Greenhouse gases are the cause of Greenhouse effect.The greenhouse gases which include CO2, Methane etc. trap the heat in the Earth's atmosphere and don't allow it to escape.
The weapon is fitted with a Leupold Mark 4. The rifle is also similar in appearance to the TAC-338 rifle that Kyle used in real life. McMillan TAC-338A sniper rifle with Harris bipod, Leupold Mark 4 scope, and AN/PVS-22 Universal Night Sight - .338 Lapua Magnum. Kyle taking aim with the rifle. Right side of the rifle.
Most tactical style scopes have an adjustment for parallax. A common misconception is that the parallax adjustment is a *focusing* knob. While it may indeed bring the image in and out of focus this IS NOT WHAT THIS IS USED FOR! It is to bring the plane of the image inside the scope into the same plane as the retical.