<em><u>The EIA forecasts that US oil petroleum and liquids consumption will increase from approximately 16 million barrels per day in 2020 to 19 million barrels per day in 2050, while at the same time natural gas consumption will increase from 84 billion cubic feet per day to 97 billion cubic feet per day.</u></em>
They are based on the chemical properties of the atmosphere. For example, the lowest level has an abundance of oxygen, while the upper levels don't, and contain more CO2.