I’ve written before about stabilising temperatures; stabilising temperatures requires getting net anthropogenic emissions pretty close to zero. See, for example, this Realclimate post, or Solomon et al. (2008). Stabilising atmospheric concentrations, however, would not require getting emissions to zero, but would still require substantial emission redutions (see this Steve Easterbrook post).Clive Best, who is a physicist, has written a blog post claiming that stabilising concentrations would simply require stabilising emissions (i.e., constant emissions will mean constant atmospheric CO2 concentrations). His basic model is explained here and here. As far as I can tell, his argument appears to simply be that if emissions are increasing, the airborne fraction will be about 50% of our total emissions, but if emissions become constant, the sinks will retain balance – taking up as much as we’re emitting – and that this will happen rather fast (a few years). This, according to Clive, is something that Climate Scientists don’t yet understand. What I suspect Clive does not understand, is why people sometimes draw cartoons like the one on the right.
It should be fairly clear that what is being suggested is wrong; we’re dealing with a coupled system, so if you add new material to one of the reservoirs, it will rise in all reservoirs. However, there is a more formal way to show this. I recently worked through the ocean carbonate chemistry. It turns out that there is a factor called the Revelle factor, which is simply the ratio of the fractional change in atmospheric CO2, to the fractional change in total Dissolved Inorganic Carbon (DIC) in the oceans:
The Revelle factor is about 10, which means that the fractional change in atmospheric CO2 will be about 10 times bigger than the fractional change in . What this tells you straight away is that you can’t change the amount of CO2 in the oceans without also change the amount in the atmosphere; stabilising emissions will not stabilise concentrations.
Now, maybe if the fractional change in is small enough, the fractional change in might also be small enough to essentially stabilise concentrations. However, we know the quantities in the various reservoirs, and we’ve already emitted enough CO2 to change the by 1 – 2%, and – hence – the atmospheric CO2 concentration by 10 – 20%. If we stabilise emissions, we could easily change the by a further 1 – 2%. In fact, we have sufficient fossil fuels to change it by more than 10% and, therefore, enough to change the atmospheric concentration by more than 100% (i.e., to, at least, double atmospheric CO2).
There is, however, something I’m slightly glossing over, so will try to clarify a little more. The above is based on an equilibrium calculation. In other words, it is the changes once the system has retained a quasi-steady equilibrium. Our emissions are continually pushing the system out of equilibrium and so the fractional change in atmospheric CO2 is actually greater than what the Revelle factor would suggest. Given what we’ve already emitted, we would expect about 20% of our emissions to remain in the atmosphere, but it’s currently more like 45%. This is because the timescale for ocean invastion is > 100 years, and so the system hasn’t yet had time to return to equilibrium.
Therefore, the Revelle factor is – in some sense – a lower limit; if we continue to emit CO2 – even at a constant rate – we’d expect at least 20% of what we emit to remain in the atmosphere and, hence, atmospheric concentations will continue to rise. So, unless Clive can find some problem with basic carbonate chemistry, his claim that stabilising emissions will stabilise concentrations is simply wrong.