NIWA
Figure 1: Concentrations of important long-lived greenhouse gases over the last 2,000 years, from measurements on air trapped in ice cores. Increases since about 1750 are attributed to human activities in the industrial era. Concentration units are parts per million (ppm) or parts per billion (ppb), indicating the number of molecules of the greenhouse gas per million or billion air molecules, respectively, in an atmospheric sample. (Figure from Forster et al., 2007).