A study published by the CO2 Coalition claims that the rise in atmospheric CO2 concentration is manmade. This claim is examined here and rejected.
There are two fundamental shortcomings in the study: It is based on an inappropriate model, that is far from reality, and it ignores that the atmosphere is an open system. In such a system the concentration cannot rise more than the inflow. Therefore, since the concentration has risen by 50 %, the inflow must have risen by at least 50 %. 5 % emissions from human activities are far too small, emissions from natural sources must have risen much more.
The study also emphasizes that the atmosphere accumulates less CO2 than humans emit. Therefore, there is a net outflow to nature, nature acts as a net sink. And, as “a sink cannot be a source”, the study concludes that the increase in concentration can only be caused by anthropogenic emissions. That sounds logical but does not stand up to scrutiny: Nature acts as both a source and a sink simultaneously, sometimes one prevails, sometimes the other. On its own, nature would have been a net source since 1750; only in combination with anthropogenic emissions has it become a net sink. Nevertheless, even as a net sink, nature can have, and actually has, contributed more to
the rise in concentration than humans.
Another weakness of the study is its assumption that natural fluxes in and out of the atmosphere have remained unchanged for 250 years. Given the high variability of natural processes, this is extremely unlikely, and given the recent warming, it cannot be correct.
The study also puts forward several additional arguments to support its view of the exclusively anthropogenic causation of the CO2 increase. It is shown that these arguments are also not viable.