Greenhouse gases have always been present in our atmosphere.

Nitrogen and oxygen by themselves make up 99% of all gases in earth’s atmosphere. Other gases are also present in very tiny amounts. Since the 1800s, we've known that some of those gases absorb and release energy. We call them greenhouse gases, and it's the changing amounts of those gases in the air that brings concern on their impact on climate.

This normal process is called the Natural Greenhouse Effect. 

As energy is given off by the earth, it is giving off long wavelengths of energy; most in the infrared category. A small range of these wavelengths can be absorbed by greenhouse gases. Once absorbed, the energy is given back off very quickly; however, the released energy goes off in all directions, like a tiny and much cooler sun.

• Some goes back to earth’s surface to be given off again. 
• If it goes in other directions, it may be absorbed by another greenhouse gas molecule. 
• The overall effect of this is to slightly slow the release of energy to space, making the lower atmosphere slightly warmer than it would be if no greenhouse gases were present. 

Earths average temperature is about 57 degrees Fahrenheit. With no greenhouse gases, earth's average temperature would be around 0 degrees F.

The Greenhouse Effect

Human activity impacts the natural process, slowing the release and warming earth.

The thing is, humans have increased the concentration of greenhouse gases in the atmosphere, which enhances the natural effect that was already there.  As more greenhouse gases are present, it takes longer for the heat to escape. The earth near the surface becomes warmer as a result.

It is a physical cause and the resulting effect.