<P> The contribution of each gas to the greenhouse effect is determined by the characteristics of that gas, its abundance, and any indirect effects it may cause . For example, the direct radiative effect of a mass of methane is about 72 times stronger than the same mass of carbon dioxide over a 20 - year time frame but it is present in much smaller concentrations so that its total direct radiative effect is smaller, in part due to its shorter atmospheric lifetime . On the other hand, in addition to its direct radiative impact, methane has a large, indirect radiative effect because it contributes to ozone formation . Shindell et al. (2005) argue that the contribution to climate change from methane is at least double previous estimates as a result of this effect . </P> <P> When ranked by their direct contribution to the greenhouse effect, the most important are: </P> <Table> <Tr> <Th> Compound </Th> <Th> Formula </Th> <Th> Concentration in atmosphere (ppm) </Th> <Th> Contribution (%) </Th> </Tr> <Tr> <Td> Water vapor and clouds </Td> <Td> 2O </Td> <Td> 10--50,000 </Td> <Td> 36--72% </Td> </Tr> <Tr> <Td> Carbon dioxide </Td> <Td> CO </Td> <Td> ~ 400 </Td> <Td> 9--26% </Td> </Tr> <Tr> <Td> Methane </Td> <Td> CH </Td> <Td> ~ 1.8 </Td> <Td> 4--9% </Td> </Tr> <Tr> <Td> Ozone </Td> <Td> O </Td> <Td> 2--8 </Td> <Td> 3--7% </Td> </Tr> <Tr> <Th_colspan="4"> notes: <P> Water vapor strongly varies locally The concentration in stratosphere . About 90% of the ozone in Earth's atmosphere is contained in the stratosphere . </P> </Th> </Tr> </Table> <Tr> <Th> Compound </Th> <Th> Formula </Th> <Th> Concentration in atmosphere (ppm) </Th> <Th> Contribution (%) </Th> </Tr>

The atmospheric gas that is mainly responsible for greenhouse effect