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. It was discovered by and is named for the Austrian chemist Hans Suess, 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.

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.

Carbon isotopes
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. 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. 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.