One of the key things I was trying to stress is that we expect something like 20% of our total emissions to remain in the atmosphere for millenia. This will, however, depend on how much we do actually emit. The more we emit, the large the fraction that will remain in the atmosphere. It was therefore a little surprising to find a paper for review called a simple model of the anthropogenically forced CO2 cycle (Weber, Lüdecke & Weiss) with the following figure
The figure shows the results from their model (blue line) compared to results from 15 other models (Grey band – Joos 2013). The grey band shows that the other models suggest – as expected – 20%, or more, of our total emissions will remain in the atmosphere for millenia. The Weber, Ludicke & Weiss model, however, suggests that it should return to almost zero within centuries.
So, why the difference? Well, this is quite easy to understand. If you go to the section describing their model, you will see that the equation describing the flux between the atmosphere and the ocean is
where is the carbon content in the atmosphere at time , is the carbon content in the atmosphere in 1750, and is the time factor of the process, which they estimate to be about 80 years. So, essentially, their model assumes that there will be a net flux into the oceans, from the atmosphere, as long as atmospheric concentrations are above pre-industrial levels. So, given that years, it’s no great surprise that their model shows atmospheric CO2 returning to pre-industrial levels within a few hundred years.
However, this assumption of theirs is simply wrong. By burning fossil fuels, we’re adding new CO2 into the system (surface ocean, atmosphere, biosphere) and so the atmospheric concentration to which the system will settle is not going to be the same as it was before we started burning fossil fuels. Eli’s animation in this post illustrates this nicely.
Essentially, given how much we are likely to emit, 20% – or maybe more – of our total emissions are likely to remain in the atmosphere for millenia. Of course, if your model assumes that the net flux into the ocean will remain positive until concentrations return to pre-industrial levels, then your model will suggest that concentrations will drop down to pre-industrial levels quite quickly (hundreds of years). This, however, is simply wrong.