<P> Methane has an atmospheric lifetime of 12 ± 3 years . The 2007 IPCC report lists the GWP as 72 over a time scale of 20 years, 25 over 100 years and 7.6 over 500 years . A 2014 analysis, however, states that although methane's initial impact is about 100 times greater than that of CO, because of the shorter atmospheric lifetime, after six or seven decades, the impact of the two gases is about equal, and from then on methane's relative role continues to decline . The decrease in GWP at longer times is because methane is degraded to water and CO through chemical reactions in the atmosphere . </P> <P> Examples of the atmospheric lifetime and GWP relative to CO for several greenhouse gases are given in the following table: </P> <Table> Atmospheric lifetime and GWP relative to CO at different time horizon for various greenhouse gases <Tr> <Th> Gas name </Th> <Th> Chemical formula </Th> <Th> Lifetime (years) </Th> <Th_colspan="3"> Global warming potential (GWP) for given time horizon </Th> </Tr> <Tr> <Th> 20 - yr </Th> <Th> 100 - yr </Th> <Th> 500 - yr </Th> </Tr> <Tr> <Td> Carbon dioxide </Td> <Td> CO </Td> <Td> 30--95 </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> Methane </Td> <Td> CH </Td> <Td> 12 </Td> <Td> 84 </Td> <Td> 28 </Td> <Td> 7.6 </Td> </Tr> <Tr> <Td> Nitrous oxide </Td> <Td> 2O </Td> <Td> 121 </Td> <Td> 264 </Td> <Td> 265 </Td> <Td> 153 </Td> </Tr> <Tr> <Td> CFC - 12 </Td> <Td> CCl </Td> <Td> 100 </Td> <Td> 10 800 </Td> <Td> 10 200 </Td> <Td> 5 200 </Td> </Tr> <Tr> <Td> HCFC - 22 </Td> <Td> CHClF </Td> <Td> 12 </Td> <Td> 5 280 </Td> <Td> 1 760 </Td> <Td> 549 </Td> </Tr> <Tr> <Td> Tetrafluoromethane </Td> <Td> CF </Td> <Td> 50 000 </Td> <Td> 4 880 </Td> <Td> 6 630 </Td> <Td> 11 200 </Td> </Tr> <Tr> <Td> Hexafluoroethane </Td> <Td> 6 </Td> <Td> 10 000 </Td> <Td> 8 210 </Td> <Td> 11 100 </Td> <Td> 18 200 </Td> </Tr> <Tr> <Td> Sulfur hexafluoride </Td> <Td> SF 6 </Td> <Td> 3 200 </Td> <Td> 17 500 </Td> <Td> 23 500 </Td> <Td> 32 600 </Td> </Tr> <Tr> <Td> Nitrogen trifluoride </Td> <Td> NF </Td> <Td> 500 </Td> <Td> 12 800 </Td> <Td> 16 100 </Td> <Td> 20 700 </Td> </Tr> </Table> <Tr> <Th> Gas name </Th> <Th> Chemical formula </Th> <Th> Lifetime (years) </Th> <Th_colspan="3"> Global warming potential (GWP) for given time horizon </Th> </Tr>

What is the largest contributor to the greenhouse effect
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