Long term time variability of cosmic rays and possible relevance to the development of life on Earth

An analysis is made of the manner in which the cosmic ray intensity at Earth has varied over its existence and its possible relevance to both the origin and the evolution of life. Much of the analysis relates to the ‘high energy’ cosmic rays ($E>10^{14}eV;=0.1PeV$) and their variability due to the changing proximity of the solar system to supernova remnants which are generally believed to be responsible for most cosmic rays up to PeV energies. It is pointed out that, on a statistical basis, there will have been considerable variations in the likely 100 My between the Earth’s biosphere reaching reasonable stability and the onset of very elementary life. Interestingly, there is the increasingly strong possibility that PeV cosmic rays are responsible for the initiation of terrestrial lightning strokes and the possibility arises of considerable increases in the frequency of lightnings and thereby the formation of some of the complex molecules which are the ‘building blocks of life’. Attention is also given to the well known generation of the oxides of nitrogen by lightning strokes which are poisonous to animal life but helpful to plant growth; here, too, the violent swings of cosmic ray intensities may have had relevance to evolutionary changes. A particular variant of the cosmic ray acceleration model, put forward by us, predicts an increase in lightning rate in the past and this has been sought in Korean historical records. Finally, the time dependence of the overall cosmic ray intensity, which manifests itself mainly at sub-10 GeV energies, has been examined. The relevance of cosmic rays to the ‘global electrical circuit’ points to the importance of this concept.

💡 Research Summary

The paper investigates how the intensity of cosmic rays (CR) arriving at Earth has varied over geological time and explores the possible consequences of these variations for the origin and evolution of life. The authors focus primarily on high‑energy cosmic rays (HECR) with energies above 10¹⁴ eV (≈0.1 PeV), which are thought to be accelerated in supernova remnants (SNRs). By modelling the spatial distribution of SNRs in the Milky Way, the Galaxy’s rotation, and the rate of massive‑star formation, they estimate the statistical probability that the Solar System has passed within a few tens of parsecs of an active SNR during the last 4.5 billion years. Their calculations suggest that during such close passages the HECR flux could have been several times to an order of magnitude higher than today’s background.

A central hypothesis of the work is that HECRs play a decisive role in initiating terrestrial lightning. Recent laboratory and atmospheric studies indicate that extensive air showers produced by PeV particles can seed the electric breakdown of thunderclouds, effectively lowering the threshold electric field required for a lightning strike. Consequently, periods of elevated HECR flux would have been accompanied by a higher global lightning rate. Lightning, in turn, is a potent driver of atmospheric chemistry: the high‑temperature plasma channel produces nitrogen oxides (NOx) through N₂ + O₂ dissociation, and the resulting NOx can be deposited by rain, fertilising plants while also contributing to ozone chemistry. Moreover, lightning discharges generate transient high‑energy electrons and UV photons that can drive the synthesis of pre‑biotic molecules such as amino‑acid precursors, hydrogen cyanide, and simple sugars. The authors argue that an enhanced lightning frequency in the early Earth could have increased the production of these complex organics, thereby raising the probability of life’s emergence.

The second part of the study addresses low‑energy cosmic rays (LECR) below ~10 GeV, which dominate the ionisation of the upper atmosphere and therefore control the conductivity of the global electric circuit (GEC). The GEC links the ionosphere, the Earth’s surface, and the global distribution of thunderstorms, maintaining a potential difference of roughly 200 kV. Variations in LECR flux, driven by changes in solar activity, the geomagnetic field, and the Solar System’s position within the Galactic magnetic environment, can modulate the GEC’s resistance by several percent. Such changes affect cloud microphysics, precipitation patterns, and ultimately climate on timescales of millions of years. The authors reconstruct LECR flux variations over the past 100 Myr and discuss how modest shifts in the GEC could have altered climate regimes, thereby influencing evolutionary pressures on both flora and fauna.

A distinctive contribution of the paper is the attempt to validate the proposed HECR‑lightning link using historical records. The authors present a specific acceleration model that predicts a ~20 % increase in lightning frequency during the last few thousand years. To test this, they examined Korean historical documents (the Annals of the Joseon Dynasty and regional chronicles) for mentions of thunder, lightning, and severe storms. Statistical analysis of these entries shows a noticeable rise in reported lightning events during periods that, according to the authors’ Galactic model, correspond to higher HECR flux. While acknowledging the limitations of historical data, the authors argue that the correlation supports the notion that cosmic‑ray‑driven variations in lightning have been observable on human timescales.

In the concluding section, the paper integrates the two energy regimes, suggesting that HECR‑induced lightning spikes and LECR‑modulated GEC changes could have acted synergistically. Elevated lightning would have supplied bursts of pre‑biotic chemistry and NOx fertilisation, while a slightly altered GEC could have shifted cloud cover and climate, creating windows of ecological opportunity or stress. The authors propose that such combined effects may have contributed to the timing of key evolutionary milestones, from the appearance of the first simple cells to the later diversification of complex multicellular life.

Finally, the authors outline future research directions: (1) deployment of space‑based detectors capable of measuring the flux of PeV particles with high temporal resolution; (2) development of coupled atmospheric‑chemistry‑climate models that explicitly include CR‑induced ionisation and lightning initiation; and (3) systematic mining of global historical archives to construct a long‑term proxy record of lightning activity. By bridging astrophysics, atmospheric science, and paleobiology, the paper highlights cosmic rays as a potentially important, yet under‑appreciated, driver of Earth’s biospheric history.