Greenhouse gases are human-made chemicals that trap extra heat in the atmosphere, raising global temperatures and changing other parts of the climate system. In the short term, higher emissions and concentrations of greenhouse gases cause harm to health through increased risks of heat-related illnesses and deaths. In the long run, they threaten the stability of the global climate, affecting food production and water availability, for example.

Since the start of industrialization, emissions of many greenhouse gases have increased. As a result, the concentrations of most of them in the atmosphere are now far above their pre-industrial levels. Carbon dioxide is the most common greenhouse gas. Its atmospheric concentration has risen by a factor of over 800 in the past century.

Different greenhouse gases have differing impacts. They also stay in the atmosphere for different amounts of time. In order to compare them, their impact is converted into a CO2 equivalent using the so-called Global Warming Potential (GWP). The GWP of a given gas is defined as the amount of energy needed to warm the Earth by 1 degree Celsius (1.8°F) in the absence of all GHGs, over 100 years.

The Annual Greenhouse Gas Index tracks the increase in the warming influence of human-made GHG emissions, including carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons. It is calculated by combining the complex scientific calculations that determine how much extra heat the individual gases trap into a number that can be easily compared to previous years, and the rate at which these numbers are increasing.