Greenhouse Gases Are the Main Reasons for Global Warming.
Cause 1: Variations in the Sun's Intensity
Cause 2: Industrial Activity
Cause 3: Agricultural Activity
Cause 4: Deforestation
Sunlight is type of EM wave. The intensity of EM follows inverse square law.

This means that the intensity at a distance that is n times greater will be n^2 times smaller.
Now we set this equation to be equal to 9 and we solve it for n:

The intensity is 9 times greater at a distance that is equal to a 1/3 of Earth's distance: