Previous studies have shed light on the importance of affect in risk perception and the role of mental imagery in generating affect. In the current study, we explore the causal relationship between mental imagery, affect, and risk perception by systematically varying the level of mental imagery in three levels (i.e., enhanced, spontaneous, or prevented). In light of the increasing environmental risk of adverse events caused by climate change, we operationalize risk as participants' perceived risk of climate change. One-thousand-fifty-five participants were recruited online and randomized to one of three levels of mental imagery. As predicted, we found a causal link between the level of mental imagery, affective experience, and perceived risk of climate change, in that enhanced mental imagery caused a larger decrease in positive affective valence and a larger increase in perceived risk of climate change. We argue that mental imagery enhances the negative affect associated with the risk event by creating a perceptual experience that mimics seeing the environmental risk events.