The main human influence on global climate is likely to be emissions of greenhouse gases (GHG). The emissions of GHG have increased sharply during the past decades due to human actions including burning of fossil fuels, land use changes, industry and agriculture. The most important greenhouse gas that human actions are affecting is carbon dioxide CO2 . Concentration of CO2 in the atmosphere has risen from the preindustrial era's 280 ppm (parts per million) to approximately 370 ppm. Annual rate of growth of CO2 concentration in the atmosphere has been about 0.5% during the last decades.
Atmospheric carbon dioxide concentrations measured at Manua Loa, Hawaii. Figure by Robert A. Rohde, Global Warming Art.
Other GHGs include methane (atmospheric concentration currently about 1.75 ppm, annual increase about 0.5%), dinitrogen oxide (N2O) (concentration in the atmosphere 13% greater than in the preindustrial era) and halogenated hydrocarbons.
Lifetimes of greenhouse gases, especially CO2, in the atmosphere are long. Even if emissions of GHGs were cut down immediately, concentrations in the atmosphere would not diminish, as the atmosphere reacts to changes slowly and it takes a long time to reach a new state of equilibrium.