The Earth 4 billion years ago used to be warmer than it is today. There was a thicker blanket around the Earth and the sun was 25% fainter than it is today which kept the sun warm. But where did the thick blanket or carbon dioxide go?
Chemical Weathering
This is where carbon dioxide and rainwater forms a weak acid called carbonic acid which dissolves silicate rocks. The carbon from the rocks in the form of bicarbonate ions washes into the ocean, where it is used by many organisms to form their shells and are deposited on the ocean bed to form carbonate rocks. This was where the carbon was kept and explains how our climate is self regulated. Even though the sun is getting brighter, we haven't seen a proportional increase in the Earth's temperature. This is an example of negative feedback. As the Earth's temperature began to increase, then so did the rate of chemical weathering.
Snowball Earth
2.2 billion and 700 million years ago our planet became a giant snowball. The growth of continents and the removal of carbon dioxide by weathering cools the planet, then ice cover increases which also means that reflectivity increases. This reflectivity reduces the absorption of sunlight by the Earth's surface and so increases the temperature further etc etc.... This produces positive feedback which is usually restricted. It amplifies the spread of ice cover away from the poles. If it reaches the tropics which is a tipping point where the process of amplification runs away and we get a Snowball Earth with the ice closing up the equator.
The Weathering process is stopped by the cover of ice,but volcanoes are still pumping carbon dioxide into the atmosphere which has no where to go as the carbon sink has been switched off. This creates a blanket warming the ice around the equator which causes the rest of the ice to melt. This kick starts the Weathering process once again and returning the carbon dioxide to the Earth's crust.
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