History of Science - 12.
Eduard Suess (1831-1914).
Alfred Wegener (1880-1930).
Arthur Holmes (1890-1965).
Walter Elsasser (1904-1991).
Edward Bullard (1907-1980).
Harry Hess (1906-1969).
Dan McKenzie (1942-).
Tuzo Wilson (1908-1993).
Louis Agassiz (1807-1873).
Joseph Adhemar (1797-1862).
James Croll (1821-1890).
William Herschel (1738-1822).
Milutin Milankovitch (1879-1958).
Radio activity was discovered in the 19th century.
It provided a source of heat that would prevent the interior of the Earth,
from cooling into a solid inert lump.
It would take the theories of relativity and quantum physics to progress,
from the discovery of radio activity, to an understanding of how the conversion of mass into energy,
would keep the stars shining.
Just like Galileo could study pendulums and rolling balls, without knowing how gravity worked,
all that geophysicists needed to know about radio activity,
was that it provided a way to keep the Earth warm inside.
Armed with this knowledge, scientists could develop geology into geophysics.
They explained the origin of the continents, ocean basins, earthquakes, volcanoes,
mountain building, land erosion, etc,.
Eduard Suess developed a synthesis model of the Earth, where contraction of the Earth,
was the driving force of the dramatic change.
The discovery of the radio activity was made at almost the same time that Suess was developing,
his synthesis model.
Suess showed that the Earth’s interior was not cooling dramatically at all.
He suggested that the present day land masses of Australia , India, and Africa were fragments,
of a greater land mass which he called Gondwanaland, that once existed in the southern hemisphere.
Suess’s synthesis model was significant for two reasons.
First it highlights the lack of any standard model of Earth’s history in the beginning of the 20th century.
Second it gave us a name Gondwana, which would become familiar as the idea of,
continental drift became established in the second half of the 20th century.
Osmond Fisher published a paper in 1882, where he took up the idea of George Darwin.
George Darwin was one of Charles Darwin’s sons.
He proposed that the moon had formed when the young Earth split into two unequal parts.
German meteorologist Alfred Wegener came up with the model of continental drift in 1912.
Though many of his detailed ideas were incorrect, his overall concept stood up to the test of time.
He is now regarded as the father of the theory of continental drift.
He was born in Berlin in 1880.
He obtained his doctorate in astronomy in 1905.
He joined the Prussian aeronautical observatory.
He took a balloon flight for 52 hours to test the instruments, which was a record at that time.
He published a meteorological textbook in 1911.
He published his ideas about continental drift in 1912.
When he was studying an atlas, Wegener was struck by the way the east coast of south America,
and the west coast of Africa, looked like they fit together, like pieces of a jigsaw puzzle.
At first he was intrigued, but he did not take it seriously.
He later came across a report discussing the paleontological similarities,
between the strata of Brazil and Africa.
The paper suggested a land bridge inking the two continents, but Wegener saw things differently.
He found evidence, for continental drift in this findings.
Wegener married in 1913, but any plans for a quiet academic life was shattered by the first world war.
He was called up to serve in the western front.
He was wounded twice, and became unfit for active services.
He worked in the meteorological service of the army, after his recovery.
He then wrote his famous book, The origin of continents and oceans, in 1915.
This made very little impact at that time, at the height of the war.
After the war, he became the lecturer in meteorology in the university of Hamburg.
He established a reputation as a distinguished meteorologist.
He continued to work on continental drift, and published new editions of his book in 1920 and 1922.
He also published an explanation of past climates based on continental drift.
In 1929, he published the fourth edition of his book.
In 1930, Wegener set out on one more expedition to Greenland to gather evidence,
in support of the drift hypothesis.
The expedition ran into some trouble, and supplies were very low.
Wegener was attempting to move from one camp to another, he got lost and died.
Wegener’s model envisaged the Earth as made up of a series of layers,
increasing in density from the crust to the core.
