Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of planets such as the Earth. What distinguishes them from other gases is that they absorb the wavelengths of radiation that a planet emits, resulting in the greenhouse effect. The Earth is warmed by sunlight, causing its surface to radiate heat, which is then mostly absorbed by greenhouse gases. Without greenhouse gases in the atmosphere, the average temperature of Earth's surface would be about , rather than the present average of .
The five most abundant greenhouse gases in Earth's atmosphere, listed in decreasing order of average global mole fraction, are: water vapor, carbon dioxide, methane, nitrous oxide, ozone. Other greenhouse gases of concern include chlorofluorocarbons (CFCs and HCFCs), hydrofluorocarbons (HFCs), perfluorocarbons, sulfur hexafluoride|, and nitrogen trifluoride|. Water vapor causes about half of the greenhouse effect, but humans are not directly adding to its amount, so it is not a driver of climate change.
Carbon dioxide is causing about three-quarters of global warming and can take thousands of years to be fully absorbed by the carbon cycle. Methane causes most of the remaining warming and lasts in the atmosphere for an average of 12 years. Human activities since the beginning of the Industrial Revolution (around 1750) have increased carbon dioxide by over 50%, up to a level not seen in over 3 million years. The atmospheric methane concentrations have increased by over 150% during the same time period. 
Without human influence, the natural flows of carbon between the atmosphere, terrestrial ecosystems, the ocean, and sediments would be fairly balanced. The vast majority of carbon dioxide emissions by humans come from the burning of fossil fuels. Further contributions come for example from cement manufacturing and deforestation. Methane emissions originate from agriculture, fossil fuel production, waste, and other sources. If current emission rates continue then temperature rises will surpass  sometime between 2040 and 2070. This is a level which the Intergovernmental Panel on Climate Change (IPCC) says is "dangerous".