Non-flammable, colourless and odourless, acidic gas (also carbon dioxide) with the molecular formula CO2. Colloquially, carbon dioxide is very often mistakenly called carbonic acid. The gas is produced during all combustion processes and also during the respiration of animal and human beings. It also occurs in volcanic rocks and at great depths of the earth. In the atmosphere it makes up only a small proportion of 0.039 percent by volume, but this tiny amount is essential for all life, because plants need it in photosynthesis, which is how oxygen is produced. Since industrialisation, this proportion has increased by around 25% due to the so-called greenhouse effect, which is directly related to climate change. The proportion is larger (probably due to the larger land mass) in the northern hemisphere.