When the atmosphere first formed, it was not made up of very much. It had hydrogen sulfide, methane, and ten to 200 times as much carbon dioxide as today’s atmosphere.
The Earth has warmed up about 1.6 degrees fahrenheit since 1906. That causes sea ice to melt and oceans to rise. carbon dioxide from the burning of fossil fuels are keeping the heat in the atmosphere and not letting it go out to space, causing the Earth to heat up.
Oxygen has not always been as abundant as it is today. Most scientists believe that for half of Earth’s 4.6-billion-year history, the atmosphere contained almost no oxygen. Cyanobacteria or blue-green algae became the first microbes to produce oxygen by photosynthesis, perhaps as long ago as 3.5 billion years ago and certainly by 2.7 billion years ago. But, mysteriously, there was a long lag time – hundreds of millions of years – before Earth’s atmosphere first gained significant amounts oxygen, some 2.4 billion to 2.3 billion years ago.
Earth's atmosphere is about 300 miles think at this point in it's life. It is composed of 78 percent nitrogen, 21 percent oxygen, 0.93 percent argon, 0.04 percent carbon dioxide, and trace amounts of neon, helium, methane, krypton and hydrogen, as well as water vapor.