Animals, including us, need oxygen from the atmospheric or air to live. Plants need the atmospheric CO2 in their photosynthesis process to produce energy to exist. The atmosphere also protects living creatures by absorbing harmful ultraviolet radiation from our sun. The atmosphere acts to retain heat through its greenhouse effect and even out the temperature differences between the day and cold dark of the night. Without the atmosphere, we and life as we know it would not exist.
The Earth's air or atmosphere is made up of different layers of gases enveloping our planet and tied to it by gravity. These layers include the troposphere, stratosphere, mesosphere, thermo-sphere, and exosphere, respectively, from the earth's surface to outer space. The troposphere, which contains about 80% of the atmosphere's mass, is made up by volume of 78% nitrogen, 21% oxygen, .9% argon, .06% water and trace amounts of other gases such as methane, and .04% carbon dioxide.
The two big green-house gases (GHG) in the atmosphere are water and CO2, however, water in the air in its various forms is part of a natural cycle, about which we can do little. Other atmospheric trace gases such as methane (CH4), an even stronger GHG than CO2, appear in the air in relatively small amounts at this time. That leaves CO2 as the major GHG that we can do somehing about. Recall from the introduction that when the CO2 level in the atmosphere goes up, so does global warmth. CO2 is a driving variable in global warming, hence a driving or forcing factor in climate change.