The concept of ‘the Anthropocene’ as a new human-induced geological epoch has made its way into IR. Debates have recently arisen between ‘post-humanists’ stressing its destruction of subject-object binaries and ‘New Anthropocentrists’ arguing that it increases the importance of the human being as planetary steward. This article moves beyond these debates to question a strange but unexplored foundation that underlies the basic discourse of the Anthropocene: the assertion that humanity must be grouped together as a collective species, ‘anthropos’, or planetary ‘We’. Using the philosophy of Martin Heidegger, it argues that the Anthropocene reveals a new and deeper shift in human subjectivity, moving from an individualistic Cartesian ‘I’ to a collective and planetary ‘We’. This argument is made in three steps. First, today's common treatment of humanity as a collective whole in Anthropocene literature is examined. Second, it details how transformations in subjectivity occur by shifting the historical boundaries of our most fundamental notion of certainty – the ‘subiectum’ – and how the technologies of Earth System Science (ESS) subtly facilitate this shift today. Finally, the article argues how this subjective transformation from the ‘I’ to the ‘We’ results from the temporal, spatial, and existential incalculability and uncertainty of the Anthropocene, thereby fostering the rise of certainty in new forms of conflictual identity politics.