A greenhouse gas is that gas which both absorbs and emits radiation within of thermal infrared range in the atmosphere. Water vapor, CO2, methane (CH4), N2O and Ozone are the primary GHGs. These GHGs are well-mixed and hence take years and years to leave the atmosphere. The lifespan of Carbon Dioxide in the atmosphere is somewhere between 5 years and 200 years.
Nitrous Oxide stays in atmosphere for about 114 years and has Global Warming Potential (GWP) of 289 over 20 years. Both Methane and HCFC-22 have the lifespan of 12 years each in the atmosphere. The GWP of methane over 20 years is 72 and that of HCFC-22 for the same period is 11,000.
It has become even more necessary to reduce the greenhouse effect produced by GHGs seeing their respective GWPs. Measures like creating more "sinks" have been taken to remove GHGs from the atmosphere. They can be either removed by natural processes or negative emissions.