The communication of anthropogenic climate change presents many challenges, for communicators, scientists, policymakers and publics alike. Particularly difficult is the issue of uncertainty, which can include ambiguity around the phenomenon of climate change, the possible impacts of this, and the timeframe within which such impacts will be seen. Previous research has established that audiences are often averse to uncertainty, and will disregard or ignore messages that contain it. This raises a theoretical and practical question of how best to manage uncertainty in climate change communication in order to maintain audience engagement. This question was the focus of this PhD research. Specifically, the aim of this thesis was to explore the process of climate change communication from the perspectives of the scientists, communicators, and the recipient. I achieved this research goal by utilising a mixed methods design. I firstly interviewed the originators (i.e., scientists) and professional communicators of climate change information to explore the process from their side (Chapter 2). This revealed a number of themes connecting to the different ways scientists and communicators understand the process of communication (e.g., as information exchange versus relationship building), the challenges of climate change communication and uncertainty in particular, and the (appropriate) role of scientists when communicating with the public about climate change. Next, in a series of studies I experimentally explored how audiences respond to variations in the informational content of climate change messages (such as the level of uncertainty) and the role of different communicative styles in further shaping audience engagement (Chapter 3). Broadly, the results of these studies suggest that while uncertainty can undermine audience engagement with climate change communications, the negative effects of uncertainty are buffered when the communicator is perceived to be high in morality and/ or when they use an open communication style. Interestingly, these effects of communication style were particularly evident among women, whereas men tended to react against this. Together, these studies show how relational factors (e.g., communication styles and perceptions of communication sources) can moderate the impact of informational content on audience responses. Finally, I ended this programme of research by looking in more detail at how audiences perceive a real scientific organisation engaged in climate change communication and the bases of their beliefs about organisation competence and morality (Chapter 4). This study combined qualitative and quantitative data to delve deeper into some of the insights gained in the experimental work, and to reconnect this to the real-world organisation context I began with. This study again showed how perceptions of communicator morality moderate responses to uncertainty, but also provide useful insights into the different origins of perceptions of morality and competence. Chapter 5 concludes by summarising the research presented in this thesis, discussing its strengths, limitations and ways forward. Here, I also consider the theoretical, methodological and practical implications of the thesis’ research findings. Briefly, it is argued that addressing the scientific uncertainties of climate change may not necessarily mean altering the form of information itself. Rather, modifying the language peripheral to the information that contains uncertainty, attending to the ways in which audiences perceive the sources of uncertainty, and considering variations amongst publics, may help to engage in effective communication around the complex issue of climate change.

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Alors que les principales activités d’un chef de projet s’effectuent à travers le processus de communication, on observe que de nombreux projets font l'objet de retards, dérives ou défauts de spécifications. Excès de mesures pour prévenir le risque ou gestion déficiente de la communication laissant trop de place à l’incertitude ? La Théorie de la Réduction de l’Incertitude développée par Berger et Calabrese (1975) dans le champ de la communication ne permet pas de totalement saisir comment un chef de projet dissipe l’incertitude existante entre les acteurs. En revisitant un projet opérationnel dans le cadre d’une recherche-action, nous nous employons à identifier comment incertitude et communication s’influencent et se structurent mutuellement. Nous avons convoqué l’approche constructiviste et la théorie de l’acteur-réseau de Callon et Latour pour accéder au sens de cette relation circulaire. Nous avançons les hypothèses selon lesquelles : - L’incertitude est un actant qui intervient dans la construction du réseau (au sens de Bruno Latour (2007) « ce qui est tracé par les traductions »).- Le processus de communication diffère suivant la nature de l’incertitude rencontrée ou ressentie. - Le processus de communication performe et scelle les relations en les rendant si coûteuses à défaire et si économiques à maintenir, qu’elles deviennent irréversibles. L’approche communicationnelle de l’incertitude mettra en évidence plusieurs caractéristiques de cet actant, comme sa capacité à peupler un « réseau de manques », la façon dont le réseau se hiérarchise pour faire sens, la description du processus d’estimation continue dont il fait l’objet (l’Incertus). Si nous concevons l’incertitude comme l’attribut d’un phénomène alors la « communication-incertitude » fabrique le sens en même temps qu’elle détermine la valeur de cet attribut. En positionnant l’incertitude comme un phénomène socialement construit, nous présentons un modèle constructiviste de « communication-incertitude » où l’observateur est un acteur intentionnel limité par des contraintes (Boudon, 2009) et proposons de distinguer la nature de l’incertitude suivant une typologie : l’incertitude de variabilité (inhérente à la variabilité des choses), l’incertitude épistémique ambiguë ou non (due à l’imperfection de nos connaissances) et l’incertitude d'échelle (en rapport avec l’imperfection de nos modèles de représentations). Dans ce mouvement vers l’irréversibilité, les processus de communication participent au remplacement des médiateurs (qui transforment, redonnent du sens, font faire des choses inattendues) par des intermédiaires (qui transmettent, transfèrent sans modifier) et les actants les plus réversibles sont évacués vers la périphérie du réseau. / While the main activities of a project manager are done through the communication process, it is observed that many projects are subject to delays, excesses or defects specifications. Excess of measurements to prevent the risk ? Defective management of the communication which leaves too much place to uncertainty ? The Theory of Uncertainty Reduction developed by Berger and Calabrese (1975) in the field of communication does not fully understand how a project dissipates the existing uncertainty between actors. By revisiting an operational project within the framework of action research, we strive to identify how uncertainty and communication influence and form themselves mutually. We used the constructivist approach and the actor-network theory of Callon and Latour to reach the meaning of this circular relationship. We advance the following hypotheses: - Uncertainty is a nonhuman actor involved in the construction of the network (as defined by Bruno Latour (2007) "which is drawn by translations"). - The communication process differs according to the nature of the uncertainty encountered or felt. - The communication process performs and seals relationships by making them so expensive to undo and so economic to maintain, that they become irreversible. The communicational approach of the uncertainty will highlight several features of this nonhuman actor, as its ability to populate a "network of gaps", the way the network ranks into a hierarchy to make sense, the description of the continuous process of estimation (the Incertus). If we conceive uncertainty as an attribute of a phenomenon, then "communication-uncertainty" makes sense at the same time it determines the value of this attribute. By positioning uncertainty as a socially constructed phenomenon, we present a constructivist model of "communication-uncertainty" where the observer is an intentional actor limited by constraints (Boudon, 2009). We propose to distinguish the nature of uncertainty following a typology: the variability uncertainty (inherent variability of things), the epistemic uncertainty ambiguous or not (due to the imperfection of our knowledge) and the scale uncertainty (in touch with the imperfection of our models of representations). In this movement towards irreversibility, the communication processes involved in replacing mediators (which transform, give meaning, make unexpected things do) by intermediaries (which transmit, transfer without changing) and the most reversible nonhumans actors are evacuated to the network edge.