Tag: milankovitch

Continental Drift

Tap for animated GIF

Public Domain

Alfred Wegener had a lot of evidence for continental drift, but he didn’t have an explanation for how they did it. He had the curious way South America and Africa looked as if they should fit together. This was noticed almost as soon as good maps were available, but it was largely dismissed as coincidence. After all, it would imply that the two land masses had moved apart, and everyone knew that couldn’t happen. The idea was ridiculous.

He also had a geological connection. The rocks of South America and Africa matched up where they would have been joined had they once been a single land mass. It is the same two billion year old rock on the two separate continents.

In a similar example, there is an old mountain range — over 400 million years old — that today has its remnants in the widely separated areas of Canada, Greenland, Ireland, Scotland, England and Scandinavia. When these areas are put together, the so-called Caledonian mountain belt re-emerges.

Old glacial deposits put down during the Permo-Carboniferous glaciation 300 million years ago are found in the present day Antarctica, Africa, Australia, India and South America. The most economical explanation for this is that these continents were gathered around the south pole at the time.

Finally there is the fossil evidence. Often the same type of fossil is found on continents that are separated today, while being found nowhere else. Either this is because the continents drifted apart after the fossils were laid down, or something more improbable happened, such as breeding pairs swimming together to another continent and establishing the species there.

Public Domain

Alfred Wegener was born in 1880 and died in 1930, but his continental drift theory, first put forward in 1912, didn’t achieve wide acceptance until the 1950s. The expanding theory was developed in the four editions of his book, The Origins of Continents and Oceans, accumulating increasingly impressive evidence as it went. The theory had a few supporters, such as Milutin Milankovich, but since Wegener couldn’t come up with a convincing mechanism for how the continents moved, most scientists were sceptical. One even argued that the continents simply couldn’t “plow through” the oceanic crust. They also found fault with the imperfect fit of the jigsaw coastlines, not realizing that he was matching them at their continental shelves, where it is a much better fit.

Paleomagnetism, a new science in the 1950s, produced much evidence to support Wegener. The ancient magnetic field was imprinted in the rocks and can be read today. India is in the northern hemisphere today, but its paleomagnetic signature shows that it was in the southern hemisphere in the past, as predicted by Wegener. As the evidence quickly mounted, and with additional evidence of seafloor spreading, scientists came to accept the theory. Eventually the theory of plate tectonics brought it all together.

Pangea animation 03.gif
Original upload by en:User:TbowerUSGS animation A08, Public Domain, Link

Today we can directly measure the movement of the continents with the Global Positioning System (GPS.) Alfred Wegener could have used that when he was exploring Greenland, the continent that eventually killed him.

rjb

Alfred Wegener

Alfred Lothar Wegener was born in Berlin, Germany on November 1st, 1880. He was the youngest of the five children of Richard Wegener, clergyman, theologian and classical language teacher. The family was well-off enough to own a vacation home, as well as to afford to educate all their children. Alfred did very well in school and went on to study physics, meteorology and astronomy. He got a doctorate in astronomy in 1905, but had formed a strong interest in the growing disciplines of climatology and meteorology.

Public Domain

Wegener made four expeditions to Greenland in his study of the polar climate, the first in 1906. He built a weather station and made observations using kites and tethered balloons, in addition to the usual instruments. He had his first experience with the killing harshness of Greenland’s climate when the expedition leader and two others died while exploring. He returned to Germany in 1908.

His second expedition to Greenland in 1913 began with a calving glacier that almost wiped it out, and ended with a fortunate and unlikely rescue as their crossing of the interior resulted in their having to eat all of their dogs and ponies before its completion.

His military service in World War One lasted only a few months. He faced fierce fighting, was injured twice and declared unfit for duty. He spent the rest of the war in the meteorological service and published 20 papers by its end. Having published on his ideas about continental drift for the first time in 1912, Wegener followed up with a major work — “The Origin of Continents and Oceans” — in 1915. Interest was small.

