Answer to Question 1
O2 and N2 are poor absorbers of IR radiation and, thus, are not considered greenhouse gases. As O2 and N2 are totally symmetric molecules, electromagnetic waves pass by such molecules without being absorbed. Technically, they have no electric dipole moment with which the radiation can interact.
Answer to Question 2
The water vapor feedback is a positive feedback that tends to amplify small temperature perturbations. The ice albedo feedback is another positive feedback that tends to amplify changes to the Earth's surface temperature. Earth's climate is stable because of the negative feedback between the surface temperature and the flux of outgoing infrared radiation.
The greenhouse effect: heat trapping by earth's atmosphere.
Blue light is scattered more than other wavelengths by the gases in the atmosphere, giving the Earth
IPCC model projections depend on greenhouse gas emissions
Greenhouse gas (GHG) and Water vapor (H2O)
Land use change and forestry are responsible for 18% of global greenhouse gas emissions
Sun's composition of Gases.