Milankovitch Cycles – Eccentricity

Posted on 2017-01-21 by arjaybe


In the post on Milutin Milankovitch I said I would be discussing the Milankovitch cycles and their impact on Earth’s climate. There are three main cycles in Earth’s relationship to the Sun that have been shown to have an effect on the recurring cycles of the Ice Ages. Two of them involve Earth’s axis, and one its orbit around the Sun. In this post, we will look at the eccentricity of Earth’s orbit.

Here’s a graphical view of the cycles:

Credit Incredio – CC-BY-SA. Tap image for large original.

The first thing to realize about planetary orbits, as demonstrated by Johannes Kepler, is that they are not circular. As a first approximation, all orbits are elliptical. They orbit the central body on an oval path, so sometimes the orbiting body is closer, while sometimes it is farther away. In other words, for part of the year Earth is closer to the Sun than it is on the other half of the orbit. Presently, we are at our closest during the part of the orbit where it is winter in the northern hemisphere. It seems quite nice that things would be set up that way, doesn’t it? The part of our planet that has more land and more people is closer to its heat source during the coldest part of the year.

How does this affect our climate, and what is orbital eccentricity anyway? As there is more land in the north and more water in the south, the current regime keeps us a little warmer. Incoming sunlight is reflected more by snow-covered land than by open water. It has a higher albedo. If the great northern continents had more snow cover, as they would in longer, colder winters, they would reflect more sunlight and cool even more. One ingredient for a possible ice age. One component in our current lack of glaciation.

Orbital eccentricity, the Milankovitch cycle in this post, is the name for how elliptical an orbit is. A nearly circular orbit is less eccentric, and a more elliptical one is more eccentric.

The astounding fact is that Earth’s orbit oscillates between more and less eccentric in a 100,000 year cycle. That variability is one of the astronomical factors that Milankovitch calculated in his quest to see if such things could contribute to the ice ages discovered by geologists. Currently the eccentricity is near its minimum — our orbit is nearly circular — with a difference of only about three percent between the smallest and greatest distance from the Sun in a year. This tends to keep us a little warmer. However, the eccentricity is increasing, which should lead to a cooling trend. When the orbit is more elliptical, with the Sun at one focus of the ellipse, the Earth spends more time farther from the Sun, out on the long end of the ellipse. This leads to a gradual cooling trend, contributing to the possibility of accumulating snow and ice. On average during this latest Ice Age, comprising the last few million years, the approximately 100,000 year cycles are roughly 80% cold and glacial, and 20% warm and interglacial.

The 100,000 year cycle in Earth’s orbital eccentricity is the dominant cycle contributing to cooling and warming. The other, shorter cycles make their contribution when they ‘sync’ up with the big one. We’ll talk about those cycles in future posts, beginning with axial obliquity.

rjb

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Milutin Milankovitch

Posted on 2017-01-10 by arjaybe

Milutin Milankovitch (Milankovic) was a Serbian scientist, born in 1879 in the village of Dalj, in present-day Croatia. It was then part of the Austro-Hungarian Empire. At 17 he took up Civil Engineering at the Vienna University of Technology, going on to earn a Ph.D in engineering eight years later. His thesis was on pressure curves, useful in the planning and construction of load-bearing structures like bridges. He got work with an engineering firm, concentrating on reinforced and armored concrete, until he was offered the chair of applied mathematics at the University of Belgrade in 1909. He began to concentrate on fundamental research, though he kept his hand in concrete as well.

In his research he became interested in celestial mechanics and astronomical effects on planetary climate. He found that, although scientists were finding convincing evidence of ice ages in Earth’s past, including some indications that they might be cyclical, they were unable to come up with a plausible theory to explain it. He decided to use his interest in astronomy, and his facility with mathematics, to see if he could find any patterns that might explain the cycles in Earth’s climate. He began in 1912, following up on the work of his predecessors in the field, including James Croll, whose pioneering work on astronomical influences on ice ages was rejected by geologists and climatologists of the day. Milankovitch would face the same scepticism.

He began by publishing some papers on the effects of solar radiation, and its distribution on the planet’s surface, bringing some mathematical rigor to the science of meteorology. Then he began the more onerous task of calculating the cyclical variability of the Earth’s rotation on its axis as well as its orbit around the Sun. These eventually came to be called Milankovitch cycles, when everyone finally caught up and realized he was right. I’ll discuss those cycles independently in future posts.