After the war he worked as a climatologist and as senior lecturer at the University of Hamburg. In collaboration with Milutin Milankovich, he did pioneering work in a field that would become known as paleoclimatology, where they reconstructed ancient climates. He published the third edition of “The Origin of Continents and Oceans,” provoking discussion of his theory of continental drift, and disparagement by the experts of the day.

By 1924 he attained a position that provided stability for his family, and he was able to concentrate on his studies for the rest of the decade. In 1926 he presented his ideas on continental drift at a symposium in New York, to near-uniform rejection. In 1929 he published the fourth and final edition of “The Origin of Continents and Oceans,” and made his third expedition to Greenland.

Wegener led the 1930 Greenland expedition, his fourth, and his sense of personal responsibility ultimately led to his death. A combination of a late thaw and harsh conditions resulted in the failure of a re-supply mission and the death of Alfred Wegener. His body remains buried where he died.

Public Domain

Alfred Wegener was accomplished at astronomy, meteorology and climatology, but what he is known for today is continental drift. We’ll cover that in more detail in future posts.

rjb

Top Ten Posts of 2017

Credit Marjaree Mason Center – CC-BY-SA

Here are the ten most viewed posts of 2017, not including permanent site components such as the home page, Downloads, Welcome, etc. Once again it seems I’ve become the Internet gateway for people wondering about spanking their wives.


1. Spanking for Love

What is it with spanking? This post has just over twice as many views as the second one.


2. Bipedal – The Savanna Theory

The interest in this continues. It spikes at the same times each year. School assignments?


3. Ants in the Devil’s Garden

After a big drop-off from #2, people seem to love these orchardist ants.


4. Bipedal – The Aquatic Ape Theory

The curve flattens from here on down. This one is probably spillover from #2.


5. Altocumulus Castellanus

The only Cloud of the Day in the top ten. That surprises me. And I wonder why this one in particular.


6. Collective Nouns

A perennial favorite, and a favorite of mine. Murders and murmurations.


7. Most Unpleasant Sounds

This one also surprises me. A quirky little list.


8. 15 Answers to Creationist Nonsense

Wouldn’t it be nice if we could just point them at this and not have to deal with them over and over?


9. Milankovitch Cycles – Obliquity

The only top ten post that I actually wrote this year. Part of a demanding series.


10. Microsculpture – The Insect Portraits of Levon Biss

Oh, good. I’m glad the list includes a tribute to beauty and hard work.

So, that was 2017. What are the odds that spanking will be #1 again in 2018?

rjb

Milankovitch Cycles – Other


See the previous posts on Milankovitch cycles: orbital eccentricity, axial obliquity and axial precession.

There are two more cycles to consider in this increasingly complex story of the astronomical cycles affecting Earth’s long-term climate. The first is another form of precession. Similarly to the precession of Earth’s axis, which marks out a circle on the star field every 26,000 years, the ellipse of Earth’s orbit around the Sun also precesses. That is, the orbit itself revolves, with the apsides of the orbital ellipse — the points of nearest and farthest approach — revolving about every 112,000 years relative to the fixed stars. Combining orbital precession with axial precession means that on average it takes about 23,000 years for the equinoxes to go through one cycle and return to the same calendar date. This affects climate by changing where on the orbit the seasons occur.

Credit Krishnavedala – CC-BY-SA – Tap for larger.

The other cycle, and the last one we will look at, is orbital inclination. It turns out that the plane of Earth’s orbit around the Sun is tilted relative to the Sun’s equator, and also relative to the Solar System’s so-called invariable plane. The invariable plane can be thought of as the rotational plane of the whole Solar System, mostly defined by the big gas giants: Jupiter, Saturn, Uranus and Neptune. The plane of Earth’s orbit is tilted relative to that by 1.57 degrees. The tilt of our orbit rocks up and down on about a 100,000 year cycle. That is, our tilt relative to the Sun and the rest of the Solar System is not constant, but changes over time. This affects climate by changing the apparent tilt of our axis relative to the Sun, affecting seasonal variation. Depending on the tilt of our orbit relative to the Sun, the same axial orientation relative to the fixed stars results in varying tilt relative to the Sun.

