Greenhouse gas (GHG) emissions are gases in the Earths atmosphere that trap heat, contributing to climate change. The main sources of GHG emissions due to human activity are:

• Burning of fossil fuels such as coal, oil, and natural gas, which is estimated to have emitted 62% of the human-caused GHG in 2015.
• Land use change, mainly deforestation in the tropics, accounts for about a quarter of total anthropogenic GHG emissions.

The largest source of GHG emissions from human activities in the United States is from burning fossil fuels for electricity, heat, and transportation. The types of GHG emissions include:

• Carbon dioxide (CO2), which is the most important factor in causing climate change.
• Methane (CH4), which accounted for 12% of all U.S. GHG emissions from human activities in 2021.
• Nitrous oxide (N2O) .
• Fluorinated gases, which are typically emitted in smaller quantities than other GHGs but are potent GHGs.

GHG emissions are often measured in CO2 equivalents, which is a metric used to quantify GHG emissions. The standard way to convert non-CO2 gases into their CO2 equivalents is to multiply their mass by their global warming potential (GWP), which measures the warming impacts of a gas compared to CO2. The standard metric used to quantify GHG emissions is carbon dioxide-equivalents.