He said that the present day continental blocks, still have essentially the same outlines,
as they have had since the breakup of a single supercontinent, called Pangea.
Pangea contained all the land surface of the planet at the end of the Mesozoic era,
about 150 million years ago.
He explained the way mountains had formed along the eastern edges of the,
north and south American continents, as they drifted away from Europe and Africa.
Mountain ranges in the heart of land masses, could be explained by the collision of continents.
He showed how glaciation had occurred in the distant past, simultaneously on continents that are now,
far apart from one and another, and far from polar regions.
Most valuable contribution, was his synthesis, gathering evidence to support,
the former existence, of the super continent of Pangea.
Arthur Holmes was a leading expert on radioactivity decay.
He was in the forefront of efforts to measure the age of the Earth using radio active techniques.
He could be said as the man, who measured the age of the Earth.
Holmes came from a modest family in England.
He went to Royal college of science in London in 1907.
He had a national scholarship of about 1.5 pounds a week, during the academic year.
This was not enough to live in 1907, but he had to make do as best he could with this.
The American Bertram Boltwood had developed the technique for dating rocks,
from the proportion of lead and uranium isotopes they contain.
The radioactive decay of uranium eventually produces lead.
A characteristic time scale measuring this ratio can reveal the age of rocks.
Holmes used this technique to date samples of Devonian rock from Norway,
as having a age of 370 million years.
He graduated in 1910, with a glowing reputation, but a burden of debt.
He was delighted to get a job as prospecting geologist in Mozambique, at 35 pounds per month.
He joined the Imperial college, where he received his doctorate in 1917.
He then worked in Burma for an oil company.
In 1924, he became professor of geology at Durham university.
He retired from the university of Edinburg in 1956.
By then he had firmly established the radioactive technique for measuring the age of rocks.
He came up with the age of the Earth as 4500, plus or minus 100 million years.
In 1944, he published, Principles of physical geology, which has become a standard text.
Sima is the material of the lower part of the Earth’s crust,
underlying both the oceans and the continents.
It is characterised as relatively heavy, and rich in silica and magnesia.
In 1917, Holmes presented a paper, where he suggested that although,
the continents floated on denser material, they did not move through the sima.
Rather this denser material itself moved around very slowly, stirred by convection currents,
produced by the heat within the Earth.
It cracks apart in some places, and pushes the continents on either side of the crack apart,
while they collided in other parts of the globe.
Apart from radioactive heating, the key component of Holmes’s model was time.
Solid rock was warmed from beneath, could stretch and flow slowly.
In 1930, Holmes produced the most detailed account of continental drift.
He described how convection currents operating inside the Earth,
as a result of heat generated by radioactive decay, could have caused the breakup of Pangea.
It first broke up into two large landmasses.
Gondwanaland in the southern hemisphere, and Laurasia in the north.
These in turn fragmented and drifted to form the pattern of land,
that we see on the surface of the Earth today.
He estimated that convection currents would move continents about at the rate of 5 cm a year.
This was enough to produce the Atlantic basin from a crack in the crust,
over a period of 100 million years.
Holmes made a case for convection as the driving force for the process of continental drift.
The evidence in support of continental drift was persuasive, rather than compelling.
There were other ideas, like Permanentism, which did not find much favour.
What made continental drift and established paradigm, the standard model of how the Earth works,
was new evidence which emerged in the 1950’s and 1960’s, thanks to emergence of new technology.
The first new evidence came from the study of fossil magnetism.
This is the magnetism found in samples of rock from old strata.
The impetus for the work came originally from the investigation of the Earth’s magnetic field,
whose origin was still a puzzle in the 1940’s.
Walter Elsasser was a German scientist, who left Germany for the United States,
when Hitler came to power.
In the late 1930’s, he developed the idea that the Earth’s magnetism is generated,
by a natural internal dynamo.
He published these ideas in 1946.
British geophysicist Edward Bullard took up these ideas.
