During the 1950s, Dave Keeling developed a set of techniques for systematically measuring atmospheric carbon dioxide. The problem was not simple. How does one create a global number, as Paul Edwards (2010) has pushed us to ask? Keeling put gas analyzers on military aircraft flying across the South Pacific, at the Little America research station in Antarctica, and most famously, at the Mauna Loa Observatory, 3600 meters above sea level in the middle of the Pacific Ocean. The problem, for Keeling, was that CO₂ varies tremendously throughout the atmosphere. Due to the effects of photosynthesis, ocean fluxes, farming, and industrial activity, any measurement of CO₂ concentration is only a local measurement. Rather than create a snapshot by averaging many different measurements all over the world (as, for instance, we do with global temperature), Keeling decided to measure as far away from anywhere as possible in order to produce a number that could speak for the planet’s atmosphere. Mauna Loa Observatory became the standard for determining global CO_2. The global number produced there became the Keeling Curve. It also became a key cultural reference: for instance, in the name of the activist group 350.org—a number that refers to the presumed safe limit of atmospheric carbon dioxide (in parts per million).

This past year, 2015, was probably the last year in which, for the indefinite future, atmospheric CO₂ will ever be below 400ppm. And so the tense of the Anthropocene may be exactly that: the indefinite future.

The geological carbon cycle is massive, many orders of magnitude larger than the paltry contribution of humans. Carbon dioxide in the air is only part of the complex of transformations that take place on timescales ranging from days to hundreds of millions of years, involving chemical forms ranging from ethereal, vibrating gas molecules to the calcium carbonate of Earth’s vast limestone reserves.

Anthropogenic carbon emissions are central to the geological claim of humans’ transformative role in planetary ecology, and therefore play an important role in the concept of the Anthropocene. Indeed, Donna Haraway (2015) has suggested that the concept of the Anthropocene fails if it delineates a previously pure nature unadulterated by human influence or if it serves to reify an undifferentiated global humanity. Yet from the vantage point of carbon, the Anthropocene does not mark a moment when geology passed into human time, but rather the inverse: when anthropos became inherently and pervasively geological. Anthropogenic is a misnomer if it only means “human-caused,” because the carbon in question is always already a multispecies communication of biomatter and stored chemical energy across geological timescales.

There is something unique about the hypothesis of anthropogenic climate change. For one thing, carbon dioxide is only a pollutant at the scale of the planet. It is only a pollutant when it changes the overall absorption of heat in the atmosphere. This effect is not without parallel (ozone-damaging refrigerants also qualify), but the volume of emissions and the magnitude of the problems caused introduce a sharp historical discontinuity. Human activity may have started this process, but at this point we are just along for the ride with, at best, a sinking premonition of how things might turn out. The event of climate change thus takes the form of a speculative question. As hypothesis—not as established, authoritative fact—climate change set off a flurry of novel research, open-ended sci-fi imaginings, and apocalyptic fantasies about the end of civilization. The fossil fuel companies were among the first to act decisively in the face of this new threat—to defend themselves, of course! But even the sober-minded United Nations negotiations tend to view climate futures as a matter of open-ended possibilities, not totalizing fact. Tracing carbon’s inexorable rise, the Keeling Curve thus invites an imagination of environment in which Earth’s surface is transformed in ways that no one, really, can yet get a grip on.

The matter of carbon traded in carbon markets cannot be reduced to substantialist features of a compressible gas. What matters, and what has financial value on the carbon markets, is the contribution of several greenhouse gases to overall planetary warming. This value is assimilated into a common metric, CO₂-equivalent, which is a measure of the incremental anthropogenic contribution to global warming. In Beijing, a bevy of actors are designing carbon data platforms that attempt to map and track carbon emissions systematically across sectors of the Chinese economy (Whitington, forthcoming). Configuring this unique historical conjuncture, carbon is quite literally a metric of the human.

No one knows what it will mean to curtail fossil energy use in the world’s economy. But no one knows what a future of unabated planetary warming will look like either. The counterpoint of carbon is the atmosphere itself, taken as a regulatory object and as the very medium in which much of life exists. Glimmers of many possible futures present themselves, inviting guesswork and grand postulates. From a certain perspective, the decarbonization of the economy is a radical historical potential. Even if unlikely, the fact that it is being debated actively by corporate boards and is the subject of several global political agreements shows the dynamism and open-endedness of climate change futures. There is a long history to the collective exploration of potential carbon futures, and yet this history of the indefinite future is only beginning. One way or another, we will continue to learn more about the role of carbon in the atmosphere than anyone ever expected.