Greenhouse gases are molecules in our atmosphere that prevent heat radiating from Earth’s surface from escaping into space. This is called the greenhouse effect, and it keeps the planet warmer than it would be without the presence of these gases.
The main long-lived greenhouse gases are carbon dioxide (CO2), methane (CH4), and nitrous oxide. They are all emitted by human activities. With the exception of water vapor, the concentrations of these gases have increased over the last one and a half centuries, thanks to humans’ industrial and land use changes. This has led to a warming of the planet’s atmosphere, and also to other changes in our climate system.
Carbon dioxide is the most important greenhouse gas because it can linger in the atmosphere for years. It enters the atmosphere from a variety of sources: outgassing from volcanoes, the combustion and natural decay of organic material (including coal, oil and natural gas), and respiration by aerobic (oxygen-using) organisms.
Most of CO2 emissions come from the combustion of fossil fuels. Transportation is the largest sector contributing to carbon emissions, but energy production (including electricity and heating) and other processes also contribute. Nitrous oxide (a byproduct of agricultural fertilizers and livestock waste), fluorinated gases used for air-conditioning and refrigerating vehicles, and methane from land use change are other major contributors.
Observations of greenhouse gases are conducted around the world by many networks, including the Global Atmosphere Watch Reference Network, based at Mauna Loa Observatory in Hawaii. These observations provide the raw data that are used to derive GAW’s Greenhouse Gas Bulletin, which reports on the atmospheric burden of the three most influential greenhouse gases.