During the war, he worked on the magnetising ships, to protect them from magnetic mines.
Later at the university of Toronto, he developed the model of the Earth’s magnetic field,
as a product of circulating conducting fluids, in the hot fluid core of the planet.
This is the convection and rotation in molten iron, in the core of the planet.
In the 1950’s, he used the electronic computer for the first numeric simulations of this dynamo process.
By that time measurement of fossil magnetism,
had shown that the Earth’s magnetic field, had had the same orientation, relative to the rocks,
for the past 100,000 years.
The rocks are magnetised when they are laid down, as molten material flowing from volcanoes,
or cracks in the Earth’s crust.
Once set, they preserve the pattern of the magnetic field, in which they formed,
becoming like bar magnets.
British researchers found that in older rocks, the direction of fossil magnetism could be quiet different,
from the orientation of the present day geomagnetic field.
Even more strange they found that in some occasions in the geological past, the geomagnetic field,
had the opposite sense to that of today, with north and south magnetic poles interchanged.
It was this paleomagnetic evidence that made the debate,
about continental drift hot up during the 1960’s.
Some scientists used the magnetic orientations of the rocks, from a particular time in the geological past,
as the ‘lines of print’ and matched it across the joins of continental reconstructions,
and found that those reconstructions broadly matched the ones made by Wegener.
2/3rd of the crust of the Earth’s surface comprises of the sea bed.
Before world war 1, this was a mysterious and unexplored world.
Due to submarines, the war encouraged development of technology for identifying,
what lay beneath the surface of the ocean.
Eco-location or sonar is one such technology which got developed.
This was used to map the sea bed.
Before 1940’s geologists had assumed that the sea floor represented the most ancient crust of the Earth.
The sea floors were assumed to be covered with huge amounts of ancient sediment,
worn off the land over eons.
It was assumed to be a featureless layer, 5 or 10 km thick.
The crust itself beneath the sediment was assumed to be about 10 km thick,
like the crust of the continents.
Research of the sea belt showed that all these ideas were wrong.
There was only a thin layer of sediment.
All of the rocks of the sea floor are young.
The youngest rocks were found next to the ocean ranges.
These had geological active features, where underwater volcanic activity,
marks the line of a crack, in the Earth’s crust.
They are young because, that is when they solidified from molten magma.
Seismic surveys showed that the thickness of the crust is 5 to 7 km under the ocean,
compared with an average of 34 km for the continental crust.
The pieces of the puzzle were put together by the American geologist Harry Hess,
of Princeton university in 1960.
According to this model the ocean ridges are produced by convection currents in the fluid material,
of the mantle.
The mantle is a layer of fluid rock, just below the Earth’s solid crust.
The warm material is not liquid, but it is hot enough to flow slowly, as a result of convection.
The volcanic activity associated with the ocean ridges marks the place,
where this hot material breaks through to surface.
It then spreads out on either side of the ridge, pushing the continents on either side,
of the ocean basin apart.
The youngest rocks solidifying next to the ridges, and the older rocks laid down,
10’s and 100’s of million years earlier.
New ocean crust, created in this way is widening the Atlantic at a rate of about 2 cm a year.
The ocean basin are now seen as the sites of action in continental drift.
The continents themselves are literally carried along for the ride,
as a result of activity associated with the crust of the ocean floors.
This does not mean that the Earth is expanding.
The second key concept of Hess’s model was, that in some parts of the world,
notably along the western edge of the pacific ocean, thin oceanic crust is being forced down,
under the edges of thicker continental crust.
This dives down into the mantle below.
This explains the presence of deep ocean trenches in those parts of the world.
It also explains the occurrence of Earth quakes in places such as Japan.
Islands like Japan are explained as being produced by the tectonic activity,
associated with sea-floor spreading.
The Atlantic ocean is getting wider, but the pacific ocean is narrowing.
Eventually if this process continues, America and Asia will collide to form a supercontinent.
