Major greenhouse gases and sources

Carbon dioxide (CO2): Carbon dioxide is the primary greenhouse gas, responsible for about three-quarters of emissions. It can linger in the atmosphere for thousands of years. In 2018, carbon dioxide levels reached 411 parts per million at Hawaii’s Mauna Loa Atmospheric Baseline Observatory, the highest monthly average ever recorded. Carbon dioxide emissions mainly come from burning organic materials: coal, oil, gas, wood, and solid waste.

Methane (CH4): The main component of natural gas, methane is released from landfills, natural gas and petroleum industries, and agriculture (especially from the digestive systems of grazing animals). A molecule of methane doesn’t stay in the atmosphere as long as a molecule of carbon dioxide — about 12 years — but it is at least 84 times more potent over two decades. It accounts for about 16 percent of all greenhouse gas emissions.

Nitrous Oxide (N2O): Nitrous oxide occupies a relatively small share of global greenhouse gas emissions — about six percent — but it is 264 times more powerful than carbon dioxide over 20 years, and its lifetime in the atmosphere exceeds a century, according to the IPCC. Agriculture and livestock, including fertilizer, manure, and burning of agricultural residues, along with burning fuel, are the biggest sources of nitrous oxide emissions.

Industrial gases: Fluorinated gases such as hydrofluorocarbons, perfluorocarbons, chlorofluorocarbons, sulfur hexafluoride (SF6), and nitrogen trifluoride (NF3) have heat-trapping potential thousands of times greater than CO2 and stay in the atmosphere for hundreds to thousands of years. Accounting for about 2 percent of all emissions, they’re used as refrigerants, solvents, and in manufacturing, sometimes occurring as byproducts.

Other greenhouse gases include water vapor and ozone (O3). Water vapor is actually the world’s most abundant greenhouse gas, but it is not tracked the same way as other greenhouse gases because it is not directly emitted by human activity and its effects are not well understood. Similarly, ground-level or tropospheric ozone (not to be confused with the protective stratospheric ozone layer higher up) is not emitted directly but emerges from complex reactions among pollutants in the air.