Solar irradiance changes and photobiological effects at Earth's surface following astrophysical ionizing radiation events

TL;DR

This study uses advanced radiative transfer modeling to refine estimates of surface-level UV and visible light changes after astrophysical ionizing radiation events, revealing lower UVB increases and a net rise in visible light, impacting biosphere and climate assessments.

Contribution

The paper improves upon previous models by providing more accurate calculations of irradiance and biological effects following ionizing radiation events, considering a wider range of organisms and conditions.

Findings

Past estimates of UVB irradiance were overestimated.

Biologically damaging radiation varies significantly among organisms.

Most regions experience a net increase in visible light after events.

Abstract

Astrophysical ionizing radiation events have been recognized as a potential threat to life on Earth, primarily through depletion of stratospheric ozone and subsequent increase in surface-level solar ultraviolet radiation. Simulations of the atmospheric effects of a variety of events (such as supernovae, gamma-ray bursts, and solar proton events) have been previously published, along with estimates of biological damage at Earth's surface. In this work, we employed the TUV radiative transfer model to expand and improve calculations of surface-level irradiance and biological impacts following an ionizing radiation event. We considered changes in surface-level UVB, UVA, and photosynthetically active radiation (visible light) for clear-sky conditions and fixed aerosol parameter values. We also considered a wide range of biological effects on organisms ranging from humans to phytoplankton. We found that past work overestimated UVB irradiance, but that relative estimates for increase in exposure to DNA damaging radiation are still similar to our improved calculations. We also found that the intensity of biologically damaging radiation varies widely with organism and specific impact considered; these results have implications for biosphere-level damage following astrophysical ionizing radiation events. When considering changes in surface-level visible light irradiance, we found that, contrary to previous assumptions, a decrease in irradiance is only present for a short time in very limited geographical areas; instead we found a net increase for most of the modeled time-space region. This result has implications for proposed climate changes associated with ionizing radiation events.