We know that the energy output of a newborn star (sun) is less than as it ages. The Standard Solar Model indicates that our sun would have been at about 70% of the current level during the first part of its existence. This means that the Earth would’ve been a ‘giant snowball’ but for the high levels of methane and carbon dioxide in the atmosphere at that time. The ‘greenhouse effect’ kept the planet unfrozen. That is, until Cyanobacteria evolved and their photosynthesizing released oxygen that broke down the methane into CO2 and then basaltic weathering caused the free CO2 to be absorbed to the point that the planet froze over.
Nowadays, the sun is much brighter while greenhouse gases are fortunately much lower. However, mankind is disrupting the balance between oxygen and CO2 by artificially unsequestering large amounts of carbon. This is causing temperatures to rise and to release ever increasing amounts of methane via melting permafrost and methane hydrate degasification. The ocean's ability to absorb excess carbon dioxide is decreasing and ocean acidification is increasing. The greenhouse effect, instead of being a giver of life, will increasingly become a taker.
Early Earth:
https://en.m.wikipedia.org/wiki/Early_Earth
Faint Sun Paradox:
https://en.m.wikipedia.org/wiki/Faint_young_Sun_paradox
Astrobiology of methane and CO2:
Note that methane breaks down into carbon dioxide which is stable -
https://www.astrobio.net/geology/earths-early-atmosphere/
What is climate change:
http://www.bbc.com/news/science-environment-24021772
