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!” I felt strangely relieved when I saw the headline that atmospheric concentrations of carbon dioxide had passed 400 parts per million. My sense of relief was, however, immediately replaced by a feeling guilt: why did this seem less troubling than the news of a terrorist attack, earthquake, or nuclear accident I had expected? There are, of course, some well established psychological paradigms that explain my initial relief. The basic psychological make-up of human beings appears to be poorly equipped to respond to the real and present danger of climate change. Paradigms suggest that we routinely ignore the threats of climate change precisely because we do not perceive it as either a real or present danger. For many living in More Economically Developed Countries, the real threats of climate change are associated with distant locations. For others throughout the world, climate change is seen as problem not of the now, but of the (near) future (Jones, et al 2013).

In these contexts, it is perhaps unsurprising that reaching levels of 400ppm of CO2 has been met with high levels of indifference. Following the 400ppm announcement on the @Keeling curve Twitter Feed of the Scripps Institute of Oceanography (on 4th May 2013), the only major national newspaper in the UK to carry the story on its front page was The Independent (the Financial Times reported on corrupt bankers, while the Daily Mail revealed the sale of drugs on Amazon) (Simms, 2013). In the context of this apparent apathy, I want to use this short review to consider the questions does 400ppm matter, and, if so, why?

400: a round number in the history of the atmosphere.

In a strange way there are some interesting parallels between the 400ppm ratio marker and the millennium celebrations (see Gould, 2011). As a society we appear to be drawn to round numbers in history and feel obliged to commemorate or reflect upon them in some way (Simms, 2013). It is thus clear that while scientifically there is no particular reason we should fixate on 400 as opposed to 401ppm, in psychological terms it is obvious why the transition from 399 to 400ppm seems to matter. As Neil Smith observed (in relation to society’s decidedly divided views on the significance of the year 2000), the fact that people appear to feel more ambiguous about the portentous nature of round numbers (such as 400) may be a popular indicator of the rise of a distinctly postmodern worldview (1999). I tend to concur with Ralph Keeling (son of Charles David Keeling), however, when he recently observed that the significance of 400ppm was that it appeared to represent our collective entry into a new atmospheric era: as we leave behind the relative stability of the 300-399ppm range and enter the more troubling 400 era (Montaigne, 2013). The fact that 400ppm also lies at the midway point between 350ppm (the concentration levels James Hansen suggested we need to achieve to ensure environmental stability) and 450ppm (the level of CO2 concentration the IPCC equates with the dangerous increase of global average temperatures beyond 2 degrees Celsius), cements its symbolic and material significance.

Atmospheric Histories of the Present and the Global Scientific Gaze.


It would appear that one of the implications of 400ppm has been an increasing desire to historicize the climate change debate. At one level, this historicization has occurred in relation to the suggestion that we may be entering a new era in atmospheric history. At another level, there has been a renewed desire to position our current situation in historical context. It has consequently been widely discussed that the last time the planet’s atmosphere contained 400ppm of CO2 was during the Pliocene, some four million years ago. There are, however, other historical comparisons we can make, and which appear instructive to our assessment of the significance of 400ppm. We could, for example, compare the 4th May 2013 with the London Fog disaster of December 1952. This was an atmospheric disaster that took thousands of lives and saw London come to a grinding halt for several days. The impact of this atmospheric event resulted in the passing of the Clean Air Act of 1956 and a transformation in the ways in which governments throughout the world regulated air pollution. It is clear that 400ppm lacks both the tangibility and immediacy of the London Fog Disaster. We also already know that 400ppm will not result in any new forms of climate change legislation or regulation. Comparisons with 1952 does, however, raise another reason why 400ppm may be significant. The fact that we collectively knew, in real time, that we had (admittedly briefly) passed 400ppm indicates the significant changes that have occurred in environmental science and monitoring. In 1952 monitoring devices were clogged-up and under-funded atmospheric scientists and medical experts were unclear for months why the fog had caused so many deaths (see Whitehead, 2009). Perhaps the long-term significance of 400ppm lies in the very fact that we knew so quickly we had reached this arbitrary, but strangely significant, point in atmospheric history. The question, of course, remains as to whether we will use this knowledge to good effect. 


Gould SJ 2011 Questioning the Millennium: A Rationalist’s Guide to a Precisely Arbitrary Countdown.Harvard: Harvard University Press.
Jones R, J Pykett, and M Whitehead 2013 Changing Behaviours: On the Rise of Psychological State.Cheltenham: Edward Elgar.
Montaigne F 2013 Record 400ppm CO2 Milestone ‘feels like we’re moving into another era. Yale Environment, 14th May 2013.
Simms A 2013 Why did the 400ppm carbon milestone cause barely a ripple? The Guardian, 30th May 2013.
Smith N 1999 Nature at the millennium: production and re-enchantment. In: B Braun and N Castree (eds) Remaking Reality: Nature at the Millenium, pp  269-282.
Whitehead M 2009 State, Science and the Skies: Governmentalities of the British Atmosphere. Oxford:Wiley-Blackwell.