The red sea with its own spreading ridge, marks the sight of a new region of upwelling activity.
This cracking of the Earth’s crust is the beginning to splinter Africa away from Arabia, to the East.
The model was able to explain the San Andreas fault in California.
Geologists realised that blocks of the Earth’s crust are moving past one another,
at a rate of few centimetres per year.
Sea floor spreading is like a slow conveyor belt.
It endlessly loops round and round.
Over the entire surface of the globe, everything cancels out, and the planets stays the same size.
Hess’s model, inspired a new generation of geophysicists, building a complete theory,
of who the Earth works.
One of the leading players in the team was Dan McKenzie of the university of Cambridge.
He and two other scientists, Fredrick Vine and Drummond Mathews,
linked the evidence for geomagnetic reversals,
with the sea floor spreading model of continental drift.
By the early 1960’s there was a growing mass of data about the magnetic history of the Earth,
obtained from the continents.
The pattern of magnetism over parts of the sea bed was also being mapped.
These surveys showed a stripy pattern of magnetism in the rocks of the sea bed.
The stripes were running more or less north-south.
In one stripe the rocks will be magnetised in line with the present day geomagnetic field.
In adjacent stripes the rocks would have the opposite magnetism.
When plotted it resembled a slightly distorted bar code.
The scientists suggested that the patterns were produced as a result of sea floor spreading.
Molten rock flowing from an oceanic ridge, and setting, would be magnetised with the magnetism,
corresponding to the Earth’s field at the time.
Continental evidence showed that the magnetic field reversed direction from time to time.
If Vine and Mathews were correct, it meant two things.
First the pattern of magnetic stripes on the ocean floor should be correlated,
with the pattern of geomagnetic reversals revealed by continental rocks.
Second, according to Hess, crust spread out evenly on both sides of an oceanic ridge,
the pattern of magnetism seen on one side of such a ridge,
should be the mirror image of the pattern seen on the other side of the ridge.
If so, this would be a striking confirmation of the sea floor spreading model.
Vine in collaboration with Hess and Tuzo Wilson, taking into account new magnetic data,
developed the idea further, and made a new version of continental drift into a coherent whole.
Wilson coined the term ‘plate’ for the rigid portion of the Earth’s crust,
that are being moved around by forces associated with sea floor spreading, and continental drift.
Clinching evidence of the sea floor spreading model came in 1965, and was published in 1966.
Bullard in 1964 came up with a new model of the old idea of Pangea.
This reconstruction used mathematical principles of Euler’s theorem,
and was carried out with an electronic computer to provide an unbiased objective matching.
The result was striking similar to Wegener’s model.
The ‘Bullard fit’ of the continents published in 1965, is a defining moment,
in the theory of continental drift.
Dan Mckenzie and Robert Parker published a paper in 1967, introducing the term,
plate tectonics, for the overall model.
It described in detail the geophysical activity of the pacific region.
It described the way plates move, on the surface of a sphere, using Euler’s theory.
The essence of plate tectonics is that seismically quiet regions of the globe,
are quiet because they formed rigid plates.
There are six large plates, and about a dozen small ones, covering the entire surface of the globe.
Most of these interesting geological activity happens at the boundary between plates,
called plate margins.
Constructive margins are regions, where new oceanic crust is being made at ocean ridges,
and spreading out on either side.
Destructive margins are regions, where one plate is being pushed under the edge of another,
and melting back into the magma below.
Conservative margins are regions, where plates are rubbing sideways, as they rotate.
The existence of ancient mountain ranges and former sea beds, in the hearts of continents today,
shows that tectonic activity has been going on, long before the breakup of Pangea.
Super continents have repeatedly been broken up and rebuilt in different patterns,
by tectonic activity.
The publication of ‘Understanding the Earth’, in 1970 in Britain, could be considered,
as the last great triumph of classical science.
