The “greenhouse gases” intercept a fraction of outgoing terrestrial infrared radiation, creating the natural greenhouse effect which was approximately 32 Celsius at the beginning of the 20th Century. Several of these gases have increased in atmospheric concentration during the past half-century, with carbon dioxide going from 315 ppmv in 1958 to 380 ppmv in 2005, and methane growing from 1.52 ppmv in 1978 to 1.77 ppmv in 2005. Nitrous oxide, the chlorofluorocarbons (CCl2F2, CCl3F, etc.) and tropospheric ozone are also GHG contributors. This process has warmed the atmosphere by about 0.7C over the past century, and will likely warm the Earth much more in the 21st, unless controls on these gases are rapidly put in place.
Feedback processes involving albedo changes from ice/snow to water/rock cause enhanced warming in the polar north, and the climate is changing rapidly in the Arctic.