What does the recent passing of the 400 parts per million (ppm) threshold in atmospheric carbon dioxide mean, in terms of the history of the Earth? It’s tempting to answer this question by simply taking a graph of atmospheric carbon dioxide concentrations over geological time, and drawing a horizontal line back to see when it was last at the current level.

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?

The wavering, saw-toothed plot of atmospheric carbon dioxide levels pushed determinedly over the threshold of 400ppm, tracing a line from the global space of the atmosphere to the hermetic spaces of geoscientist Charles Keeling’s early CO2 observations. As one of millions of virtual witnesses to this traversal, I was reminded that Keeling’s deployment of his 5-litre flasks was not the first time that a glass orb had changed science and, with it, the world.

When coming across the figure of 400ppm, I was struck by how difficult this was. 400ppm – what is one supposed to do with this figure? Tellingly, it is not only the mediators and audiences of science, but the scientists themselves that are struggling.

Millions of carbon dioxide molecules passed through the Mauna Loa measuring equipment on the 9th May 2013 to generate the milestone daily average of 400ppm. I will focus on one – the 400thpart per million – and the story of its carbon atom. There are many such science fictions that could be told; this one is indicative and personal.

Do you remember where you were when you first heard the news? I was invigilating in an examination with a physical geography colleague. About halfway through the examination my colleague was consulting his iPad when he turned the screen to me saying, “this is not good!”

In the year of 400 ppm, the politics of carbon accounting seem to have reached a dead end. However, rather than interpreting this historical juncture as the failure of climate politics, we may use it to foster more productive political imaginaries.

The concentration of carbon dioxide in the atmosphere has never before in human evolutionary history exceeded this level. But what does 400 ppm symbolise? And why might it cause dread and panic?

The symbolic threshold of 400ppm is a poignant reminder that we are well past the point where climate change response can be a planned, gradualist transition. It is much more likely that profound and unwanted change in the next few years (perhaps not even decades) will make a mockery of current policies on climate change and other issues.

Amongst all the hubbub about CO2 concentrations at Mauna Loa reaching 400 parts per million, another important point is being overlooked—were it not for the fortuitous presence of natural carbon sinks, the CO2 concentration would already have been much higher. We are being buffered from the full impact of our CO2 emissions, but this may not continue indefinitely.

The fact that, in mainstream environmentalism, nature appears increasingly as a set of dynamic, creative, and resilient social-ecological processes is symptomatic of a moment of real possibility for the creation of alternative future natures – and one that, we would argue, a critical project must actively embrace.