Richard, CO2 absorbs infrared. IR seeking missiles need to be able to "see" the gaps in the infrared band to hit what they're aiming for. I'm not making this up. The HITRAN database (the physics built into climate models) is the result of decades of measurements that started with the Air Force wanting to be able to hit targets with IR viewing missiles and be able to image ground targets from space through all sorts of weather. 

https://scienceofdoom.com/2011/02/09/understanding-atmospheric-radiation-and-the-%E2%80%9Cgreenhouse%E2%80%9D-effect-%E2%80%93-part-seven/

And the arsenic comparison is apt, it goes to show that an increase in a very tiny concentration of something can have a huge effect.