CO2 makes up less than 0.04% of the earth's atmosphere... 33 molecules of CO2 for every 85,800 molecules of air. 32 of those CO2 molecules are from totally natural sources and only one molecule of CO2 in every 85,800 molecules of air is from human activity. To believe the global warming hoax you have to subscribe to the notion that molecular level changes in a single component of the atmosphere can cause catastrophic change in the earth's climate and that the variations in solar output or other natural inputs into out climate have no net effect.
Fossil fuels such as gasoline, methane and propane contain mostly carbon. When these fuels are burned, they react with oxygen and produce carbon dioxide.
Because of our heavy use of fossil fuels, the amount of carbon dioxide in the atmosphere has been increasing since the industrial revolution. The destruction of forests which use carbon dioxide also contributes to the increase in carbon dioxide.
Most of the light energy from the sun is emitted in wavelengths shorter than 4,000 nanometers (.000004 meters). The heat energy released from the earth, however, is released in wavelengths longer than 4,000 nanometers. Carbon dioxide doesn't absorb the energy from the sun, but it does absorb some of the heat energy released from the earth. When a molecule of carbon dioxide absorbs heat energy, it goes into an excited unstable state. It can become stable again by releasing the energy it absorbed. Some of the released energy will go back to the earth and some will go out into space.
So in effect, carbon dioxide lets the light energy in, but doesn't let all of the heat energy out, similar to a greenhouse.
Currently, the amount of carbon dioxide in the atmosphere is increasing at the rate of about one part per million per year. If this continues, some meteorologists expect that the average temperature of the earth will increase by about 2.5 degrees Celsius. This doesn't sound like much, but it could be enough to cause glaciers to melt, which would cause coastal flooding.