The amount of CO₂ released from combustion of fossil fuels, land use changes (i.e. deforestation, agriculture), and concrete production, as opposed to natural ocean and land CO₂ sinks, is fairly well known and the atmospheric CO₂ increase is in quantitative agreement with the anthropogenic sources minus natural sinks (Le Quéré et al., 2016). On the short time scale since atmospheric CO₂ concentrations started increasing beyond the typical Pleistocene CO₂ values known from ice cores (i.e. 280 ppm, Bereiter et al., 2015), only major and extended volcanic eruptions could release a similar amount of CO₂, but no eruptions have occurred that could explain the magnitude and continuous rise of atmospheric CO₂.

In addition to these quantitative assessments, atmospheric CO₂ has an unusual geochemical signature that links it to an organic carbon source. Specifically, plants and algae prefer the lighter carbon isotope ¹²C over the heavier isotopes ¹³C and ¹⁴C, and plant material ends up being isotopically lighter (i.e. enriched in ¹²C over ¹³C and ¹⁴C; note that ¹⁴C is radioactive and carbon molecules ~50,000 years of age are radiocarbon-dead, i.e. they no longer contain any ¹⁴C). Because trees and algae form the basic food source for animals and bacteria, and the organic matter of plants, algae and other organisms accumulates in the geological record to form coal, oil and gas, the isotopic signature of fossil fuels is also relatively poor in ¹³C and devoid of ¹⁴C, but rich in ¹²C. Burning ¹²C-rich fossil fuels therefore adds a disproportionate amount of ¹²C to the atmosphere, and this signal can be traced instrumentally today, but also in ancient air trapped in ice cores and in the carbon isotopic composition of marine and terrestrial fossils. This so-called Suess effect (Keeling, 1979) shows that increasing amounts of fossil fuel burning since the industrial revolution have contributed more and more ¹²C to atmospheric CO₂ and the isotopic composition of atmospheric CO₂ (i.e. δ¹³C_CO2) thereby documents the organic carbon origin of the ongoing CO₂ increase. Similarly, the oxygen concentration in the atmosphere decreases as more and more (fossil) organic carbon is burned (i.e. oxidized), confirming that CO₂ is being emitted by oxidation of reduced (i.e. organic) carbon. Finally, there is also a strong correlation between rising CO₂ concentrations and the world population since at least the Industrial Revolution (i.e. since the 1850s), supporting the causal relationship determined isotopically and quantitatively from observations of natural and anthropogenic CO₂ sources and sinks described above.

Figure caption: Atmospheric CO₂ and world population are tightly correlated – more people on the planet emit progressively more CO₂. Data points represent individual years in sequential order, with the first (lowest) presenting the year 1800 and the last (highest) presenting 2018. Data sources are: world population and mean annual CO₂ from 1959-2018, and Bereiter et al. (ice core CO₂ data highlighted with turquoise box for the years 1800-1958, 2015).

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