Suess effect

The Suess effect is a change in the ratio of the atmospheric concentrations of heavy isotopes of carbon (¹³C and ¹⁴C) by the admixture of large amounts of fossil-fuel derived CO₂, which contains no ¹⁴CO₂ and is depleted in ¹³CO₂ relative to CO₂ in the atmosphere and carbon in the upper ocean and the terrestrial biosphere .^[1] It was discovered by and is named for the Austrian chemist Hans Suess,^[2] who noted the influence of this effect on the accuracy of radiocarbon dating. More recently, the Suess effect has been used in studies of climate change. The term originally referred only to dilution of atmospheric ¹⁴CO₂ relative to ¹²CO₂. The concept was later extended to dilution of ¹³CO₂ and to other reservoirs of carbon such as the oceans and soils, again relative to ¹²C.^[3]

Although the ratio of atmospheric ¹⁴CO₂ to ¹²CO₂ decreased over the industrial era (prior to atmospheric testing of nuclear weapons, commencing about 1950), because of the increase, due to fossil fuel emissions, in the amount of atmospheric CO₂ over this period, roughly 1850 to 1950, the amount of atmospheric ¹⁴CO₂ actually increased over this period. ^[4]

Carbon has three naturally occurring isotopes. About 99% of carbon on Earth is carbon-12 (¹²C), about 1% is carbon-13 (¹³C), and a trace amount is carbon-14 (¹⁴C). The ¹²C and ¹³C isotopes are stable, while ¹⁴C decays radioactively to nitrogen-14 (¹⁴N) with a half-life of 5730 years. ¹⁴C on Earth is produced nearly exclusively by the interaction of cosmic radiation with the upper atmosphere. A ¹⁴C atom is created when a thermal neutron displaces a proton in ¹⁴N. Minuscule amounts of ¹⁴C are produced by other radioactive processes; a large amount was produced in the atmosphere during nuclear testing before the Limited Test Ban Treaty. Natural ¹⁴C production and hence atmospheric concentration varies only slightly over time.

Plants take up ¹⁴C by fixing atmospheric carbon through photosynthesis. Animals then take ¹⁴C into their bodies when they consume plants (or consume other animals that consume plants). Thus, living plants and animals have nearly the same ratio of ¹⁴C to ¹²C as the atmospheric CO₂. Once organisms die they stop exchanging carbon with the atmosphere and thus no longer take up new ¹⁴C. This effect is the basis of radiocarbon dating, with the proviso that mass-dependent fractionation and the decrease in ¹⁴C due to radioactive decay and are accounted for.

Photosynthetically fixed carbon in terrestrial plants is depleted in ¹³C compared to atmospheric CO₂.^[5] This fractionation of carbon isotopes is caused by kinetic isotope effects and mass dependence of CO₂ diffusivity. The overall effect is slight in C4 plants but much greater in C3 plants which form the bulk of terrestrial biomass worldwide. Depletion in CAM plants vary between the values observed for C3 and C4 plants. In addition, most fossil fuels originate from C3 biological material produced tens to hundreds of millions of years ago. C4 plants did not become common until about 6 to 8 million years ago, and although CAM photosynthesis is present in modern relatives of the Lepidodendrales of the Carboniferous lowland forests, even if these plants also had CAM photosynthesis they were not a major component of the total biomass.

Fossil fuels such as coal and oil are made primarily of plant material that was deposited millions of years ago. This period of time equates to thousands of half-lives of ¹⁴C, so essentially all of the ¹⁴C in fossil fuels has decayed.^[6] Fossil fuels also are depleted in ¹³C relative to the atmosphere, because they were originally formed from living organisms. Therefore, the carbon from fossil fuels that is returned to the atmosphere through combustion is depleted in both ¹³C and ¹⁴C compared to atmospheric carbon dioxide.

• Environmental isotopes
• Bomb pulse

• Cabaneiro, A.; Fernandez, I. (October 2015). "Disclosing biome sensitivity to atmospheric changes: Stable C isotope ecophysiological dependences during photosynthetic CO₂ uptake in Maritime pine and Scots pine ecosystems from southwestern Europe". Environmental Technology & Innovation. 4: 52–61. Bibcode:2015EnvTI...4...52C. doi:10.1016/j.eti.2015.04.007. (a 25-year-long dendrochronological study (1978–2002) using stable C isotope ratio mass spectrometry in growth rings of perennial trees from the Southern Atlantic Europe that explores the Suess Effect-ecosystem relationships to examine the biome sensitivity to ¹³C-CO₂ atmospheric changes)
• Suess, H. E. (September 1955). "Radiocarbon Concentration in Modern Wood". Science. 122 (3166): 415–417. Bibcode:1955Sci...122..415S. doi:10.1126/science.122.3166.415-a. S2CID 177102578. (in Northern hemisphere)
• Lerman, J. C.; Mook, Wim; Vogel, J. C. (1970). Olsson, Ingrid U. (ed.). Radiocarbon Variations and Absolute Chronology: Proceedings of the Twelfth Nobel Symposium held at the Institute of Physics at Uppsala University. New York: Wiley. pp. 275–301. LCCN 73115769. (in the Southern Hemisphere)

• An anomalous Suess effect above Europe
• Magnitude and Origin of the Anthropogenic CO₂ Increase and ¹³C Suess Effect in the Nordic Seas Since 1981
• Are arable soils of urban areas influenced by the atmospheric Suess-Effect?
• The need to correct for the Suess effect in the application of δ¹³C in sediment of autotrophic Lake Tanganyika, as a productivity proxy in the Anthropocene