CO2 is the most abundant greenhouse gas in the atmosphere (excluding water vapor). In its natural occurrence it helps keep the earth at a comfortable and life-sustaining temperature. However, it is also believed to be responsible for the majority of global warming. Some man-made sources of CO2 are the burning of fossil fuels, deforestation, biomass burning and cement production. Natural sources include animal respiration and volcanic eruptions. CO2 is removed from the atmosphere by photosynthesis and by the oceans absorbing it. It is not well known how long the CO2 produced can linger in the atmosphere because there are so many processes that produce and remove CO2. From 650,000 years ago until the industrial revolution the concentration of CO2 never rose above .000280%, or 280ppm, in the atmosphere. Since then the level of CO2 in the atmosphere has grown gradually over time. The CO2 concentration, as of 2009, is around .000385%, or 385ppm, and is increasing at an average global rate of 1.9ppm per year.
Since carbon dioxide is used in plant growth, an excess amount can act as a fertilizer to increase the growth of vegetation. However, this only happens if the plants have sufficient water and nutrients. In dry conditions or poor soil, the excess carbon dioxide will have no effect on the plants.
In the video below, Peter Griffiths, a NASA earth scientist explains how carbon is transferred to and from the atmosphere and the effects of the carbon crisis.