As of March 2009, carbon dioxide in the Earth's atmosphere is at a concentration of 387 ppm (parts per million) by volume. Atmospheric concentrations of CO2 fluctuate slightly with the change of the seasons;concentrations of carbon dioxide fall during the northern spring and summer as plants consume the gas, and rise during the northern autumn and winter as plants go dormant, die and decay.

Many scientists and climate organisations have stated that 350ppm is the safe upper limit for carbon dioxide. The organization 350.org promotes 350 PPM as the number humanity needs to get back to as soon as possible to avoid runaway climate change.

The percentage of carbon dioxide (or CO2) in the atmosphere is about 0.0387%, or 0.04% when rounded off. Though it is important to note that carbon dioxide can concentrate itself in certain areas--such as hot springs and urban areas, where it is produced heavily by cars and factories, and there are less plants to turn it back into oxygen. In such areas, concentrations of this gas can reach up to 10 times the normal amount. This 0.04% doesn't sound like much, until you compare it to statistics from before/during the beginning of the industrial revolution that begun in the late 1700s and early 1800s. Since then, the amount of C02 in the air has risen by about 35% to 36%, depending on what sources you quote.

It also might be important to note that CO2 isn't ALL bad! Plants use carbon dioxide in photosynthesis, making the gas necessary to their survival and longevity. In turn, they switch the CO2 back into oxygen to help us breathe.