I once again managed to get involved in a discussion on Judith Curry’s blog about the rise in atmospheric CO2. This time was slightly better than it has been in the past, as most seemed to at least agree that the rise was anthropogenic. The dispute seemed to be as to whether or not a particular line of evidence was conclusive or not. Let’s clarify something first, though. There are many lines of evidence indicating that the rise in atmospheric CO2 is anthropogenic; this is not really in dispute.
However, a particularly elegant way to illustrate that the rise is anthropogenic (which Dikran Marsupial used during the discussion on Climate Etc.) is to simply consider mass balance. If is the rise in atmospheric CO2, is the anthropogenic emissions, is the natural emissions, and is the natural uptake, then
which we can rewrite as
The rise in atmospheric CO2 is smaller than our emissions, so the left-hand side is negative. Therefore, the right hand side is negative, nature is net sink, and therefore cannot be the source.
However, some were arguing that this simple mass balance argument does not preclude the possibility that some component of nature could be a net source. Okay, but we can go a bit deeper and consider the different components of the system. We have the oceans (which both releases and absorbs CO2) we have the biosphere (which both releases and absorbs CO2) we have the lithosphere (which releases CO2 via volcanic activity and absorbs CO2 via the slow carbon cycle) and we have fossil fuels, which we burn to release CO2 (there is no relevant anthropogenic sink).
Well, the oceans are taking in more CO2 than they release, the biosphere is taking in more CO2 than it releases, and the lithosphere is – we think – roughly in balance, with volcanoes releasing as much CO2 as is being absorbed via the slow carbon cycle. What’s left? Us; the release of CO2 via the burning of fossil fuels. Therefore, the rise in atmospheric CO2 is anthropogenic.
However, I don’t think the above extension is really necessary. If a component of nature could be, or has been, a net source of atmospheric CO2 that would imply a couple of things. There should have been a time when atmospheric CO2 rose faster than our emissions, and – similarly – there should have been a time when atmospheric CO2 would have continued rising were we to stop all our emissions. I don’t think either of these is true. I think atmospheric CO2 has always risen more slowly than our emissions and, if we were to stop emitting, concentrations would drop, not rise. Hence, it seems that the basic mass balance argument is sufficient to show that nature cannot be a source. Even if one component of the natural system is a net source, that would simply imply that other parts are an even bigger sink, so that – overall – nature is a net sink.
There is, however, one possibility. What about, for example, the conditions today being such that nature would be a source were we never to have emitted CO2. Well, we do know that there is a relationship between temperature and atmospheric CO2. If the temperatures had risen in the absence of our emissions, we would expect atmospheric CO2 to rise by between 10 and 20 ppm. One might, therefore, argue that a small part of the rise in atmospheric CO2 is natural and due to the rise in temperature. However, this is a bit of a cheat, given that the rise in temperature is mostly a consequence of our emissions anyway. Also, given our emissions, the concurrent rise in temperature really acts to slightly reduce the uptake of anthropogenic CO2; nature is still a net sink.
This, however, does lead to an interesting issue. As we continue to warm, we expect the uptake by the natural sinks to decrease; the ocean uptake being constrained by Henry’s Law, and the biosphere being constrained by nutrient availability. However, we don’t expect either to ever become a net source of atmospheric CO2. It is, however, quite possible that other natural sources may start to operate, such as permafrost release. These would then be a net source of atmospheric CO2. However, they would be feedback responses to the warming that will be largely a consequence of our emissions (assuming we do continue to emit CO2), hence to suggest that this would mean that nature has somehow become a net source would seem rather disingenuous.
So, as far as I can tell, the mass balance argument pretty conclusively shows that nature cannot be a source and, hence, that the rise is almost certainly anthropogenic. Of course, there are plenty of other lines of evidence, so we certainly don’t need to simply rely on the mass balance argument, but I think the basic mass balance argument is still sufficient precludes nature being a source.