Colorless gas, formula CO2, molecular weight 44; the fourth most abundant gas in dry air. The end product of the combustion or oxidation of organic material, including fossil fuels, CO2 is a very strong greenhouse gas and has very important radiative effects. Carbon dioxide is soluble in water, and the atmospheric concentration is buffered by dissolution in seawater. Carbon dioxide is taken up by the biosphere during photosynthesis, and large amounts of carbon can be stored in decaying organic matter.