about Croll’s work. He stated that he had “never in [his] life been so deeply interested by any geological discussion” (so he clearly should have spent more time looking at rocks).

However, despite these glowing endorsements, James Croll’s theory was abandoned just a few decades later.

What was wrong with his theory?

It’s safe to say that James Croll didn’t get everything right. He used equations from Leverrier and Lagrange to find the maximum orbital eccentricity, and he also accounted for the precession of the equinoxes (the slight wobble of Earth’s rotational axis) and changes in obliquity (the tilt of the rotational axis). However, he stated that ice ages would only occur in one hemisphere at a time. He predicted that when orbital eccentricity was high, the northern hemisphere winter would be particularly cold and glaciated, but the southern hemisphere winter would be warmer and ice-free (and vice versa). This assertion was his undoing.

In the late nineteenth century, geological research was booming. Geologists soon found evidence of ice ages impacting both hemispheres at once, and this discovery led them to disregard James Croll’s theory – even the bits which were right.

Milankovitch cycles

The underlying laws of James Croll’s theory weren’t revisited until 1920, when Serbian scientist Milutin Milankovitch ran the calculations again. He estimated the energy received from the sun at various latitudes over the last 650,000 years based on orbital eccentricity, precession and obliquity (see the diagram above). He found that periods with cool summers coincided almost perfectly with the last four Alpine glaciations – and these findings were published in 1924, in a work by Wladimir Köppen and Alfred Wegener. Milankovitch’s theory was the opposite of what James Croll had predicted: glaciation wasn’t triggered by high eccentricity creating very cold winters, but by low eccentricity creating cool summers, reducing the amount of glacier melting each year.

However, scientists in the mid-twentieth century still weren’t keen on the astronomical theory. It was only widely accepted in 1976, following the work of James Hayes and others, who found evidence for changes in climate in a number of independent proxy records. This was 50 years after Milankovitch’s initial publication, and 10 years after his death – and it was a staggering 112 years after James Croll’s theory was published. These days, the concept of astronomical climate forcing is widely accepted, to the point where it is taught in schools. We refer to these changes as “Milankovitch cycles”.

Important ideas left behind

To me, one of the most important aspects of James Croll’s work was his appreciation of climate feedbacks. He knew that “although an increase of eccentricity could have no direct tendency to lower the temperature… it might bring into operation physical agents which would produce this effect.” The idea of runaway positive feedback is a crucial concept for climate science.

Unfortunately, Croll’s name and ideas were quickly forgotten. Perhaps this was due to him entering the scientific sphere relatively late, or due to his reserved nature and dislike of scientific showmanship. It wouldn’t surprise me if he felt like an outsider, having only made it into a university as a janitor, and coming from a working class background. People didn’t see the value of his work, and he had nobody to promote his ideas after he died. Had people only investigated his theories sooner, we might have understood the fragility of our climate before it was too late.

In summary…

Croll was a visionary. He was the first to unite geology and astronomy in a convincing manner (literally uniting heaven and Earth), and even if he got some things wrong, his ideas were well ahead of their time. Nowadays, the concept of positive feedback is pretty mainstream, with the discovery of climate “tipping points” making headlines. I think it’s a real shame that James Croll’s name has been lost to history.

Happy reading, and have a lovely week!

(NOTE: James Croll’s book, Climate and Time, is available for free online. His 1864 paper, on the other hand, is available to rent for an entirely reasonable and ethical £45 for 48 hours, courtesy of the benevolent Taylor and Francis. Where does the money go? Who knows. James Croll certainly could have done with it in the 1880s.)