The following graph represents the changing atmospheric levels of the GHGs carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) over the past 2,000 years. It is taken from the Fourth Assessment Report [PDF] (2007) by the Intergovernmental Panel on Climate Change (IPCC).
Source: IPCC Fourth Assessment Report: Climate
Change 2007 Working Group I Report: The
Physical Science Basis Cambridge University
Press FAQ 2.1, Figure 1.
All GHGs impact the earth's climate, but some have a greater impact than others on a per-molecule basis. For example, CO2 is the most common GHG, but one molecule of CH4 has a global warming potential (GWP) 25 times that of a CO2 molecule over 100 years, and a molecule of N2O has a GWP 298 times that of a CO2 molecule over 100 years. Because scientists use CO2 as the baseline against which all other GHGs are measured, the GWP of CO2 is 1. Thus, we say that CH4 has a GWP of 25, while N2O has a GWP of 298.
Although most GHGs come from both natural and man-made (anthropogenic) sources, it is the fast-growing anthropogenic sources that are pushing the earth's climate out of balance. The IPCC Fourth Assessment Report states that "The atmospheric concentrations of CO2 and CH4 in 2005 exceed by far the natural range over the last 650,000 years. Global increases in CO2 concentrations are due primarily to fossil fuel use, with land-use change providing another significant but smaller contribution. It is very likely that the observed increase in CH4 concentration is predominantly due to agriculture and fossil fuel use. The increase in N2O concentration is primarily due to agriculture."