Greenhouse gases are chemicals that help trap heat in the atmosphere by absorbing, reflecting or re-emitting long-wavelength (infrared) solar radiation. This warms the Earth and alters other parts of the climate system, with important implications for the human and natural environment.
There are many different greenhouse gases, and they each have a different effect on the atmosphere. For example, carbon dioxide has a very high global warming potential, or GWP, and is responsible for most of the current climate change. Methane, on the other hand, has a much lower GWP but is still important.
A large part of the GHG emissions come from the burning of fossil fuels for electric power and transport, and other industries, like agriculture, waste management and treatment activities, and various industrial processes. For these reasons, energy-related activities are the largest sector of greenhouse gas emission worldwide.
The concentration of these gases in the atmosphere has been changing rapidly due to human activity. This has led to the accumulation of high amounts of greenhouse gases in the atmosphere, especially carbon dioxide and methane. The concentrations of these gases are also much higher than they have been over the past 800,000 years, as evidenced by ice core records.
Observations of greenhouse gases are made in surface measurement networks and airspace observations, such as the NOAA Global Atmosphere Watch (GAW) programme. These observations are used in combination with models to better understand atmospheric chemical composition and processes. The information is used to guide decisions on mitigation measures.