In order for us to understand the process of terraforming, it is important for us to understand that Earth was not always the lush, green, life-bearing planet that it is today. .
When Earth was formed it was a ball of molten rock with a thin atmosphere consisting of hydrogen and helium. These gases quickly escaped from the atmosphere because of solar wind, extreme heat and the ultra-light weight of these elements. When the gases escaped, Earth was left without an atmosphere. However, Earth was slowly evolving.
The Earth had constant geological activity that created extreme volcanism. The volcanoes spewed out gases that went into the atmosphere. The two most important gases were H2O and CO2.
The H2O collected in the atmosphere and started to condense into clouds. The immense cloud cover allowed the planet to cool down. When the temperature decreased to 100 degrees Celsius, it began to rain. The rain pooled up and over time and formed the oceans.
The CO2 dispersed into the atmosphere and created a greenhouse effect. The greenhouse effect was caused by the trapping of infrared heat by the CO2 molecules. This benefited the Earth because it prevented a continual cooling process that would leave Earth a frozen ball of ice. The CO2 kept the Earth at a suitable temperature for microbial life to develop.
The next stage of the Earth's atmosphere was created by the blue-green algae that produced oxygen as a byproduct of photosynthesis. The oxygen collected in the atmosphere and allowed animals, that use respiration and not photosynthesis to evolve. Also, the oxygen bonds were broken up by ultra violet rays from the sun. The loose oxygen molecules reformed into O3, or ozone. The ozone formed a layer around Earth that blocks out the harmful UV rays, protecting the life below.