The natural greenhouse effect has been keeping the earth warm for millions of years. Greenhouse gases in the atmostphere trap some of the earth's rays and raise the earth's temperature to a comfortable level.
However, things like cars, planes, agriculture and energy production are all dramatically increasing the amount of greenhouse gases in our atmostphere.
This produces a thick blanket of greenhouse gases in the atmostphere.
This means the greenhouse gases trap to much of the suns rays and the earth heats.