Human sources of carbon dioxide are much smaller than natural emissions but they upset the balance in the carbon cycle that existed before the Industrial Revolution. The amount of carbon dioxide produced by natural sources is completely offset by natural carbon sinks and has been for a very long time. Before the influence of humans, carbon dioxide levels were quite steady because of this natural balance.
Over the past 400,000 years, CO₂ concentrations have varied regularly from about 180 parts per million during the deep glaciation periods to 280 parts per million during the interglacial periods (measurements taken from ice cores). However following the start of the Industrial Revolution, atmospheric CO₂ concentration has increased to 400 parts per million and continues to increase. A natural change of 100ppm normally takes 5,000 to 20,000 years. The recent increase of 100ppm has taken just 120 years. This increase is having a significant effect on our planet.
The Royal Society said that the 400 ppm milestone should “jolt governments into action”. The National Geographic noted that the concentration of carbon dioxide in the atmosphere is this high “for the first time in 55 years of measurement – and probably more than 3 million years of Earth history”.
But consider what happens when more CO₂ is released from outside of the natural carbon cycle . Although our output of 38.2 gigatons of CO₂ is tiny compared to the 770 gigatons moving through the carbon cycle each year, it adds up because the land and ocean cannot absorb all of the extra CO₂. About 40% of this additional CO₂ is absorbed. The rest remains in the atmosphere, and as a consequence, atmospheric CO₂ is at its highest level in millions of years.