Earth’s atmosphere protects the biosphere from nearby supernovae

Supernovae (SN), the explosive deaths of massive stars, are the primary source of galactic cosmic rays (GCRs), including high-energy protons and ions, and can generate intense gamma-ray bursts. The potential for nearby supernovae to cause mass extinctions has been a topic of considerable debate. Concerns center on the possibility of significant stratospheric ozone depletion leading to increased ultraviolet (UV) radiation exposure, and a rise in aerosol particles and cloud cover, resulting in global cooling. Previous studies, using lower-dimensional models, yielded varying results regarding the severity of these impacts. Some suggested that ozone depletion from a nearby SN could be catastrophic, while others argued the effects would be less severe. The current study aims to provide a more comprehensive assessment using advanced Earth system models (ESMs) that account for complex atmospheric dynamics, chemistry, and feedback processes, resolving discrepancies in previous research and offering a more accurate picture of the Earth's resilience to nearby supernova events. The hypothesis is that Earth’s atmospheric and biogeochemical processes possess sufficient buffering capacity to mitigate the adverse effects of even relatively close supernova events.

Early research hypothesized that increased cosmic radiation from nearby supernovae could severely deplete Earth's ozone layer, potentially leading to catastrophic consequences. Subsequent studies using two-dimensional atmospheric models corroborated this idea but also suggested a less severe impact than initially predicted. Threshold distances for catastrophic effects were estimated, with some studies suggesting a supernova needed to be within 8 parsecs to pose a significant threat. However, there is no geological evidence of such a close supernova. The relationship between GCRs and cloud formation is another area of active research. It has been proposed that increased GCRs could increase cloud condensation nuclei (CCN) concentrations, influencing climate and potentially marine biodiversity. The existing literature highlights significant uncertainties and inconsistencies, primarily due to limitations in the modeling approaches used. This current study aims to address these limitations by employing more sophisticated and comprehensive ESMs.

This study utilizes the Earth system model with Atmospheric Chemistry (EACM) model, incorporating the aerosol process from the CLOUD experiment, to simulate the impact of nearby supernovae on Earth's atmosphere and climate. The model incorporates detailed atmospheric circulation dynamics, chemistry, and feedback processes to assess stratospheric ozone loss in response to elevated ionization. A 100-fold increase in GCR intensity, representing a nearby supernova scenario, was simulated. The model includes processes such as ion-induced nucleation and particle growth to CCN. Simulations were conducted under present-day atmospheric conditions, as well as for a low-oxygen atmosphere (2%), reflecting conditions during the early Cambrian period, to evaluate the potential effects on early life. The model also incorporates the Multicomponent Aerosol model for neutral and charged particles (ion-UHMA) to simulate aerosol dynamics, including condensation, coagulation, deposition, and ion dynamics. Two sets of simulations were performed, one with high and one with low ion-pair production rates to account for varying sulfuric acid concentrations. The model considers heliospheric modulation and geomagnetic field coefficients to account for variations in cosmic ray influx. The simulations assess changes in atmospheric composition (NOx, HOx, O3), aerosol concentrations (CCN), cloud cover, radiative forcing, and ozone column thickness. Specific submodels within the broader ECHAM/MESSy framework were employed for aerosol nucleation, polar stratospheric cloud formation, and the computation of the geomagnetic cut-off rigidity.

The study's key findings demonstrate that the Earth's atmosphere is remarkably resilient to the effects of nearby supernovae. Despite a simulated 100-fold increase in GCR intensity, the impact on stratospheric ozone is considerably less severe than previously predicted. While the model reveals a maximum ozone depletion of approximately 10% globally, with the most pronounced effects occurring in the polar regions, this is comparable to the ozone loss caused by current anthropogenic emissions. The increase in ozone in the tropics is also within the range observed due to recent anthropogenic pollution. Analysis of low-oxygen atmosphere conditions during the early Cambrian also showed that even with only 2% oxygen, the ozone layer maintained sufficient thickness to absorb short-wave UV radiation, suggesting that life on land would still be protected. The increase in aerosol and cloud cover due to increased cosmic rays resulted in radiative forcing comparable in magnitude to current anthropogenic forcing but with an opposite sign, indicating a potential moderating effect on climate change. The model also reveals that the increase in CCN concentration is non-linear and affects buffering for new particles, implying that the overall increase in clouds is relatively modest (2-5%). The increase in cosmic rays is localized largely to the polar regions due to the geomagnetic field's shielding effect. Ion lifetimes are significantly shorter than the time needed for ion-induced nucleation of sulfuric acid, limiting the formation of new particles. Although a 100-fold increase in cosmic rays is simulated, the actual increase in surface level radiation was only a few percent, with the highest increase found in the polar regions. Despite elevated radiation, the effects on the biosphere were minimal, particularly at lower and middle latitudes.

The findings directly address the research question by demonstrating that the Earth's atmosphere is robust against the atmospheric impacts of nearby supernovae. The relatively minor ozone depletion and the compensating effects of chemical cycles underscore the atmosphere's resilience. The comparable radiative forcing from increased aerosols and clouds, but in the opposite direction to current anthropogenic effects, suggests a potentially mitigating factor. The results suggest that while a nearby supernova might cause localized stress, it's unlikely to cause a global catastrophic event. The findings challenge previous estimations of the threat posed by supernovae, refining our understanding of the Earth's response to extreme cosmic events. This research significantly advances our understanding of the interplay between cosmic radiation and Earth's climate system, offering valuable insights into the resilience of the biosphere. The relatively modest impact contrasts sharply with earlier predictions of catastrophic consequences, necessitating a reassessment of the long-term risk posed by these events. The comprehensive nature of this model enables the integration of various atmospheric processes, significantly improving the accuracy of predictions.

This study, using a sophisticated Earth system model, provides strong evidence that the Earth's atmosphere effectively protects the biosphere from the potentially harmful effects of nearby supernovae. Even a 100-fold increase in galactic cosmic rays, a significant increase compared to pre-industrial levels, resulted in only modest ozone depletion and a minor impact on global climate. Future research should explore potential direct health effects on organisms, from elevated radiation exposures resulting from supernovae, focusing on the effects on diverse species and ecosystems. While the focus here was on atmospheric effects, other areas such as biogeochemical cycling and direct radiation effects also warrant further study. Further refinements to the model could explore potential synergistic interactions between supernova effects and other environmental stressors.

The study's simulations, while comprehensive, involve certain simplifying assumptions, particularly regarding the low-oxygen atmosphere used to represent the early Cambrian period. This simplified representation might not fully capture the complexity of atmospheric conditions during that era. Additionally, the model's ability to predict the exact level of GCR increase from a supernova is limited, as the exact mechanism and the intensity of cosmic ray increase is highly variable depending on the properties of the supernova. While the model accounts for many processes, the interactions of other environmental factors, such as volcanic eruptions, may also affect the outcomes. The study focuses primarily on atmospheric effects; direct biological consequences of radiation exposure are not fully investigated. The study does not address the direct health risks to humans and animals resulting from exposure to elevated radiation levels.