Ans: Most of the sun's energy reaches the Earth as visible light(0.40-0.76 microns wavelength). Part of the energy is absorbed by the surface of the Earth and converted into heat. Now the heated surface of the earth radiates this heat energy back into the atmosphere and back towards the space as infrared radiation (higher wavelength than visible light), but greenhouse gases like CO2, CFCs(chlorofluorocarbons), HCFCs(hydrochlorofluorocarbons), water vapor, etc absorb this infrared radiation which causes the heat to get trapped near the surface of the earth. This warming process caused due to atmosphere's absorption of heat energy radiated from Earth's surface is called the 'greenhouse effect' or 'global warming'.
If leaked into the atmosphere some chemicals directly affect the atmosphere and cause global warming. These effects are measured by an index called Global Warming Potential (GWP). The higher the global warming potential the more it can hold the heat into the atmosphere for a longer amount of time.
The GWP of CO2 is 1. The GWP of other chemicals is compared with CO2.
Some of the new generation refrigerants such as HFCs, HCs, and HFOs, have lower to a moderate values of GWP.
R134A has a GWP of 1430 which is considered a moderate GWP.
R290 has a GWP of 3.0 which is considered a very low GWP compared to other old generation refrigerants like R22, R12, etc.