The largest solar flare ever recorded...

A peak in atmospheric radiocarbon measured in tree rings reveals the largest solar flare ever known, 14300 years ago

An article published in Philosophical Transactions of the Royal Society A from 9 October 2023

• An unprecedented atmospheric radiocarbon peak has been measured in the rings of subfossil trees discovered in the French Alps (Gap region) and dated to 14,300 years ago.
• This sudden increase in radiocarbon in the atmosphere was probably caused by an exceptional solar flare, the largest ever identified.
• Similar solar eruptions would be catastrophic for modern society, wiping out satellite systems and power grids, destroying telecommunications and causing damage at exorbitant cost.
• For the future, it is crucial to understand these solar phenomena so that we can be better prepared and try to mitigate their effects on our societies, which are dependent on advanced technological infrastructures.

Variations in solar activity can have an impact on the Earth's climate, but they can also give rise to serious technological risks. By releasing large quantities of particles and electromagnetic waves, solar flares can destroy electricity and telecommunications networks, including GPS navigation satellites, bringing air, sea and land transport to a standstill.

Systematic measurements of solar activity began in the early 17th century, with the observation of sunspots using astronomical telescopes. From the 19th century onwards, astronomers showed that solar activity fluctuates according to 11-year cycles, and that its intensity can fall for several decades at a time, such as the Maunder Minimum (1645-1715). Over the last century, sunspot records have been supplemented by other data from ground-based observatories, space probes and satellites. However, these short-term instrumental records are insufficient to document and understand the Sun's behaviour and predict its future activity. Cosmogenic isotopes such as carbon-14 (¹⁴C) in tree rings and beryllium-10 (¹⁰Be) in polar ice are produced in the upper atmosphere by cosmic radiation modulated by solar activity. These isotopes are in fact the best indicators for reconstructing solar activity before the period of instrumental measurements.

This new study, involving three research teams, is based on an analysis of the content of ¹⁴C of subfossil trees discovered in the southern French Alps. For the past 25 years, the IMBE has been carrying out regular field campaigns along the rivers and streams of the Gap region and has assembled an exceptional collection of perfectly preserved subfossil trunks in Aix en Provence. Their dendrochronological study has made it possible to build floating chronologies covering the last 15,000 years. The Drouzet torrent chronology covers the period 14400-13700 cal BP (calendar years before the present, defined as 1950 AD). Measuring the ¹⁴C of the annual growth rings in this chronology was carried out using the AixMICADAS spectrometer in CEREGE's radiocarbon unit. This unit, inaugurated in 2016, produces accurate and reliable results, which have been confirmed in 2020 and 2023 as part of specific international intercomparisons on wood dating.

Researchers discovered a peak in ¹⁴C, which took place in a single year between 14300 and 14299 cal BP. By comparing these results with the ¹⁰Be anomaly in Greenland ice cores thanks to carbon cycle modelling calculations carried out at CEREGE and a sophisticated statistical analysis carried out at the University of Leeds in the United Kingdom, the researchers attribute this anomaly of ¹⁴C to a solar flare on an exceptional scale, the largest ever recorded. In addition, another anomaly with a longer duration was detected between 14000 and 13900 cal. years BP. It can be attributed to a Maunder-type solar phenomenon linked to the modulation of galactic cosmic particles by the heliomagnetic field.

Previous research had detected nine anomalies in the ¹⁴C, probably linked to major solar eruptions over the last 15,000 years. The most important of these date from 774 BC, 660 BC, 5259 BC and 7176 BC. The solar event of 14300 cal BP discovered by this new study is much more intense. It is 10 to 100 times more intense than the solar storms known for the instrumental period, such as the famous Carrington event in 1859, which caused major disruptions to telegraph networks and northern lights as far away as the latitudes close to the equator, or the March 1989 event that plunged the province of Quebec in Canada into chaos for several hours.

This discovery opens up new perspectives for understanding the history of solar activity and its impact on the Earth. It also illustrates the crucial role played by natural archives such as tree rings in modern scientific research. The implications of these results for our understanding of climate and our ability to predict future solar events are fascinating and are arousing growing interest in the scientific community and the general public.

Radiocarbon research at CEREGE in Aix-en-Provence (UMR Aix-Marseille Université, CNRS, IRD, INRAE, Collège de France) is supported by the Collège de France and the French National Research Agency (EQUIPEX ASTER-CEREGE, ANR CARBOTRYDH and MARCARA projects).

To find out more (free access):

Bard E, Miramont C, Capano M, Guibal F, Marschal C, Rostek F, Tuna T, Fagault Y, Heaton TJ. A radiocarbon spike at 14,300 cal yr BP in subfossil trees provides the impulse response function of the global carbon cycle during the Late Glacial. Philosophical Transactions of the Royal Society A, 381: 20220206 (2023). https://doi.org/10.1098/rsta.2022.0206

Contacts : Edouard Bard, Collège de France and CEREGE, Aix-en-Provence
Cécile Miramont, University of Aix-Marseille and IMBE, Aix-en-Provence
Timothy Heaton, University of Leeds, United Kingdom

Press review:

. Le Monde, 16 Oct.2023
. Washington Post, 11 Oct. 2023
. Phys.org
. REUTERS
. Scientific American, 19 Oct. 2023