Much of the carbon dioxide that enters the atmosphere is a by-product of the burning of fossil fuels, solid wastes, trees, and other biological material. According to the EPA, the transportation sector generated 27% of 2020 greenhouse gas emissions, which was the most of any sector. Electricity production produced approximately 25% of greenhouse gas emissions in 2020. Another sector that is a top contributor to global greenhouse gas emissions is commercial and residential real estate. Construction of the various forms of buildings, along with the heating and cooling needs of homes and businesses, are another major source of pollution. In 2020, direct greenhouse gas emissions from homes and businesses accounted for 13% of total greenhouse gas emissions in the U.S.