If you placed a carbon dioxide monitor directly in front of your mouth and breathed out, the level of carbon dioxide emitted would be around four per cent or 40,000 parts per million.
This is perfectly normal. It is the carbon dioxide that we generate as part of our metabolism. Respiration is our method of generating the molecules that dissipate energy to the various parts of our cells.
But such a measurement would give you a false but very high impression of the levels of carbon dioxide in the atmosphere.
However, the air that you breathe out rapidly diffuses into your surroundings. Typically, gas molecules hurl along at speeds of 300 metres per second.
This means that the carbon dioxide in your breath quickly establishes an equilibrium distribution in the space that you occupy. In a small crowded classroom or boardroom, this can actually be a problem as the increasing levels of carbon dioxide tend to make people drowsy.
In the great outdoors, the carbon dioxide that you and all living organisms expel travels throughout the atmosphere. Over time, a state of equilibrium is achieved with the carbon dioxide levels in the lower atmosphere a careful balancing act controlled by three major influences.
The first is all living things on the planet. They breathe out carbon dioxide but plants and other organisms with chloroplasts also breathe in carbon dioxide and water vapour to generate sugars.
The second major influence is the oceans. Water absorbs carbon dioxide gas. Reactions between the water and carbon dioxide result in the formation of carbonate, bicarbonate, and carbonic acid. These reactions are all reversible resulting in the loss of carbon dioxide from the oceans as well.
The third major influence is the combustion of organic material and in particular fossil fuels. Fossil fuels represent carbon that was removed from the atmosphere through biological processes millions of years ago. As they have been buried for all this time, they haven't influenced the balancing act for carbon dioxide until we recently started digging them up and burning the carbon content.
The net result of all three major influences - and a few others - is the overall carbon dioxide content in the atmosphere.
To measure this, scientists required a location that is far from any major disturbing source of carbon dioxide - such as cities or heavy industry. Since the mid-1950s, the NOAA has been measuring carbon dioxide levels on Mauna Loa in the Hawaiian Islands.
Being in the middle of the Pacific Ocean has the benefit that the readings are truly a measure of the average carbon dioxide levels for the planet, free from local effects. Indeed, the data is so good that one can follow the seasonal carbon dioxide levels as spring comes to the northern hemisphere.
On May 9, the carbon dioxide levels at Mauna Loa passed 400 ppm for the first time.
There is nothing particularly magical about the number 400 ppm but it is a milestone along the way as we increase carbon dioxide levels in the atmosphere. It represents a 120 ppm increase since pre-industrial levels.
And it shows us that the balancing act between the oceans, the living world, and the atmosphere has been altered. Perhaps irreversibly.
Discussions of carbon dioxide in the atmosphere are most often linked with climate change. But a changing climate isn't the only consequence of increasing carbon dioxide levels. The ocean acts as a tremendous sink for atmospheric carbon. Indeed, there is about 50 times as much carbon in the ocean as in the atmosphere.
To put that in perspective, there is about the same amount of carbon in the living world as the atmosphere and about twice as much in the soil.
The consequence of increasing atmospheric carbon dioxide for the oceans is dire. The chemical equilibria that balance carbonate, bicarbonate, and carbonic acid are very sensitive to the equilibrium distribution between the water and the air.
Over the past 150 years, the average pH level at the ocean's surface has dropped from 8.18 to 8.07. That might not sound like a big deal but pH is a logarithmic scale - each unit represents an order of magnitude. The shift in the ocean's pH represents a 30 per cent increase in hydrogen ion concentration.
Such an increase is interfering with the formation of aragonite which is a form of calcium carbonate incorporated by marine organisms for the production of shells. Changing the pH means that it is much more difficult for corrals and molluscs to form strong shells. Indeed, existing coral beds might dissolve completely away.
The impact to the food chain in the ocean and the viability of reef ecosystems is severely threatened by the changing balance between carbon dioxide in the atmosphere and carbon dioxide dissolved in the ocean.
Simply put, 400 ppm maybe not be a landmark but it is more than we can bear.
