Where does carbon dioxide come from?

Carbon dioxide (CO₂) comes from both natural sources (including volcanoes, the breath of animals and plant decay) and human sources (primarily the burning of fossils fuels like coal, oil and natural gas to generate energy). Human activities have been the main cause of rising carbon dioxide levels in our atmosphere since the 1800s.

The amount of carbon dioxide in the atmosphere is determined by the carbon cycle - a system of 'sources' and 'sinks' of the gas that add and remove it, respectively. One part of the cycle involves rocks, starting with volcanoes, which belch CO₂. This is countered by 'weathering', a process where atmospheric CO₂ mixes with rainwater to make an acid that reacts with rocks, locking the CO₂ away.

The emergence of life on our planet added a new layer to the carbon cycle. As plants grow, they take CO₂ out of the atmosphere, and when they die, it is released again. Animals that consume the plants also store the CO₂ for a while, before they too die and decompose.

Some dead plants don't decompose and instead become layers of coal, oil and other organic-rich sediments such as peat. Eventually, these layers would naturally burn or be recycled through volcanoes, returning the CO₂ to the atmosphere over many thousands (if not millions) of years.

However, humans have been digging up these layers and burning them at a rate the planet has never seen before, releasing vast amounts of CO₂ in a geological blink of an eye. Estimates show that by burning these fossil fuels, humans have essentially taken millions of years of carbon uptake by plants and returned it to the atmosphere in less than 300 years.

Natural sinks of carbon are unable to keep up with this rate of change. This causes CO₂ to build up in the atmosphere, which rose from a concentration of around 280 parts per million (ppm) in 1750 to more than 415ppm in 2021.