Greenhouse gases (GHGs) are molecules in the atmosphere that absorb and re-radiate heat from Earth, keeping it from leaving into space. They include water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and a suite of fluorine-bearing compounds (HFCs, PFCs and SF6) emitted by industrial activities. Increasing concentrations of these molecules in the atmosphere have been linked to global warming, referred to as climate change.
Emissions of GHGs have been rising steadily over the past one and a half centuries, driven by human activities such as burning fossil fuels for energy. Currently, human-induced CO2 emissions account for about half of the world’s total.
Concentrations of these gases build up in the atmosphere when they are emitted into it, and they stay there for long periods of time. As concentrations increase, the average surface temperature of the Earth rises, which has effects on other parts of the climate system such as precipitation patterns and sea level.
Methane is a powerful greenhouse gas, and its atmospheric concentrations have been rising rapidly in recent years. It is emitted from natural and human sources such as wetlands, tundra, oceans and their bottom sediments, termites and livestock farming. The most important source of anthropogenic methane is the production, transportation and use of fossil fuels. Nitrous oxide is a strong greenhouse gas with high emissions from both natural and human sources including soils under natural vegetation, agriculture (nitrous oxide fertilizers, rice farming), biomass combustion, fossil fuels combustion and industrial processes.