Periods of glaciation

 

Precambrian Climate-Earth's Early Environment

 

4.5 to 2.5Ga

The first half of earth history is significantly different from later times. The aerial extent of continents during the first half of the earth history was probably much less than present. Between 4.6 and 3.9Ga (billion years) the crust was probably much warmer than at present as the Earth was cooling down.

Large impacts by cosmic debris may have annihilated early ecosystems by producing globally lethal conditions which also resulted in loss of water due to evaporation during these events.

The formation of the Earth's core (iron) due to gravitational sinking may have released tremendous amounts of heat, raising the Earth's surface temperatures to 1200oC.

 

At around 4 billion years ago we have the first evidence of life that was once living in the oceans of Earth. These micro-organisms were capable of photosynthesis. The presence of these organisms indicates that by 4 billion years ago the Earth's surface temperature must have cooled to less than 100oC, the boiling point of water. Furthermore, the oceans must have been above 0oC, the freezing point, in order for these organisms to have lived. Therefore, we know that by about 4.0 billion years ago the Earth's surface temperature was between 100 and 0oC.

 

 

But how warm or cold was the Earth 4 billion years ago?

We believe that heat transport via the oceans was much less than today. See the diagram above. The upper diagram shows what the heat transport towards the poles via the oceans was in the Archean compared to the present (lower diagram).

We also know that based on physical and chemical principals, the Sun would have emitted less solar radiation 4 billion years ago and has increased ever since. So how cold was it in the Precambrian?

 

We know that by at least the late Pre-Cambrian, the climate was cold enough to have glaciers.

Our best evidence suggests that these glaciers (see below) were located close to the equator (based on paleomagnetic reconstructions). So why was the temperature of the Earth cold enough to allow ice to exist near the equator?

 

We believe one reason is that in addition to lower heat flux by the oceans, the greenhouse gases may have been lower. The reason for this was that plants were photosynthesizing and using the greenhouse gas (CO2). This lowered the amount of greenhouse gas in the atmosphere and promoted a colder climate condition, even near the equator.

 

 

Paleozoic Climate