Setting an atmoshperic carbon dioxide baseline value

[old man emu]

Everyone goes on about rising CO2 levels in the atmoshere. But rising compared to what? How does science determine a base value for comparison. There must be some way to determine exactly what "fresh air" is.

 

Just south of the isolated north-west tip (Woolnorth Point) of Tasmania, at a place called Cape Grim, the Bureau of Meteorology and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) fund and operates the Cape Grim atmospheric observational facility. The Cape Grim Baseline Air Pollution Station first began measuring the composition of the atmosphere in April 1976 and has been in continuous operation since that date. The air sampled arrives at Cape Grim after long trajectories over the Southern Ocean, under conditions described as ‘baseline’. This baseline air is representative of a large area of the Southern Hemisphere, unaffected by regional pollution sources. 

 

Air samples are analysed at the station to determine concentrations of greenhouse and ozone-depleting gases, other air pollutants, including aerosols and reactive gases, and radon. Since sampling began at Cape Grim, more that 3 billion measurements have been taken. Among these are measurements of greenhouse gases (GHGs), including carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and synthetic GHGs such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF6). The number of atmospheric greenhouse gases measured at Cape Grim now exceeds 50, as well as concentrations of natural and anthropogenic aerosols (particles such as sea salt, mineral dust, carbon particles etc). Some of the air samples collected at Cape Grim have been archived for further analyses as required. This picture shows some of the samples stored in "fresh air archives".

Because we have these measurements and samples going back 50 years, we are in the position of being able to say that the atmoshere has changed over that time. Happily, all is not doom and gloom. The growth of some GHGs (for example methane) has slowed recently and some are in decline (CFCs and halons for example).

 

Read more here: https://capegrim.csiro.au/

 

[octave]

We can also go further back by using ice cores.

 

"Continuous ice core records from Antarctica provide direct measurements of atmospheric \(\text{CO}_{2}\) going back 800,000 years. [1, 2]

However, specialised, discontinuous ice samples (specifically "blue ice" from the Allan Hills, Antarctica) have enabled scientists to measure \(\text{CO}_{2}\) from air bubbles trapped as far back as 2 million years ago, with experimental studies extending even further. [1, 2, 3]"

 

Two million-year-old ice cores provide first direct observations of an ancient climate

[old man emu]

Ice cores are great for paleo-meterology, but contemporaneous sampling is what is needed for contemporary analysis. Besides, ice cores can't help us monitor CFCs since they did not exist before the middle of tthe 20th Century.