WHAT IS THE SNOWBALL EARTH HYPOTHESIS?

The snowball Earth hypothesis proposes that at least twice in Earth’s history, sometime between 2.4 billion and 580 million years ago, the planet became entirely frozen from pole to pole, including the oceans. Although no single driving process has so far been identified, some scientists believe this best explains deposits of material of potential glacial origin found in low latitudes.

The hypothesis is an example of how feedbacks in Earth’s climate geosystem – in this case the ice albedo feedback – can create extreme change on a global scale. The basic idea is that when a planet cools it becomes more reflective – ice is more reflective than land or open ocean – so it absorbs less solar radiation and becomes cooler still, and a runaway cooling effect takes place.

The term ‘snowball Earth’ was originally coined by the American geologist Joseph Krschvink in 1992, but perhaps unsurprisingly, the hypothesis is hotly contested among experts in this field. It has led to an alternative theory being proposed which challenges the suggestion that Earth was completely frozen over – the slushball Earth hypothesis.

Stage 1

Initial global cooling caused the ice sheets at the poles to spread, and the larger white surfaces reflected increasingly more sunlight away from Earth.

Stage 2

This loss of sunlight amplified the cooling effect, which allowed the ice sheets to expand even further across Earth’s surface.

Stage 3

Continued positive feedback caused the ice sheets to extend to the tropics, encasing the Earth entirely in ice.

Stage 4

While Earth was buried in ice for millions of years, volcanoes slowly pumped carbon dioxide (CO2) into the atmosphere.

Stage 5

The CO2 accumulated in Earth’s atmosphere and trapped heat, which started to trigger a greenhouse warming effect.

Stage 6

CO2 levels became high enough to cause global temperatures to rise and break Earth out of the glaciated state, melting the ice.