In this article I discuss three “Warnings to Humanity” about the state of the global environment, signed by global networks of scientists and published in 1992, 2017 and 2019. I place these in the context of the long practice in human culture of separating and relating different registers of time: the human time of communication and recollection, and ‘inhuman’ times such as the time of the gods, culture heroes, or latterly Earth history. I suggest that in the Anthropocene the ability of geological and meteorological tropes to control the semiotic relations between lived human time and deep, planetary time is being disrupted. I then use speech act theory to analyze how the language of the three “Warnings” works to position the scientist signatories as accredited “watchmen” monitoring the changing relations between human and Earth time, and wider humanity as exposed to knowing culpability in ongoing global environmental deterioration. I conclude by suggesting that the meshing of human and Earth time is stretching the representational capabilities of the natural sciences to breaking point, and that the environmental humanities should also play an important role.