Monday, January 31, 2011

The study of Global Warming Part 5: Joseph Adhemar and James Croll

Joseph Adhemar (1797-1862) was the first person to develop an astronomical theory for the ice ages and interglacials. Aggasiz's publication of Etudes sur les glaciers stimulated a great interest in the causes of climate change, now that it was shown to have occurred.

Joseph Adhemar was the first to propose an astronomical cause for the ice ages. Adhemar's theory was wrong, but it did point later scientists in a useful direction.

Adhemar knew about the precession of the equinoxes (which made such a splash a few weeks ago when it was revealed that astrological signs are no longer correct). The Earth's orbit is elliptical, and as a result the earth moves more slowly when it is further away from the sun. Now, the Earth is closest to the sun during the first week of January, and furthest during the first week of July. As a result, the polar night is 178.8 days at the North Pole and 186.5 days at the South Pole.

As a result, Adhemar believed that ice ages alternated by hemisphere, with the Southern Hemisphere currently in an ice age, and the Northern Hemisphere in an interglacial. He published his ideas in Revolutions of the Sea (1842). He believed the length of the polar night was the determining factor. This was wrong, he didn't take into account that when the polar night is longer, the sun is closer to the Earth during the summer, and actually delivers more heat. And of course it turned out that ice ages are global, not alternating by northern and southern hemisphere.

And yet, Adhemar did stimulate new investigations into astronomical influences on climate change.

James Croll (1821-1890) was the first scientist to develop a good astronomical mechanism for causing the ice ages. His ideas were valid--although his calculations contained many errors and his timing was way off. However, these errors were not his fault.

Croll was not from a wealthy background and was not formally educated. Many bright people in the 19th century never were able to obtain a high school, let alone university education. He was apprenticed as a wheelwright when he was 16, but being very bright was not happy with his lot in life. He read when he could and self-taught himself physics and astronomy, and became a tea merchant, then a manager of a hotel, and then an insurance agent. In 1859 he became caretaker to the museum of the Andersonian college (presently the University of Strathclyde), so as to have access to the college library, where he could get information on his pet project---solving the mystery of the ice ages!

Croll worked for over a decade, working out how variations in the eccentricity of the orbit of the Earth, the procession of the equinoxes, changes in the tilt of the Earth's axis and variations in sunlight received at the poles that these variations caused could cause the ice ages.

An objection to astronomical influences on climate had been that the changes in sunlight in the polar regions were so small and subtle. Croll realized that these variations could be more than 10% and therefore significant. Croll also had a key insight. The albedo variations caused by changes in snow cover could amplify the changes in climate--a positive feedback! This new idea when combined with orbital and axial variations enabled them together to account for all the temperature change needed to plunge the Earth into an ice age, and bring it out!

Unfortunately, information was not available to calculate accurately how the gravitational attraction of other planets changed the eccentricity of the Earth's orbit. The distances of the planets from each other and the Earth were known to a relative degree, but the absolute distances were still not known to within a few percent. The masses of the planets were also not known very well. Jupiter and Venus influence the orbit of the Earth more than other planets (although all the planets from Saturn inwards have measurable effects). In particular, while the mass of Jupiter could be determined with some accuracy due to the orbital motions of the Galilean satellites---Venus has no satellites, and therefore there was no way to determine its mass. The same thing with Mercury.

Another factor was that relativity had not been discovered. In relativity, mass and energy are equivalent, and the energy of gravitation has its own mass---which changes the orbit of Mercury enough so that its orbit had motions not accounted for in Newtonian mechanics. In fact, there was a widespread belief that there had to be a planet closer to the sun than Mercury to account for its orbital behavior, and astronomers were on the hunt for the planet Vulcan, believed to be between one-third and half the distance from the sun that Mercury is.

The result of all this is that while Croll calculated accruately with the best available information he had, his solutions to the equations for determining variations in the orbit of the Earth were wrong. Way wrong.

Croll thought the last ice age had ended 80,000 years ago. Before carbon dating emerged after World War II, it was impossible to date material accurately. But there were two clues that indicated 80,000 years was far too long.

First, Niagara Falls. By the late 19th century, Niagara Falls had been observed for more than 200 years, and its rate of erosion back along the Niagara River was well known. Timing back from when the falls first started on the Niagara Escarpment yielded estimates of around 10,000-12,000 years for the age of the falls when the ice sheet receded. Of course the river flow and the condition of the rocks could be different in the past, and the extreme outlier estimates were as young as 6,000 years and a few estimates were beyond 20,000 years---30,000 at the very extreme. But there was no way, geologically, that Niagara Falls was 80,000 years old.

There were also annual deposits of silt in lakes that left thin layers---and these could be investigated. None of these lake deposits went back more than 20,000 years. And during the oldest lake deposits, investigations of pollen in lake bed sediments revealed that the vegetation was from colder adapted plants more than 10,000 years ago, and no lakes in areas were ice sheets occurred seemed to be older than 19,000 years.

Croll's ideas about the influence of orbital variations, axial tilt and procession of the equinoxes were much discussed, and regarded as interesting. But they just didn't fit the direct evidence of erosional rates of Niagara Falls and other falls, and the lake deposits. And as more and more lake deposits were studied, the evidence against such an ancient end to the last ice age became untenable.

It was not until Milutan Milankovitch did his work well into the 20th century, with accurate figures for the mass and orbit of the planets that a workable astronomical theory of the ice ages was developed.

Croll was just ahead of his time--he had the right ideas, but didn't have the right information to make his theory stand up.

Despite this, Croll was well respected among scientists. He had regular correspondences with Charles Lyell from 1864 on, and Charles Darwin. Croll published his work Climate and Time, in Their Geological Relations in 1875, and reviews from other scientists were very positive. (it took over a decade for him to make all the astronomical calculations!) In 1876, despite never having completed a middle school education, Croll was elected as a fellow of the Royal Society, and granted an honorary degree from St. Andrew's university. It may have helped that Croll was friendly, funny, and well liked. Croll retired in 1880 due to ill health but at least he got to the pinnacle of scientific respect. Quite an achievement for a wheelwright with a 6th grade education!

If you interviewed a geologist or physicist from 100 years ago they would probably tell you that Croll was a brilliant man who had an interesting idea about the ice ages, researched it well, and was just plain wrong.

No he wasn't wrong--he just had inaccurate information to work with. Croll was ahead of his time.

1 comment:

  1. A disturbing article about the drought last year in the Amazon basin: http://www.bbc.co.uk/news/science-environment-12356835

    ReplyDelete