Through the decomposition of organic matter, CO₂ is released into the atmosphere as part of this natural process. At the same time, photosynthesis helps to significantly reduce atmospheric CO₂ . The solubility of CO₂ in water depends on the temperature, so the oceans both produce and reduce CO₂.
Anthropogenic (man-made) carbon dioxide sources primarily relate to the combustion processes of fossil fuels. CO₂ is an inert end product that accumulates in the atmosphere and is in constant exchange with the biosphere and the oceans. Carbon dioxide reacts with water to form carbonic acid or hydrogen carbonate, which leads to a change in the pH value, i.e. the acidity of the water. Other anthropogenic CO₂ sources that intensify the greenhouse effect include changes to land usage, e.g. the clearing of forests or draining of wetlands such as moors
Sources: IPCC 2014 Synthesis Report; IPCC 2007 Assessment Report 4