There are two major sources of 14C in the atmosphere:
- solar radiation
- affected by sun spots (small magnitude, decadal time scales)
- earth's magnetic field can deflect solar radiation (effect has a larger magnitude and occurs on millennial time scale)
- nuclear weapons tests (large magnitude effect, can change quickly, affected by technology and politics)
There are also sources of CO2 that is depleted in 14C relative to the atmosphere. Atmospheric 14C can be reduced by the release of low 14C carbon, because this input dilutes the atmospheric signature. Because 14C has a half life of 5000 years, only 0.5^(age_x/5000) times the initial 14C will remain at age_x.
CO2 that is depleted in 14C comes from:
- The burning of fossil fuels is the most important source of 14C depleted CO2
- Historically, increased soil temperatures and reduced glacial coverage have released depleted CO2 from soils
- The circulation of the ocean brings 'old' CO2 from the deep ocean to the surface, where it exchanges with 'new' CO2 in the atmosphere.
There are two major sinks of CO2 from the atmosphere, these sinks remove 14C CO2 in proportion to its atmospheric concentration:
- photosynthesis (plant and algae), some of which is buried or sink in the ocean
- being dissolved in the ocean
Finally, the average time that a molecule of CO2 remains in the atmosphere is between 5 and 200 years (IPCC 2007 table 2.14), so after an anomalous change, the concentration of 14C in the atmosphere will return to equilibrium with sources and sinks over time. This can be seen in first figure below.
In the figures below, the y axis is Δ14C. Δ14C is the per-mil (parts per thousand) deviation in the 14C content of CO2 relative to the pre-weapons testing atmosphere (c. 1950).
Further introduction and technical references can be found at Dr. Fiona Petchy's website, references with a global change emphasis can be found at the Radiocarbon in Earth System Science Short Course website.
[14Catm] has been declining since the bomb spike
14C increased sharply (first figure) from the 1940s through the early 1960's due to the peak in nuclear tests. Since the atomic weapons ban treaty, atmospheric 14C levels have been steadily declining, but the rate of this decline has been slowing as atmospheric 14C reaches equilibrium with other sources and sinks. This is primarily the result of:
- the uptake of CO2 by the biosphere (plants, plankton)
- exchange with older CO2 in the ocean and
- the release of CO2 from fossil fuels into the atmosphere (radiocarbon depleted in fossil fuels).
A classic paper on this is Nydal and Lövseth (1983), who report records of 14C in the atmosphere - observed at a range of sites from 78N to 21S between 1960 to 1981. They demonstrate that the shape of the 14C 'bomb spike' is consistent across latitudes as well as the expected effects of atmospheric mixing between hemispheres and between the free atmosphere and the ocean surface.
Measurements at the ocean surface are relatively lower because this is where CO2 from the atmosphere is being exchanged with CO2 from the ocean, which much older and more depleted on average.
[14C_atm] continues to decline
Plenty of recent studies demonstrate that the decline continues to present. For example, Levin and Kromer (2004) provide a more recent analysis demonstrating that this trend continued into the 2000s:
The looooong (50ky) view
14C has been declining over longer time scales as well, as explained by Hughen et al 2004, resulting from a combination of changes in the earth's magnetic field, the release of 14C depleted CO2 from the deep ocean, and the coupling of atmospheric 14C-CO2 uptake by plants and old C release from soils that occurred during glacial retreat from 12ky ago to present.
For consistency with the above figures (keeping time from left to right), the direction of the x-axis in the figure below (years before present) is reversed. Original data available from NOAA. You can see the decline in 14C_atm that has accompanied the melting of the ice sheets (the last glacial maximum occurred approximately 20-25kya).
Hughen, K.; Lehman, S.J.; Southon, J.R.; Overpeck, J.T.; Marchal, O.; Herring, C.; Turnbull, J. 2004. 14C Activity and Global Carbon Cycle Changes over the Past 50,000 Years. Science 303(5655)
Nydal, R., and K. Lövseth (1983), Tracing Bomb 14C in the Atmosphere 1962–1980, J. Geophys. Res., 88(C6), 3621–3642, doi:10.1029/JC088iC06p03621.
Levin, I., B. Kromer, H. Schoch-Fischer, M. Bruns, M. Münnich, D. Berdau, J.C. Vogel, and K.O. Münnich, 1994. δ14CO2 record from Vermunt. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.
Levin, I., Kromer, B., 2004. The tropospheric and delta 14C level in mid-latitudes of the Northern Hemisphere (1959–2003). Radiocarbon 46, 1261–1272