How and When Incivility Unfolds in Online Heatwave Communication: Characterizing Public Discussions on Reddit

The increasing frequency of heatwaves has sparked widespread concern and emotionally charged, often uncivil, discussions on social media. Drawing on heatwave communication and online incivility research, this study investigates how incivility manifests, arises, and spreads in Reddit discussions about heatwaves between 2023 and 2024. Specifically, it examines the forms of incivility expressed, contexts in which incivility is more likely to occur, and how incivility spreads across comment layers. Using natural language processing (i.e. topic modeling and emotion analysis) as well as a large language model-assisted incivility detection approach, we analyzed 1,369 posts, 10,800 first-layer comments, and 7,297 second-layer comments from eight climate- and country-related subreddits. Results revealed that incivility was more prevalent in comments than in posts, with first-layer incivility significantly predicting second-layer instances, suggesting a contagion effect that persisted across comment layers. Comments expressing anger, disgust, and sadness were more likely to be uncivil, and controversial topics like climate change attribution, geopolitical conflicts, and regional impacts correlated with higher incivility. Theoretical and practical implications were discussed.