A few days ago, I was somewhat less than civil in response to a comment by Nic Lewis, which then lead to a typically juvenile post on Bishop Hill. It was, though, quite amusing to have Anthony Watts complaining about my lack of civility and suggesting that I could learn from Nic Lewis, who has published more climate papers than I have. It’s clear that Anthony Watts puts great stead in those who are both civil and well-published.
My frustration was largely because I had thought that Nic was simply being pedantic about my not qualifying something sufficiently. However, it seems that he was actually suggesting that not only could the equilibrium climate sensitivity (ECS) be low, but that – in fact – the carbon cycle could draw down CO2 rapidly enough, that only a small fraction would remain in the atmosphere for a very long time. Depending on how one defines “small fraction”, I think that this is not what is expected. I thought, therefore, that I would try to discuss our current understanding.
There is, however, rather a lot to this whole isssue, so my plan is to do it in two posts, this being the first. One of the first things to recognise is that what is of interest is not the residence time for an individual CO2 molecule, but the decay time for an enhancement in atmospheric CO2 concentration. In a sense, one can envisage the system as consisting of a number of CO2 reservoirs; the ocean, the biosphere, the atmosphere, and the planet’s lithosphere. There are continuous fluxes into and out of each reservoir. Some of the fluxes are large enough that an individual CO2 molecule will typically only spend a few years in the atmosphere, before being absorbed by the ocean, or the biosphere. However, this is considerably faster than the timescale over which an enhancement in atmospheric concentration would decay.
There are, also, essentially two carbon cycles; a fast carbon cycle, and a slow carbon cycle. The fast carbon cycle involves the ocean, the biosphere, and the atmosphere, and is associated with fluxes of 10s of GtC per year. The slow carbon cycle is associated with the sequestration of CO2 into the deep ocean, into rocks via weathering, and the emission of CO2 back into the atmosphere via volcanoes. It can involve fluxes of only about 0.1GtC per year, orders of magnitude smaller than those associated with the fast carbon cycle. However, it’s essentially the slow carbon cycle that sets the quasi-steady atmospheric CO2 concentration (i.e., the concentration that could be sustained for thousands of years with little change).
Prior to the industrial revolution, the atmospheric CO2 concentration was pretty steady at around 280ppm. The reason for this is that this is the concentration at which the rate of sequestration into the slow carbon sinks, matches the rate at which it is released through volcanic activity. Small perturbations away from this would change the sequestration rate, so that the atmospheric concentration would then return to about 280ppm. As long as these perturbations were small, this quasi-steady concentration could be maintained. However, what’s happened since the industrial revolution is that we’ve emitted a large amount of CO2 into the atmosphere (about as much as was in the atmosphere prior to the industrial revolution) at a rate far in excess of the rate at which it could then be sequestered back into the slow carbon sinks.
Given sufficient time, the CO2 that we’ve emitted will be sequestered into the slow carbon sinks and the concentration will return to pre-industrial levels. However, since it relies on the slow carbon cycle, this will take 10s of thousands of years, if not longer. We even have evidence for this kind of timescale. According to Archer et al. (2009)
Sediment cores from the deep ocean reveal a climate event 55 million years ago that appears to be analogous to the potential global warming climate event in the future. Isotopes of carbon preserved in CaCO3 shells reveal an abrupt release of carbon to the atmosphere-ocean system, which took about 150 thousand years to recover.
However, there is something I haven’t discussed. Even though it seems clear that it will take 10s of thousands of years for atmospheric CO2 to return to pre-industrial levels, what’s of interest to us is what fraction of our total emissions will remain in the atmosphere once the fast cycle has returned the ocean, biosphere, and atmosphere into a state of quasi-equilibrium. It’s this that I will try to discuss in another post.