dioxide. HFC's and PFC's are not known exactly how much heat they absorb, but it is
             known that they absorb the most heat.
             (http://www.epa.gov/globalwarming/emissions/index.html)
             Carbon dioxide is the most important of the greenhouse gases released by human
             activities. It is the main contributor to global warming because of the quantities released.
             Carbon dioxide levels in the atmosphere have increased 25 per cent since 1850 from 280
             parts per million to 350 parts per million today. The rate is increasing by 2 or 3 per cent
             per decade and accelerating. Consequently, if current predictions are accurate, carbon
             dioxide concentrations in the atmosphere could increase to 970 parts per million by the
             year 2100. Carbon dioxide is released into the atmosphere through many natural sources.
             ...