hat 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. Such an approach could suggest past analogues for the present situation – for example 4.5 million years ago, in the Pliocene epoch, when CO2 also stood at around 400 ppm, and when the Earth was around 4 degrees hotter and the oceans up to 40 m higher than at present.

But this kind of approach is not consistent with the complex, historical nature of the Earth.  Absolute values such as levels of atmospheric CO2 mean radically different things at different stages of the Earth’s history. Over the 4.5 billion-year lifetime of the Earth, atmospheric CO2 has followed a broad secular downward trend from its early preponderance, and one that is likely to continue as the planet’s mantle cools and eventually absorbs all the planet’s CO2 (and indeed water); against this longer perspective, CO2 levels today are very low indeed.  But life took this downward curve and did something wild with it, creating a carbon metabolism and – with a bit of help from tectonics and celestial collisions – a very complex history.  The geochronological division of this narrative into aeons, eras, periods, epochs and so on have largely been established piecemeal, and reflect human conventions, but a broad-brush summary of them – starting at the longest timescale and moving ever finer – can suggest various ways of interpreting the 400 ppm event as heralding a possible Earth-system transition.

Transitions at the scale of aeons – between the Hadean, Archaean, Proterozoic and Phanerozoic – have been major transitions in the Earth’s mode of becoming, such as the emergence of life and continents in the Archaean, the establishment of a far-from-equilibrium state as life started to alter the chemical and thermodynamic balance of the Earth in the Proterozoic, and the emergence of complex, multicellular life in the Phanerozoic.  The transitions between eras within a given aeon have been less spectacular, but can still be quite extraordinary in their implications – for example the shift to an atmosphere with free oxygen that occurred in the Palaeoproterozoic (about half-way through the existence of the Earth so far), or the other great surge in oxygen levels that happened a billion and a half years later in the Neoproterozoic, or the later colonisation of land in the Palaeozoic half a billion years ago. These developments have, if anything, accelerated the downward trend in atmospheric CO2, sequestering it in soil, rock and ocean, leading to the low levels typical of our current era, the Cenozoic.

But at the finer level of periods, such as the Carboniferous, Permian and Triassic, the Earth has often experienced huge oscillations in the level of CO2 – often between hundreds and thousands of ppm.  Our own Quaternary period, the last period of the Cenozoic, has been 2.5 million years of low CO2, and thus a relatively cool planet with polar ice caps, but within that an oscillation between ice ages and warm interglacials – until the advent of the Holocene epoch, the ‘wholly recent’ epoch of the Quaternary, stopped that cycle.

Because of the indeterminacy of the Earth’s evolving story, we can only speculate how the current change in the state of the Earth measures against this hierarchy of transitions.  Perhaps the on-going massive spike in carbon dioxide – along with all the other changes that the human-machine-economy complex are wreaking on the global system – will not, in the end, be enough to constitute even a change of Epoch.  However disastrous for human society, in geological terms perhaps it will rank as no more significant that the sudden, transient spike in temperature caused by the release of massive amounts of CO2 and methane in the early Eocene, and the Earth will eventually return to the extended interglacial of the Holocene.

Or perhaps those that suggest we have entered the Anthropocene are right – that the observable changes in the state of the Earth suggest that we are entering a new epoch within the Quaternary period.  Yet modellers have suggested there is no stable third state in the Quaternary, beyond the familiar glacial and interglacial states.  If this is so – and if CO2 concentrations end up, say, topping the 1000 ppm mark – the Earth may be pushed too far from the Quaternary to return to the glacial-interglacial pattern, and tip into a state which would warrant classification at least as a new period within the Cenozoic era.

To go further and imagine that current environmental change could be part of a transition in Earth’s history at the level of eras or even aeons might seem absurd.  Yet this kind of speculation is surely necessary if our response to these changes is to be adequate.  The advent of the Cenozoic, for example, was occasioned by the celestial collision that ended the reign of the dinosaurs and made possible the rise of mammals, but also by the closing of the straights of Panama, which had the effect of gradually cooling the planet. If anything like the scenarios imagined by some writers of science fiction – and indeed proponents of new technologies such as biotechnology and geoengineering – came to pass, then the Earth could arguably undergo changes in its mode of becoming at least as significant as those at the start of the Cenozoic.  And then perhaps it will have to be said that the Earth has entered the Technozoic – the era or aeon of technological life.