This figure shows that the atmospheric concentrations of naturally occurring greenhouse gases—carbon dioxide (CO2, red), methane (CH4, blue), and nitrous oxide (N2O, green)—have varied over the past 650 millennia as the Earth has cooled (glacial periods, minima in the black curve) and warmed several times (interglacial periods denoted by the grey bars). Concentration units are parts per million (ppm) or parts per billion (ppb)—the number of molecules of the greenhouse gas per million or billion molecules, respectively, in a dry atmospheric sample. Until the past two centuries, the concentrations of CO2 and CH4 had never exceeded about 280 ppm and 790 ppb, respectively. Current concentrations of CO2 are about 390 ppm and CH4 levels exceed 1,770 ppb. Both numbers are much higher than at any time during the last 650,000 years.
Data for the past 2000 years show that the atmospheric concentrations of CO2, CH4, and N2O – three important long-lived greenhouse gases – have increased substantially since about 1750. Rates of increase in levels of these gases are dramatic. CO2, for instance, never increased more than 30 ppm during any previous 1,000-year period in this record but has already risen by 30 ppm in the past two decades.
These increases in greenhouse gas concentrations and their marked rate of change are largely attributable to human activities since the Industrial Revolution (1800). The increases and current atmospheric levels are the result of the competition between sources (the emissions of these gases from human activities and natural systems) and sinks (their removal from the atmosphere by conversion to different chemical compounds--for example, CO2 is removed by photosynthesis and conversion to carbonates). Brief summaries of these factors for several important greenhouse gases are given in Greenhouse Gas Sources and Sinks with graphics showing the human and natural contributions to their emissions (and sinks).