A colourless, odourless gas which is the fourth most abundant component of the air in the homosphere, amounting to about 0.035 per cent by volume. It is released by volcanoes, part of the source material being carbonate rocks, and produced by metabolic processes in animals and by the combustion of hydrocarbons. It is broken down by photochemical processes in the upper atmosphere and by photosynthesis in plants. The most important greenhouse gas, it is the major factor in global warming. It is readily soluble in water, and the oceans act as the principal reservoir (containing approximately 99 per cent of terrestrial carbon dioxide), from which it is deposited as carbonate rocks such as limestone. Its solubility is strongly dependent upon temperature, leading to considerable variation in oceanic concentration at different times and places. Its role in global warming was first suggested by Svante Arrhenius in 1896.