Credit Lasunncty – CC-BY-SA – Tap for larger.

The Milankovitch cycles, and the others that weren’t known in his time, are well understood, and their effect on Earth’s climate is well accepted by climatologists. That doesn’t mean that we’ve got it all sewed up. There is the matter of the well established 100,000 year cycle in glaciation matching the 100,000 year orbital cycles, while those cycles have the weakest effect on climate. Then there’s an unexplained change from a 41,000 year ice age cycle that lasted for two million years, to the 100,000 year one that’s been in force for the last million. And there are others. There is still plenty of work for climatologists.

To recap: We have looked at the astronomical events that Milutin Milankovitch considered to be implicated in Earth’s Ice Ages. We began with a brief look at his life, then followed up with the three main cycles examined by him: orbital eccentricity, axial obliquity, and axial precession. And now we have tacked on orbital precession and orbital tilt. All of these things, and possibly others not yet discovered, interact in a complicated dance that results in the recurring cycles of glaciation. Milankovitch pointed out that the most important effect is the amount and intensity of insolation at the mid-latitudes, particularly in the northern hemishpere’s summer. Warmer summers tend to prevent the buildup of snow and ice on the big continental land masses.

A note of caution: the Milankovitch cycles are seized upon as an example of natural forces that affect climate change, often with the hope that they will negate the reality of our current climate change, or at least absolve us of our responsibility for it. This is a false hope. If it were true, then the cycles should be trending toward warmer, but they’re not. Orbital eccentricity is increasing, which should promote a cooling trend. Axial tilt is decreasing, also normally leading to cooling. If anything, Earth should be cooling. The fact that it’s warming should make it clear that something else is counteracting the astronomical effects. The most economical hypothesis would seem to be that we are releasing the solar energy captured and buried by plants hundreds of millions of years ago, by burning the resulting fossil fuels. Compounding this is the insulating effect of the carbon dioxide released in the process, leading to a warming of the atmosphere. Don’t blame Milutin.

Now that I’ve put you through all this, here’s a link to a nine minute video that brings it all together.

rjb

Milankovitch Cycles – Precession


In the two previous posts on Milankovitch cycles — Eccentricity and Obliquity — we looked at the changing shape of Earth’s orbit around the Sun, and the changing tilt of Earth’s axis respectively. In this post we look at the other axial cycle: precession. In addition to changes in the tilt of the axis relative to the plane of our orbit, the direction that the axis is pointing in space changes over time. Over a period of about 26,000 years, it draws a circle on the star field. Presently the north end of the axis is pointing almost directly at Polaris, the pole star. About 13,000 years ago, near the end of the last great glaciation, it was pointing at the star Vega. The spinning planet is wobbling like a spinning top.


The effect of precession on climate is to continuously change where on our orbit the seasons occur. You can imagine that if we were on the opposite side of our wobble’s circle, then our summers and winters would be on the opposite sides of our orbit. Instead of winter in the northern hemisphere occuring during the part of the orbit when we are closest to the Sun, it would be at the farthest away. Northern hemisphere winters would be colder, but summers would be warmer, perhaps preventing the accumulation of ice and snow from season to season. Precession doesn’t affect the tilt of the axis, only where it’s pointing, so we still have seasons and they’re still affected by the obliquity cycle.

Now we have three cycles interacting: eccentricity at about 100,000 years, obliquity at about 41,000 years, and precession at about 26,000 years. You can see how complex this interaction is, and appreciate the work it took for Milankovitch to untangle it. He was doubted for a time, but these astronomical cycles are now accepted as important drivers of Earth’s climate. They came together to pull us out of the last glaciation, and now they are conspiring to push us into the next one. If we follow the established cycles, we should see a large part of the northern hemisphere’s continents covered in gigantic glaciers in the future.

Next up: some bonus extras.

rjb

%d bloggers like this: