Abstract: Climate change and the Covid-19 pandemic are global challenges in which scientists play a crucial role, and immediate political actions are necessary. However, in contrast to climate change, strong governmental actions have been taken during the pandemic. While climate change has been on the public agenda for several decades, the pandemic is a rather new issue. In such cases, social media offer scientists the potential to disseminate scientific results to the public and express calls to action and their personal views towards politics. Thus far, little is known about the extent to which scientists make use of this option. In this study, we investigated the similarities and differences between visible German climate experts and visible German Covid-19 experts regarding advocacy and assessments of policies and political actors on Twitter. We conducted a manual content analysis of tweets (N = 5,915) from 2021 of the most visible climate experts (N = 5) and the most visible Covid-19 experts (N = 5). The results show that climate experts addressed politics more often than Covid-19 experts in their tweets. The selected climate experts more often expressed negative evaluations, the degradation of competence and blaming. The Covid-19 experts, however, made more political calls for action. We assume that an issue’s history and context will affect scientists’ public assessments of politics. Our comparative study provides insight into the interrelations between science and politics in digital communication environments and elucidates visible scientists’ communication behaviours towards different socio-scientific issues.