Following a presentation in London by Dr. Murry Salby, there has been much discussion at several sites: No Tricks Zone, Climate Etc. and WUWT. These threads are always a challenge for a reader because there are exchanges debating various issues between highly convinced people who are seldom explicit about the assumptions underlying their relative positions.
Interesting in this case are the reactions to Salby’s assertion that the observed rise in atmospheric CO2 is caused by effects of rising temperatures upon natural sources/sinks, and not by rising fossil fuel emissions. (Leaving aside for today the whole other issue of climate sensitivity to changes in CO2).
Attacks have been mounted both by supporters of IPCC, and also by skeptics of IPCC alarms who nevertheless accept the notion that all or most of the measured rise in CO2 is from humans, fossil fuels and cement in particular. Still others find flaws in Salby’s argument, but are not convinced by the alternative.
I recently posted a review of Salby’s textbook which touched on this topic. Firstly, I agree with those who say you cannot use static calculations on a dynamic and open system like the atmosphere. That is, both inputs and outputs are interactive and vary in response to each other. The most obvious example is increasing CO2 causing plant growth which in turn consumes more CO2. Thus algebra can mislead us, since it is the differentials over time that accumulate the object of interest at changing rates.
Secondly, it seems to me that the atmosphere itself is too small a subsystem to draw any meaningful conclusions. The ocean and land sources/sinks are orders of magnitude larger than the amount in the atmosphere, and the errors in estimating those flows far exceed the man-made emissions (which are also estimates with larger uncertainties than is usually admitted).
Rather than thinking of the air as a reservoir of CO2, it is more like a tidal pool. Imagine a scientist concerned that this tidal pool is changing volume because of water (unpolluted) leaking from a nearby landfill. So a measuring cup sample is taken periodically and tested. All the while, the pool is repeatedly drenched and drained by waves, currents and tides, along with occasional rains and storms. Whatever the test results, the effect of additional water from the landfill can not be discerned in the absence of markers distinguishing it from ocean and rainwater.
I don’t say Salby has all the answers. I agree with him that at the current state of information, atmospheric CO2 from human sources can not be identified apart from much larger natural fluxes of CO2.