400ppm is a milestone conjured from much-deliberated empirical data and contested political imperatives. `Constructed’ though it may be, the figure is spun around a referential core, dare we say an essence. While it may be impossible to identify it with precision, scientists insist that there is a given greenhouse gas concentration in the Earth’s atmosphere whose exceeding will trigger an irreversible transition in the global climate system: a boundary condition as real as the onset of spring, the boiling point of a fluid, the tripping of a landslide.  As expressions of scientific authority, knowledge claims about climatic thresholds demand the scrutiny of social studies of science. As the bearing witness to coming catastrophe, they also cry out for acts of commitment and fidelity.

The passage across the 400ppm mark has met with frighteningly little fanfare, a situation that cannot be detached from a political context in which the negotiation of obligations and rights regarding climate change still pits nation against nation. In contemporary social thought, as in everyday political practice, the thresholds loaded with the most significance are those that delineate one social formation or political unit from another.  Under conditions of intensifying globalization, the idea of passing over or otherwise negotiating the boundaries between nation states has taken on such significance that it figures not only for what is of political salience but for the most dramatic forms of cultural and subjective change. In much critical and popular thought, promise lies in spatial mobility, in border crossing, in the transgression of boundaries.

In other words, the assumption is still that human subjects mobilise themselves to move across an immobile Earth. But have we been too short-sighted, too horizontal, too human-centred in our imaginings of borders? What if the most crucial borders turn out to be not so much the lines which demarcate one political unit from another across our planet’s frenetically criss-crossed surface, as those boundaries which separate one `regime’ or `state’ of the Earth system from another?  To put it another way, what if the most critical thresholds are the ones that define strata rather than those which delineate territory?

What is now at stake is not only the likelihood that human agency has nudged the Earth out of the brief interglacial known as the Holocene, but the possibility of a departure from the geoclimatic conditions constitutive of the two and half -million year-old Quaternary period. The more momentous transitions between states of the `Earth system’ leave their mark in the geologic layerings of the terrestrial crust, the evidence of significant shifts being taken by geoscientists as the boundary markers of the different geologic epochs, periods, eras and eons.

Gathering evidence of past climate change reveals how many times our ancestors and related hominids had to negotiate significant shifts in Earth systems.  Not only might the threshold transition we are barreling towards exceed even these changes, but it would occur on a planet far more densely occupied and tightly subdivided than ever before.

We face the prospect of stratal upheaval – of geo-climatic regime change – equipped with political procedures and visions that were shaped in response to territorial contests.   Such spatial imaginaries assume a largely flat Earth, a planet devoid of compositional depths or deep-temporal dynamics.  This raises the issue of what form our politics might take if it were genuinely oriented towards the potential for state shifts or regime changes in vital Earth systems.  It poses the question, in other words, of what a politics of strata might look like.

Such a politics would need to set out from the assumption that the very conditions of territorial occupation are contingent. It is not enough to say that socio-political inscriptions on the Earth’s surface are artificial and might be done differently.    There are no territories without exposed strata, no exposed strata without multiple layers of subtending strata, no subtending strata without deep, temporal dynamics.  And by the same logic, there is no social formation which is not also a geo-social formation.

Even if its own tensions and inconsistencies could be smoothed out, Kant’s dream of `communal possession of the Earth’s surface’ would fall far short of the politics needed for a stratified planet, for an Earth that destratifies on the spot. At issue are not only the temporary congealings and arrested flows of the uppermost stratum, but the essential capacity of Earth systems to reorganize themselves, with or without human intervention. Of course it is not simply a question of replacing a politics of territory with a politics of strata, but a matter of working through the manifold ways in which territorial inscriptions converge with, cut across, depend upon the dynamical properties of strata. All of which means that struggles over geo-social formations – however they unfold – are emerging as the foremost political and socio-material challenge of our times.