Alfred Russel Wallace, during his time in the islands of Malay Archipelago,
noticed that there was a distinct difference between the species to the northwest,
and the species to the southeast.
A narrow band on the map, known as the Wallace line, has distinctive Asian fauna to the northwest,
and the Australian fauna to the southeast.
This was a great puzzle at that time, but can be explained with plate tectonics.
When the super continent Gondwanaland broke up, Indonesia broke away first,
and moved to the northwest, and evolved differently from Australia-Antarctica moved northwards,
catching up with Asia.
The two continents have only recently come into close proximity once again.
This explains the Wallace line.
Continental drift is relevant to many aspects of the evolution of life on Earth.
Continental drift and climate have come together to shape our species.
It begins with the story of the ice ages.
It was thought that glaciation in Europe had been much more extensive in the past, then it is today.
One of the first people to draw attention to this so called ‘erratics’, was the Swiss Bernard Kuhn.
He was a clergyman, at a time when it was believed that such phenomena could be explained,
by the effects of the Biblical flood.
Only some scientists believed otherwise.
One of them, the German Reinhard Bernihardi, published an article in 1832,
suggesting that the polar ice cap, had once extended as far south, as central Germany.
The Swiss mountaineer Jean Pierre Perraudin, observed rocks in ice free mountain valleys.
They did not weather easily, and were scarred by something pressing down strongly on them.
He realised, it was most likely that they have been gouged,
by rocks scrapping over them, by ancient glaciers.
In 1829, he presented the case for former glaciation to Swiss society of natural sciences.
The geologist, Johann von Charpentier was convinced about the idea.
Charpentier gathered more evidence, and presented a more advanced case to the society, in 1834.
Nobody seemed to have been convinced this time.
One member, Louis Agassiz who thought it was nonsense, took upon himself, to disprove the theory.
Louis Agassiz was born in Switzerland in 1807.
He studied medicine in Zurich, and moved to Paris in 1831.
He turned his attention to palaeontology, and soon became the world’s leading expert on fossil fishes.
In 1832, he was appointed as professor of natural history,
at the college and natural history museum in Switzerland.
Agassiz’s knew Charpentier, who tried to convince him that there had been a great glaciation.
Agassiz tried to find evidence against it, but finally got convinced, about glaciation.
In 1837, he presented this to the Swiss society of Natural sciences.
He used the term Ice age, in his presentation.
Agassiz setup a small observation station, on the Aar glacier, to measure the movement of ice.
He discovered that the ice moved faster than he anticipated, and it could carry very large boulders with it.
He published his book, ’studies on glaciers’, which brought the Ice age model,
into the arena of public debate.
He argued that the whole planet had once been covered with ice.
He made some exaggerated claims.
Charpentier presented a more sober Ice age model, in 1841.
In 1840, Agassiz presented his Ice age model to the British association for the advancement of science.
The model was also presented to the geological society in London.
This could have been the turning point, when the Ice age model was first accepted.
It took another 20 years to gain widespread acceptance.
In 1833, Agassiz married Cecile Braun.
The couple were initially happy, and had a son and two daughters.
In the mid 1840’s the relationship deteriorated.
In 1845, Cecile left Switzerland, and went to Germany, to stay with her brother.
Agassiz at this time was in severe financial difficulties,
which was one of the factors for the breakup of the marriage.
In 1846, Agassiz went to the U.S.
He saw geological features which gave abundant evidence of glaciation.
He discovered that American geologists had already accepted the Ice age model.
In 1847, a chair was established at Harvard for his benefit.
This solved his financial problems, and gave him a secured academic base.
He became professor of geology and zoology, and stayed at Harvard for the rest of his life.
He established a museum of comparative zoology in 1839,
the same year as Darwin’s origin was published.
Agassiz was a popular lecturer, who emphasised the need for hands on investigation,
of natural phenomena.
However, he did not accept the theory of natural selection, to the end of his life.
In 1848, Cecile died of T.B.
