The evidence for and against astronomical impacts on climate change and mass extinctions: A review

Abstract: Numerous studies over the past 30 years have suggested there is a causal connection between the motion of the Sun through the Galaxy and terrestrial mass extinctions or climate change. Proposed mechanisms include comet impacts (via perturbation of the Oort cloud), cosmic rays and supernovae, the effects of which are modulated by the passage of the Sun through the Galactic midplane or spiral arms. Supposed periodicities in the fossil record, impact cratering dates or climate proxies over the Phanerozoic (past 545 Myr) are frequently cited as evidence in support of these hypotheses. This remains a controversial subject, with many refutations and replies having been published. Here I review both the mechanisms and the evidence for and against the relevance of astronomical phenomena to climate change and evolution. This necessarily includes a critical assessment of time series analysis techniques and hypothesis testing. Some of the studies have suffered from flaws in methodology, in particular drawing incorrect conclusions based on ruling out a null hypothesis. I conclude that there is little evidence for intrinsic periodicities in biodiversity, impact cratering or climate on timescales of tens to hundreds of Myr. Furthermore, Galactic midplane and spiral arm crossings seem to have little or no impact on biological or climate variation above background level. (truncated)

Analysis of Astronomical Impacts on Climate and Mass Extinctions

The paper by Coryn A.L. Bailer-Jones provides a comprehensive overview of the potential connections between astronomical phenomena and terrestrial climate change or mass extinctions. This topic spans a range of mechanisms, including perturbations from celestial bodies and cosmic events, that may have influenced geological and biological records over the past 545 million years (Myr) of the Phanerozoic Eon. The paper critically evaluates various hypotheses, data sources, and statistical methods used to support or refute the existence of such connections.

The review begins with the examination of purported periodicities within geological and biological data. Despite claims over the years that certain patterns—such as those occurring on 26 Myr or 62 Myr cycles—indicate extraterrestrial causes for mass extinctions and climate shifts, Bailer-Jones concludes that there is scant evidence supporting intrinsic periodicity on these timescales.

Critique of Periodicity Claims

While some studies have suggested a 26 Myr cycle related to mass extinction events, these assertions have been criticized for methodological flaws and data selection biases. The paper argues that many studies failed to adequately account for substantial date errors, incomplete inference, and insufficient hypothesis testing. For example, statistical analyses were often unable to definitively disentangle periodic signals from noise or random variations in the data, leading to exaggerated claims.

Similarly, claims for a 62 Myr periodicity in biodiversity, identified during the period 520–150 Myr BP, hold slightly more credibility yet remain controversial. While one might theoretically associate such a cycle with the Sun's vertical oscillation through the Galactic plane, which could hypothetically modulate cosmic ray exposure and affect biodiversity, the robustness of these findings remains in question due to dating uncertainties and fossil preservation biases.

Examination of Astronomical Mechanisms

The paper critically evaluates several astronomical mechanisms that have been proposed as influencers of earthly phenomena:

1. Impacts from Celestial Bodies: The potential for Oort cloud perturbations to increase cometary impacts on Earth is explored, while noting the lack of strong evidence supporting regular, periodic impacts tied to solar motion patterns.

2. Cosmic Rays: The influence of cosmic rays as drivers of climate change, through the modulation of cloud formation, is dismissed as largely unsupported on geological timescales. Rather, shifts in cosmic ray flux are downplayed as minor compared to other climatic influences, such as CO$_2$ levels.

3. Supernovae and Gamma-Ray Bursts: While nearby supernovae or gamma ray bursts could theoretically cause biological damage through radiation, causing mass extinctions, direct evidence linking these events to documented extinction epochs remains sparse.

4. Solar Motion and Spiral Arms: Although the solar system's passage through galactic structures like spiral arms and plane crossings were hypothesized to coincide with extinction events, the paper asserts that empirical evidence is lacking. Furthermore, the dynamical nature of galactic structures complicates periodic connections due to uncertainties in their morphology and rotation speeds.

Methodological Insights

Bailer-Jones highlights fundamental issues in hypothesis testing and data analysis that undermine many past studies. Chief among these is the incorrect interpretation of $p$-values: rejecting a null hypothesis does not inherently validate an alternative hypothesis. There is a need for robust statistical frameworks that provide comparative assessments between competing models—something that orthodox hypothesis testing fails to achieve.

Given the complexities of geological data interpretation and the astronomical models proposed, future research should aim for improved methodologies and more comprehensive model comparison using Bayesian approaches, which offer clearer probabilistic interpretations that account for prior information.

Implications and Future Directions

The paper concludes that while astronomical influences on Earth's climate and biological evolution remain an intriguing possibility, suggesting a singular, periodic cause for significant extinction events or climate shifts lacks solid evidence and theoretical support. Understanding the role of astronomical phenomena will require better geological data for dating events, more precise measurements of the solar motion through the Galaxy, accurate models of spiral arms, and rigorous statistical methods—especially in context-driven comparisons for alternative hypotheses.

Upcoming astronomical missions, like Gaia, promise improvements in measuring Galactic structure and dynamics, which may resolve some uncertainties. Meanwhile, terrestrial factors such as volcanism, plate tectonics, and ecosystem dynamics should be considered alongside astronomical influences to explain historical climate changes and mass extinctions.