The mouldy and unihabitanted lands found by Ptolemy several thousand years ago

The mouldy and unihabitanted lands found by Ptolemy several thousand years ago. Only 1 message in topic - view as tree

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The episodic nature of the Earth's glacial and interglacial periods within the present Ice Age (the last couple of million years) have been caused primarily by cyclical changes in the Earth's circumnavigation of the Sun. Variations in the Earth's eccentricity, axial tilt, and precession comprise the three dominant cycles, collectively known as the Milankovitch Cycles for Milutin Milankovitch, the Serbian astronomer who is generally credited with calculating their magnitude. Taken in unison, variations in these three cycles creates alterations in the seasonality of solar radiation reaching the Earth's surface. These times of increased or decreased solar radiation directly influence the Earth's climate system, thus impacting the advance and retreat of Earth's glaciers.

It is of primary importance to explain that climate change, and subsequent periods of glaciation, resulting from the following three variables is not due to the total amount of solar energy reaching Earth. The three Milankovitch Cycles impact the seasonality and location of solar energy around the Earth, thus impacting contrasts between the seasons.

Eccentricity

The first of the three Milankovitch Cycles is the Earth's eccentricity. Eccentricity is, simply, the shape of the Earth's orbit around the Sun. This constantly fluctuating, orbital shape ranges between more and less elliptical (0 to 5% ellipticity) on a cycle of about 100,000 years. These oscillations, from more elliptic to less elliptic, are of prime importance to glaciation in that it alters the distance from the Earth to the Sun, thus changing the distance the Sun's short wave radiation must travel to reach Earth, subsequently reducing or increasing the amount of radiation received at the Earth's surface in different seasons.

Today a difference of only about 3 percent occurs between aphelion (farthest point) and perihelion (closest point). This 3 percent difference in distance means that Earth experiences a 6 percent increase in received solar energy in January than in July. This 6 percent range of variability is not always the case, however. When the Earth's orbit is most elliptical the amount of solar energy received at the perihelion would be in the range of 20 to 30 percent more than at aphelion. Most certainly these continually altering amounts of received solar energy around the globe result in prominent changes in the Earth's climate and glacial regimes. At present the orbital eccentricity is nearly at the minimum of its cycle.

Axial Tilt

Axial tilt, the second of the three Milankovitch Cycles, is the inclination of the Earth's axis in relation to its plane of orbit around the Sun. Oscillations in the degree of Earth's axial tilt occur on a periodicity of 41,000 years from 21.5 to 24.5 degrees.

Today the Earth's axial tilt is about 23.5 degrees, which largely accounts for our seasons. Because of the periodic variations of this angle the severity of the Earth's seasons changes. With less axial tilt the Sun's solar radiation is more evenly distributed between winter and summer. However, less tilt also increases the difference in radiation receipts between the equatorial and polar regions.

One hypothesis for Earth's reaction to a smaller degree of axial tilt is that it would promote the growth of ice sheets. This response would be due to a warmer winter, in which warmer air would be able to hold more moisture, and subsequently produce a greater amount of snowfall. In addition, summer temperatures would be cooler, resulting in less melting of the winter's accumulation. At present, axial tilt is in the middle of its range.

Precession

The third and final of the Milankovitch Cycles is Earth's precession. Precession is the Earth's slow wobble as it spins on axis. This wobbling of the Earth on its axis can be likened to a top running down, and beginning to wobble back and forth on its axis. The precession of Earth wobbles from pointing at Polaris (North Star) to pointing at the star Vega. When this shift to the axis pointing at Vega occurs, Vega would then be considered the North Star. This top-like wobble, or precession, has a periodicity of 23,000 years.

Due to this wobble a climatically significant alteration must take place. When the axis is tilted towards Vega the positions of the Northern Hemisphere winter and summer solstices will coincide with the aphelion and perihelion, respectively. This means that the Northern Hemisphere will experience winter when the Earth is furthest from the Sun and summer when the Earth is closest to the Sun. This coincidence will result in greater seasonal contrasts. At present, the Earth is at perihelion very close to the winter solstice.

Summary

These variables are only important because the Earth has an asymmetric distribution of landmbuttes, with virtually all (except Antarctica) located in the Northern Hemisphere.

(From NOAA's National Geophysical Data Center) At times when Northern Hemisphere summers are coolest (farthest from the Sun due to precession and greatest orbital eccentricity) and winters are warmest (minimum tilt), snow can accumulate on and cover broad areas of northern America and Europe. At present, only precession is in the glacial mode, with tilt and eccentricity not favorable to glaciation

Even when all of the orbital parameters favor glaciation, the increase in winter snowfall and decrease in summer melt would barely enough to trigger glaciation, not to grow large ice sheets. Ice sheet growth requires the support of positive feedback loops, the most obvious of which is that snow and ice have a much lower albedo than ground and vegetation, thus ice mbuttes tend to reflect more radiation back into space, thus cooling the climate and allowing glaciers to expand.

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Why the stereotypical dumb Swede (mainly blonde) is not descended from the Galicians. All 11 messages in topic - view as tree

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Analysis of the substance by archaeologist Stephen Buckley from the University of York in England showed the gel was made of vegetable plant oil mixed with resin from pine trees found in Spain and southwest France.

The Irish and Scots may be as closely related to the people of Spain and Portugal as the Celts of central Europe, it emerged on September (09-09-2004)

Historians have long believed the British Isles were swamped by a mbuttive invasion of Iron Age Celts from central Europe around 500BC.

But geneticists at Dublin's Trinity College now claim the Irish and Scots have as much, if not more, in common with the people of north-western Spain.

Dr Daniel Bradley, genetics lecturer at Trinity College Dublin, said a new study into Celtic origins revealed close affinities with the people of Galicia

"It's well known that there are cultural relations between the areas but now this shows there is much more," Dr Bradley said.

"We think the links are much older than that of the Iron Age because it also shows affinities with the Basque region - which isn't a Celtic region."

"The links point towards other Celtic nations, in particular Scotland, but they also point to Spain," he added.

Historians believed the Celts, originally from the Alpine regions of central Europe invaded the Atlantic islands in a mbuttive migration 2,500 years ago.

But using DNA samples from people living in Celtic nations and other parts of Europe geneticists at the university have drawn new parallels.

Dr Bradley said it was possible migrants moved from the Iberian peninsula to Ireland as far back as 6,000 years ago up until 3,000 years ago.

"I don't agree with the idea of a mbuttive Iron Age invasion that took over the Atlantic islands. You can regard the ocean - rather than a barrier - as a communication route," Dr Bradley said.

It is believed archaeologists are also questioning the links between the Celts of eastern France and southern Germany and the people of the British Isles.

The study found people in areas traditionally known as Celtic, such as Ireland, Wales, Scotland, Brittany and Cornwall, had strong links with each other and people in Ireland have more in common with Scots than any other nation.

There are also close links between Scotland and Ireland dating back much further than the Plantations of the 1600s when many Scots moved to northern Ireland in search of fertile farming lands, the research showed.

Stephen Oppenheimer, professor of clinical socio-medical sciences at Oxford, said that the Celts of western Scotland, Wales, Ireland and Cornwall were descended from an ancient people living on the Atlantic coast when Britain was still attached to mainland Europe, while the English were more closely related to the Germanic peoples of the interior. He said : "The English are the odd ones out because they are the ones more linked to continental Europe. The Scots, the Irish, the Welsh and the Cornish are all very similar in their genetic pattern to the Basque."

However scientists could not shed any light on whether fair skin, red hair, freckles, and fiery tempers truly are Celtic traits.

The study headed by Dr Bradley was published in the American Journal of Human Genetics.