His son Alexander joined Cambridge, Massachusetts, and eventually became a distinguished naturalist.
In 1850, Agassiz married for the second time, and brought his two daughters to join the family.
He enjoyed 25 years of domestic happiness, and academic success.
He died in 1873.
What is called as the astronomical theory of Ice ages, has its roots,
to Kepler’s discovery of elliptical orbits of the planets around the Sun.
In 1842, French mathematician Joseph Adhemar, published revolutions of the sea.
Due to elliptical orbit, the Earth is closer to the Sun, for some part of the year.
The Axis of the spinning Earth, is at an angle of 23 and half degrees to the vertical.
On a time scale of centuries, this tilt always points in the same direction, relative to the stars.
As we go around the Sun, first one hemisphere, then the other is leaning towards the Sun.
That is why we have seasons.
On July 4th the Earth is at its furthest distance from the Sun.
On January 3rd it is at the closest.
The difference is less than 3% of the 150 million kilometers average distance from the Sun.
Adhemar reasoned correctly, the hours of total darkness at the south pole,
is longer than the daylight hours, in the southern summer.
He believed that this meant, that the south polar region was getting colder as the centuries passed.
He believed that the Antarctic Ice cap was still growing, because of this.
Like a spinning top, the Earth wobbles as it rotates.
The wobble is known as the precession of the equinoxes.
It causes the axis of the rotation of the Earth , relative to the stars, describe a circle every 22000 years.
So, 11000 years ago the pattern of seasons, relative to the elliptical orbit was reversed.
Adhemar envisaged the alternating cycle of the Ice ages.
First the southern hemisphere, and then the northern hemisphere,
11000 years later was covered with Ice.
At the end of the ice age, the seas gnawed at the base of the huge ice cap,
until the remaining mass collapsed into the ocean.
This sent huge waves rushing into the opposite hemisphere.
Adhemar’s ideas were wrong.
There is no pattern of alternating northern and southern glaciations, 11000 years apart.
James Croll was born in Scotland in 1821.
His father was a stonemason, who travelled a lot, leaving his family to cope with the farming.
Croll received only a basic education, but read avidly, and learnt the basics of science from books.
He got a job as a janitor in the Andersonian museum of Glasgow.
His salary was small, that he had access to the excellent scientific library of the museum.
One of the books he read, was by Adhemar.
The French mathematician, Urbain Leverrier did a detailed analysis of,
how the Earth’s orbit changes in time.
His work led to the discovery of the planet, Neptune.
This was a profound piece of work , which predicted the presence of Neptune,
based on the perturbations on other planets, by a unseen gravitational influence.
This was far more profound than the discovery of Uranus in 1781, by William Herschel.
Herschel discovered Uranus with a powerful telescope.
Neptune’s existence was predicated mathematically.
It was a great vindication of Newton’s laws and the scientific method.
The prediction involved terribly laborious calculation with pencil an paper.
One of the fruits of this labour, was an accurate analysis of how the shape of the Earth’s orbit changes,
on a timescale of 100 thousand years.
Sometimes the orbit is more elliptical, and sometimes more circular.
The total amount of heat received by the planet over the entire year, is always the same.
When the orbit is circular, the amount of heat received by the planet, each week is the same.
When the orbit is elliptical more heat is received in a week when the Earth is closer to the Sun.
Croll wondered whether this could explain the Ice ages.
He combined, the changes in ellipticity calculated by Leverrier,
and the effect of the precision of the equinoxes, to produce new model of the Ice ages.
In this alternating the Ice ages are embedded in a Ice Epoch, hundreds of thousands of years long.
According to this model the Earth had been in an Ice Epoch from about 250 thousand years ago,
until 80000 years ago.
Since then there has been an warm period, between Ice Epoch, called as the inter glacier period.
He published his Ice age model, in the philosophical magazine, in 1864.
The work attracted considerable attention, and Croll realised his lifelong ambition,
of becoming a full time scientist.
