You can think of the greenhouse effect as a car sitting in the sun. You probably know that if you leave your car in the sun for a few hours the inside will be much hotter than the outside. When heat enters through the windows some of it is absorbed by the seats, armrests, ect. and some is reflected back away from the inside and out the windows. But since the heat being reflected from the inside of the car is a different wavelength than what entered the car some of it can't escape and is trapped within the car raising the temperature.
Co2 is a greenhouse gas along with water vapor, methane, nitrous oxide and CFC's. What happens is the greenhouse gases absorb some of the radiation and reflect is back to earth making the earth warmer. The warm temperature on earth evaporates the water in the oceans creating water vapor (which is a greenhouse gas) making the earth even hotter. Scientists have actually found a direct correlation between the amount of Co2 in the atmosphere and the temperature of the earth. Most of the temperature increase has happened after the industrial revolution which leads me to believe we are the cause of it.
The greenhouse effect is pretty much proven science at this point.
