n Thursday 9 May 2013, for the first time, the daily-average measured atmospheric concentration of carbon dioxide at the Mauna Loa Observatory in Hawaii exceeded 400 parts per million (ppm). (When first measured by Charles Keeling in 1957/8 it was about 315 ppm.) This signature measurement drew a fair amount of attention across the world’s media, with headlines such as ‘carbon dioxide passes symbolic mark’, ‘CO2 crosses dreaded 400 ppm milestone’, ‘NOAA debunks 400 ppm CO2 panic’. 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?

One might argue that the global CO2 concentration is symbolic because it tells us something important about the changing composition of the air that we all breathe.  CO2 is a reasonably well-mixed gas in the lower atmosphere and so a single number has global resonance since pretty much all 7.1 billion of us are breathing this air with its extra molecules of CO2.  (With a few very rare exceptions, the concentration of CO2 in the lower atmosphere only varies from the global average by plus/minus 5 per cent.)  And since this change is brought about by humanity’s collective industrial metabolism the rise of CO2 concentrations reveals both the global and the intimate nature of the change being wrought.  So symbolic, yes; but dread and panic, no: the direct effects of this modestly elevated concentration of CO2 in the atmosphere are mostly beneficial – ceteris paribus it leads to stronger plant growth and higher crop and timber yields.

To explain dread and panic maybe we have to see the symbolism of 400 ppm as being less about the air that we breathe than it is about its effect on our weather.  Weather arises from the interactions of sun, air, land, ice, ocean and life.  Changing the concentration of CO2 in the air alters these interactions and so changes the weather experienced by people in places.  So here again the rise of CO2 concentration speaks of something that is both global and intimate: global, since weather results from an interacting worldwide system; and intimate, since each human life is conditioned by the weather.

But how might 400 ppm induce dread and panic?  There is a far from linear relationship between CO2 concentration and the nature of weather experienced by people in places.  400 ppm as a global index of weather change therefore both distorts and obscures.  Weather is affected by too many other changing influences, both natural and human, that are not well represented by a single number – dust, soot, aerosols, clouds, surface albedo and so on.  These climate-forcing agents are not well-mixed in the global atmosphere or are variable across the Earth’s surface and contribute in complex ways to the interacting dynamics of the climate system.  A global number would be inadequate as a measure of their change in status.

No, the dread and panic of the media headlines reflects a different symbolism.  400 ppm of CO2 speaks not of the air we breathe, nor even the weather we experience.  Rather, it symbolises the inexorable human alteration to one of the basic biogeophysical conditions of life on Earth, the composition of the Earth’s atmosphere.  The full consequences of this change are not clear, but the dread and panic arises from this symbolic lack of knowledge and control about the future, a sense of helplessness that the scale of human agency in the world has outstripped our capabilities for understanding and guiding it.

But if this is so, then I wonder why is this notice of 400 ppm any more significant than, say, the eradication of smallpox in 1979, the human population reaching 7 billion in 2011, the number of Facebook users reaching 1 billion in 2012 or the number of airline passengers in the world passing 3 billion for the first time in 2013?  These examples also symbolise the astonishing presence, agency and power of human beings to transform the planet and the forms of life on it.  But do they generate headlines of dread and panic?