In 1876, he published his book, Climate and Time.
In the same year he was elected as a Fellow of the Royal Society.
He was possibly the only former janitor, who received this honour.
He published Climate and Cosmology in 1885.
His Ice age model became widely accepted,
even though there was very little hard geological evidence to back it up.
He died in 1890.
Croll pointed the way for further improvements to the astronomical model,
by suggesting that changes in the Earth’s tilt, might also play a part.
The tilt now is 23 and a half degrees, which causes the seasons.
Over a long time, this tilt varies 22 to 25 degrees.
The cycle actually takes about 40000 years.
Croll speculated that the Ice age would be more likely when the Earth was more upright,
since both the polar regions would be getting less heat.
This was only a guess.
By the end of the 19th century, these ideas began to fall into the disfavour.
Geological evidence indicated that the latest Ice age ended, not 80000 years ago,
but 10 to 15000 years ago.
Instead of warming 80000 years ago, the northern hemisphere was plunging into the coolest period,
of the latest Ice age.
It was just the opposite of what Croll’s model predicted.
Metrologist calculated that the amount of solar heating produced by the astronomical effects,
were too small to explain the great temperature differences, between the interglacials and the Ice ages.
There was evidence that there had been a succession of Ice ages.
It was a Serbian engineer, who took up the daunting task,
of seeing whether the cycles fit the geological patterns.
Milutin Milankovitch was born in 1879 in Serbia.
Serbia had become independent in 1882, after centuries of foreign rule, mainly Turkish.
Milutin graduated with a Phd in 1904, from the institute of technology in Vienna.
He became the professor of applied mathematics, at the university of Belgrade.
He taught mechanics, theoretical physics, and astronomy.
He also became obsessed with climate.
He decided to develop a mathematical model to describe the changing climate of Earth,
Venus, and Mars.
He had no mechanical aids, but used only his brain power to calculate the model, for three planets.
He took about three decades to complete the task.
The mathematical description of present day climates of Earth, Venus and Mars,
was published in 1920, and received widespread acclaim.
The book included mathematical evidence that the astronomical influences,
could alter the heat arriving at different latitudes, sufficiently to cause Ice ages.
Milutin had not worked out the full details.
This was done by Wladimir Koppen, who published a book on climate, with Alfred Wegener.
Koppen realised that what matters is not the temperature in winter, but the temperature in summer.
In northern latitudes it is always cold enough for snowfall in winter.
What matters is how much of the snow stays unmelted in summer.
So the key to the Ice ages is cool summers, and not extra cold winters.
This is exactly the opposite of what Croll had thought.
This is why the latest Ice age was most intense 80000 years ago, and ended 10 to 15000 years ago.
Milutin did more detailed calculations, and published a book, Canon of Insolation and the Ice age problem, which summed up his life’s work.
He died in 1958.
By then his model had fallen out of favour, since geological evidence did not match it very well.
In the 1970’s the astronomical model, now called Milankovitch model, had been improved to an accuracy,
he can never have hoped for, using electronic computers.
The key geological evidence came from cores of sediment of the sea bed.
These sediments have been laid down year by year, on top of the other.
They can be dated using radioactive and geomagnetic dating.
They have traces, in the form of chalky shells, of tiny creatures,
that lived and died in the oceans long ago.
They reveal the species that flourished at different times.
That in itself is a guide to climate.
At another level, analysis of isotopes of oxygen from the shells, can give a direct indication,
of the temperature, at the time, those creatures were active.
All three astronomical rhythms show the pulse-beat of the changing climate,
over the past million years or more.
The summary of this evidence was published in the journal, Science, in 1976.
There was one intriguing question left.
Why is the Earth so sensitive for small changes in the amount of sunshine reaching different latitudes.
Geological records tells us that the natural state of the Earth, throughout most of its long history,
has been ice free.
As long as warm ocean currents get to the polar regions,
it doesn’t matter how little sunshine they receive.
