As you go higher up in the atmosphere the air gets thinner and there is less oxygen to breath.
The atmosphere has 4 layers, Troposphere, Stratosphere, Mesosphere, and Thermosphere.
Troposphere is the layer that we live in, its the lowest layer, and weather happens here.
stratosphere protects us from the sun so we don't burn up, as you go higher in this layer it gets warmer, and its the 2nd layer in the atmosphere.
Mesosphere protects us from meteors, and here it gets colder and is the middle layer of the atmosphere.
thermosphere is the last layer of the atmosphere, it gets hoter, its were the northern lights happen, and its were the sun strikes it first.
The atmosphere was still held by earths gravity.
The green house effect is what keeps earth warm with out it we will be too hot or too cold.