There is a hypothesis that the Earth at one or several points would have been entirely covered by ice.  This is called Snowball Earth and is a controversial idea.  All the suggested points have their own problems.  I will go through each of them and explain what shortcomings I see.  To me Slushball Earth appear more probable.  In such a scenario the seas near the Equator are ice-free.  Same applies to low-lying coasts they border to.  However, this is of cause cold enough.

The Earth is 4,550 million years old.  Over the course of thousands of millions of years the Sun has gradually increased in brightness.  It has as such produced more energy the older it gets.  At the same time the greenhouse effect has decreased.  After the emergence of photosynthesis life has produced large amount of oxygen.  At first it was absorbed by sediments rich in iron and aluminium.  Then it started to accumulate in the air were it reacted with methane.  Also, the fraction of carbon dioxide in the air gradually decreased because it was absorbed by photosynthetic organisms.  After life established itself on land this process has got even quicker.  Land plants and fungi has broken down rocks and increased the rate of erosion.  Later peat has formed and petrified into coal.  When limestone has formed the fraction of carbon dioxide has decreased too.  The composition of the Earth’s atmosphere has for these reasons changed over geological time.  Such we have to consider when calculating past climate.

The oldest suggested Snowball Earth was during the Huronian Icehouse.  This was 2,500 – 2,200 million years ago.  During these 300 million years there are traces of at least three ice ages.  The increase in fraction of oxygen in the atmosphere led to global cooling.  It has been suggested it got so cold the entire Earth was covered by ice.  The melt-down would then have been triggered by a large meteorite impact.  This would have punctured a hole in the ice and thrown out darker rock.  The Earth would then have become darker which warmed the climate.  The question is which one of the layers correspond to the Earth freezing over.  That the same sequence of events would have happed three times appear unlikely to me.

The second suggested Snowball Earth would have been during the Cryogenian.  This was 720 – 635 million years ago.  During almost the entire period it seem to have been Icehouse Earth.  We have a fairly good idea of where the landmasses of the time were.  Based on this it is hard to make the Earth freeze over.  Succeeding to freeze the Earth over instead makes it very hard to break out of a frozen state.  It would require a fraction of carbon dioxide which the scientists themselves saw as implausible.  So I don’t think the Earth froze over during this period.

At some later points it definitely become really cold.  The Earth’s average temperature has varied from 7°C to 29°C (45°F to 84°F).  Now it is 15 – 16°C (59 – 61°F) which allows for ice sheets.  The question is just how large they have been in the past.  During the three last ice ages they were evidently much larger.  Maps showing how large they were are found here and here.  Black is current glaciers, and shaded is largest extent.  I think it was a comparable temperature at the transition Ordovician – Silurian.  In late Carboniferous and early Permian the climate was cold too.  Yet if it was colder back then I don’t think it was even Slushball Earth.  So many organisms survived that woods must have been found at the Equator.  A Slushball Earth is too cold for that.

Altogether I don’t think the Earth has frozen over during the last 635 million years.  Claiming it has earlier one needs better arguments.  It is clear there were ice ages 2,500 – 2,200 million years ago.  Traces of them has been found in several places in North America.  Such has been found in Australia, India and South Africa too.  That they would mean Snowball Earth could be an over-interpretation.  Especially since at least one of the places have traces of three ice ages.  For the Cryogenian the problem lies in the result of climate models.  If the problems remain despite improvements I don’t think it was more than a Slushball Earth.

Uploaded on the 5th of June 2024.