The warm water prevents sea ice from forming.
But occasionally, at intervals separated by 100’s of millions of years, one or the other hemisphere,
is plunged into a period of cold, lasting several million years.
We can call this an Ice Epoch.
For example, there was an Ice Epoch, which lasted for 20 million years, in the Permian Era.
This Ice Epoch ended around 250 million years ago.
The explanation is that from time to time, continental drift carries a large land surface,
over or near one of the poles.
This does two things.
It impedes the supply of warm water from lower latitudes.
So, in winter the affected regions get very cold.
Second, the continent provides a surface for snow to fall and accumulate,
to build up a great ice sheet.
Antarctic today provides a classic example of this process at work, which produces an Ice Epoch.
After the Permian Ice Epoch ended, the world enjoyed 200 million years of warmth.
This is the time the dinosaurs flourished.
But a gradual cooling began to set in about 55 million years ago.
10 million years ago, the glaciers returned.
They came first in the mountains of Alaska, and soon afterwards on Antarctica.
In Antarctica the ice sheet grew so much, that by 5 million years ago, it was bigger than what it is today.
The fact that glaciers spread in both hemispheres at the same time, is an important insight.
Antarctica covered the south pole, and glaciers built up, the way we just described.
The north polar region cooled and eventually froze, even though the Artic ocean,
not land covered the pole.
The reason for this is that continental drift, gradually closed off an almost complete ring of land,
around the Artic ocean.
This shut out much of the warm water that would otherwise have kept it ice free.
The presence of Greenland today, deflects the Gulf stream eastwards,
where it warms the British Isles, and northwestern mainland Europe instead.
A thin ice sheet formed over polar ocean.
About 3 million years ago, more ice lay over surrounding land.
This situation is particularly sensitive to the astronomical rhythms.
For the last 5 million years, the Earth has been, in what may be a unique state, for the entire history,
with ice caps over both poles, produced by two different geographical arrangements of land and sea.
This, and in particular the geography of the northern hemisphere,
makes the planet sensitive to the astronomical rhythms.
Within the current Ice Epoch, the effect of the pulse-beat of climate,
is to produce a succession of full Ice ages.
Each Ice age is roughly 100,000 years long, separated by warmer conditions like those of the present day,
Interglacials, some 10000 years long.
By this reckoning, the present Interglacial would come to an end, in the couple of thousand years.
The sequence of these Ice ages set in 3.6 million years ago.
At that time, our ancestors were living in the great rift valley of East Africa.
An ancestral form of hominid was giving rise to 3 modern forms,
the Chimpanzee, the Gorilla, and ourselves.
It is just at this time that the fossil record provides direct evidence of a hominid, that walked upright.
It is possible that the pulse-beat of climate played a key role, in the evolution of Homo sapiens.
During a full Ice age, the oceans are so chilled, there is less evaporation, and therefore less rainfall.
The Earth is drier, and the forests retreat.
This would have forced out some hominids from the forests into the plains.
If the situation had continued unchanged indefinitely, they might have died out in competition,
with better adapted plain dwellers.
After a 100,000 years, the conditions eased, and the descendants of the survivors,
had a chance to take advantage of the expanding forest.
They were able to breath in safety from the plains predators, and buildup their numbers.
Repeat the process 10 or 20 times, and it is easy to see, how a ratcheting effect, would have selected for intelligence and adaptability, as the key requirements for survival, on the fringes of the forest.
Back in the centre of the forest, the most successful hominid lines adapted,
more closely to life in the trees, to become chimps and gorillas.
This is only a hypothesis, but is plausible.
If it is correct, a combination of a number of factors like continental drift, astronomical cycles,
Earth’s climate, evolution etc., contributed to the evolution of human beings.
It makes us realise that we are just one form of life.
In the 20th century, the emergence of quantum revolution,
completely altered the way scientist thought about the world, on very small scales.