Radiocarbon: studying past climate and predicting the future

Fanny Trifilieff: Can you explain the principle of radiocarbon (¹⁴C) dating and its links with climate?

Edouard Bard: Radiocarbon dating is the most widely used dating method for the last 50,000 years. It is based on the radioactive decay of ¹⁴C, originally produced in the upper atmosphere by interaction with cosmic ray particles. The ¹⁴C content of a fossil sample is compared with the atmospheric ¹⁴C content, which constitutes the starting reference, before its regular disappearance by radioactive decay over a period of 5,700 years. In its original form, radiocarbon dating is not accurate, as the atmospheric ¹⁴C content does not remain constant over time. This content has varied due to changes in the rate of production by cosmic rays, as well as rearrangements in the biogeochemical carbon cycle.

F.T: Where does the need to record past variations in atmospheric ¹⁴C come from?

E.B : Recording natural variations in ¹⁴C is essential to our understanding of fluctuations in the Sun's and Earth's magnetic fields, as well as the climate-related biogeochemical carbon cycle. The precise series of atmospheric ¹⁴C dating back 50,000 years enables us to study and simulate terrestrial processes, and to improve the computer models used for current climate change projections. Indeed, the climate simulations compiled by the Intergovernmental Panel on Climate Change (IPCC) rely on ¹⁴C as an indicator of solar activity and as a tracer of the global carbon cycle, as well as being the chronometer for most paleoclimate series over the past 50,000 years. ¹⁴C is therefore crucial for understanding and preparing for future climate change.¹⁴C also tells us about the occurrence of extreme solar flares in the past, with amplitudes far greater than those observed by astronomers. Such events could cause catastrophic damage to our satellites, power grids and telecommunications networks.

F.T: What are the prospects for this research at CEREGE?

E.B: The radiocarbon unit at CEREGE continues to work on new subfossil tree series from the southeastern Alps (in collaboration with IMBE) and marine archives. We are focusing on the variability of ¹⁴C at the ocean surface, which is an indicator of air-sea gas exchange and carbon mixing in the ocean. We are starting an international collaborative research project (ANR MARCARA with Bremerhaven's Alfred Wegener Institute) focusing on observation and numerical modeling for key sites in the main oceans during contrasting climate periods over the last 50,000 years, whose role on the carbon cycle is still poorly understood.

Edouard Bard also addressed this subject in a podcast for Science.