His work was interrupted, though only briefly, by the beginning of World War One. He was imprisoned as a Serbian enemy of the Austro-Hungarian Empire, but was soon released upon the intervention of his friend and mentor, Emanual Czuber. He subsequently was allowed to work at the Hungarian Academy of Science for the duration of the war. After the war he returned to Belgrade where he continued to create the foundation for the mathematical treatment of climate science. He calculated the curve for variations in solar radiation impinging on the Earth going back 130,000 years, extending it to 650,000 years at the urging of climatologist Wladimir Koppen. He also impressed Alfred Wegener, of continental drift fame.

Milankovitch published many more papers, as well as popular science books, including a series on the history of science. The publication of his collected works on the problem of the Ice Ages was interrupted by the Second World War, and it ended up being published in German. It was almost lost in the bombing of Belgrade when the printing house entrusted with it was destroyed. Fortunately the warehouse where the printed sheets were stored was spared.

Milutin Milankovitch died in Belgrade in 1958. After his death his work was disputed and it languished for ten years. But it slowly gained support and is now accepted by most climatologists and geologists as an accurate theory. The Milankovitch cycles have been shown to bear a close relationship with the cycles of the Ice Ages. He shares the honor of being one of Serbia’s great scientists with the legendary Nikola Tesla.

Read the series on Milankovitch cycles, beginning with orbital eccentricity.

rjb

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Evolution – Part Three

Posted on 2016-12-18 by arjaybe

Photo by Michael D. Gumert. – CC-BY – No larger image available

Some years ago I published a series of articles about evolution in my local newspaper. It generated some interest and a spate of letters to the editor, and my publisher liked it. There was even a creationist who challenged me to a debate over it. I decided to reproduce it here. This is part three, which I called Walk This Way. See also Part One and Part Two.

The first primates show up in the fossil record about 50-55 million years ago. They are part of the resulting explosion of new species that evolved to fill environmental niches vacated by the non-avian dinosaurs, after they went extinct sixty-five million years ago. Many of the new species are mammals, and we see a mammal-dominated landscape right now. Of the mammals, it is the primates which interest us the most, as they are our ancestors. Primates spread out and evolved into many different species, including lemurs, monkeys and apes. They can be found in most parts of the world, but it was in Africa where the line led to humans.

Between seven and eight million years ago a primate living in Africa split into two species. Such splits normally result when two populations of one species get separated somehow. Something like that happened to the common ancestor of chimpanzees and humans. A fossil from that period shows definite signs of an upright, two-legged gait, but the oldest hominin fossil with extensive bipedal adaptations is Ardipithecus ramidus. That was the beginning of the hominin, or human-like primate. Since then it evolved and radiated out into many different hominin species, most of which have gone extinct.

The first evidence of stone tools shows up at least 2.6 million years ago, with some evidence that pre-homo hominins were using them as early as 3.3 million years ago. Undoubtedly they were using tools made of other materials like wood and grass, but only stone can survive long enough for us to find.

Homo erectus is thought to have tamed fire by about 1.8 million years ago. Others put the use of hearths beginning later, at about 800,000 and even only 300,000 years ago. Much of the discussion revolves around whether we were simply burning grasslands to improve hunting, or actually sitting around a hearth cooking food on a regular basis. Those favoring the earlier date cite the shrinking jaw and growing brain of H. erectus as evidence that they were cooking their food. The pinnacle of upright hominids seemed to have been reached, but their brains were only about half the size of ours. Larger than the brains of similar-sized animals, but still too small by our standards.

Increasing brain size was the next big step. By the time we reach the age of Neanderthals, about four hundred thousand years ago, and modern humans at about half that, our brains were as big as they are now.

Since then it’s been a matter of social and technological evolution. The first jewelry shows up about 75-100 thousand years ago. The first garments appear to have been manufactured about 100 thousand years earlier, based on the evolution of body lice. The tool set became extremely sophisticated.

Neanderthals died out about thirty thousand years ago, leaving only a single hominid species on Earth for the first time in millions of years. Our cultural evolution continued to accelerate, as evidenced by sophisticated cave paintings, bringing us to the present state of high civilization.

Let’s hope we’re not due for another mass extinction.

rjb

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