Answer: Decreases the transparency of the atmosphere to infrared light.
Explanation: Green House gases are gases such as Carbondioxide,Methane,Water Vapor etc which are present in the Atmosphere,they trap and emit heat in the form of radiations. Green house gases act like a barrier preventing some of the radiations especially of the IR(INFRA-RED) to leave the Earth,which eventually leads to increase in the overall Temperature of the Earth.
The higher the concentration of the Green house gases,the lesser the transparency of the Atmosphere to the Infrared radiation.