The climate during the last 550 million years is rather well-documented.  There have been several periods of tens of millions of years without any ice sheets.  The greenhouse effect was then considerably stronger than it is today.  This evened out the climate between the poles and the Equator.  Naturally there was not the same climate everywhere.  But the difference in temperature was smaller between different latitudes.

When there is ice sheets at the poles it is called Icehouse Earth.  Such ice sheets are today found in Antarctica and on Greenland.  Antarctica’s ice sheet has existed for 34 million years.  Greenland has been more or less ice-covered for a couple of million years.  (The name Greenland was likely made up to make it sound better.)  During such an Icehouse Earth climate vary cyclically over tens of thousands and up to a hundred thousand years.  It is the colder periods which are called ice ages.  Warmer periods between them are called interglacials.  This change is due to astronomical cycles called Milanković cycles.  They are named for Milutin Milanković who fist described these cycles.

There are three such Milanković cycles:

The degree of the Earth’s axial tilt.  The discrepancy from 90 degrees to the plane of Earth’s orbit varies from 22 to 24½ degree.  This vary in a cycle of 41,000 years.

How elongated the Earth’s orbit is.  Although its orbit is not particularly elongated there is some difference over time in exactly how elongated it is.  This varies over a period of about 100,000 years.

In which direction the Earth’s axis points.  The direction the Earth’s axis points moves in a circle against the stars.  A full circle takes 26,000 years.

The seasons are mainly determined by the Earth’s axial tilt against the plane of its orbit.  However, they can be strengthened or weakened by how elongated the Earth’s orbit is.  This depends on which seasons the extremes of the Earth’s orbit falls.  Does the extremes coincide with the seasons the differences become larger or smaller.  Warmer summers make more snowmelt while colder summers make less.  If the seasons are especially small in the northern hemisphere the climate becomes colder.  Why particularly in the northern hemisphere?  Because there is more land to the north where the ice sheets can spread.  When the differences are the largest to the north the climate in the same way becomes warmer.  Both processes are strengthened by changes in the size of the ice sheets.  Colder seas can absorb more carbon dioxide which strengthens the effect too.

Before Greenland started becoming covered in ice the differences were not that great.  Antarctica’s size set a natural limit to how much the ice could grow.  After the Greenland ice sheet started forming a rather even ice age cycle arose.  The climate become colder and warmer in a cycle of 41,000 year.  But then the ice age cycle changed 1.25 – 0.7 million years ago.  The cycle become gradually longer with proportionally longer ice ages.  700,000 years ago the ice age cycle had stabilised again.  Now the cycle was around 100,000 years mostly consisting of ice age.  Warmer periods between them consisted less than one fifth.  The question is just what caused the ice age cycle’s length to change.  The best explanation is ice sheets having eroded away sediment from the areas they had so far covered.  The ice could then glide more easily on mostly bedrock.  This in turn made it easier for the ice to spread.  A small cooling’s effect was strengthened and increased the cycle’s length.

The last three ice ages have been particularly cold.  When it was as coldest nearly all of Northern Europe was covered in ice.  The Alps and parts of Central Europe become covered in ice too.  Northwestern Siberia was also ice-covered as well as the mountains further east.  Nearly all of Canada was covered and adjacent parts of the US.  In the southern hemisphere it was a little less cold.  There the ice mostly spread over parts of Patagonia and southern New Zealand.  Parts of Tasmania was also ice-covered.  Mountain glaciers all over the world where far larger than now.  All ice took up so much water the seas shrunk by at least a hundred meters (328 feet).  In general the world during the ice ages were tangibly colder and dryer.  There were considerably less forest and woods, and the deserts were larger.  Thanks to traces of such changes we can know how the climate was.

I don’t think it is a chance occurrence our civilisation has arisen during an interglacial.  The last ice age ended 11,700 years ago.  During the last 10,000 years climate has been stable.  The increase or decrease in temperature has not exceeded 2°C (1.1°F) per century.  This is about as quickly as societies has time to adapt.  Particularly large volcanic eruptions have caused climate disasters.  However, their effect on climate has only lasted for a few years.  Agrarian societies survive this if there are no other factors like environmental destruction and pandemics.  This in contrast with the climate change taking place during an ice age.  These were at least twice as fast as the changes within an interglacial.  Moreover, they happened 2,000 – 5,000 years apart.  The climate was simply too unstable.

Will the ice ages return?  This depends on how far global warming goes.  If it was not for global warming the next ice age would begin around the year 3500.  Due to human activities this will not happen.  What we don’t know is how much warmer it will becomes first.  The fraction of carbon dioxide is already so high the Greenland ice sheet is likely doomed.  If not the ice in Antarctica disappears there would still be Icehouse Earth.  This would make it possible for the ice ages to return.  But then the fraction of carbon dioxide has to decrease which take at least tens of thousands of years.  Climate change will if so jump at least one cycle.  This does not stop smaller changes from taking place before that.  Particularly if a supervolcano explode and cause 5 – 10 years of severe cooling.  The ash fall on land from such might hasten the process.  However, it would not cause a real ice age if the fraction of carbon dioxide is still too high.


Uploaded on the 31st of May 2024.