Detecting long-term changes in point-source fossil CO2 emissions with tree ring archives

We examine the utility of tree ring C-14 archives for detecting long-term changes in fossil CO2 emissions from a point source. Trees assimilate carbon from the atmosphere during photosynthesis, in the process faithfully recording the average atmospheric C-14 content in each new annual tree ring. Using C-14 as a proxy for fossil CO2, we examine interannual variability over six years of fossil CO2 observations between 2004-2005 and 2011-2012 from two trees growing near the Kapuni Gas Treatment Plant in rural Taranaki, New Zealand. We quantify the amount of variability that can be attributed to transport and meteorology by simulating constant point-source fossil CO2 emissions over the observation period with the atmospheric transport model WindTrax. We compare model simulation results to observations and calculate the amount of change in emissions that we can detect with new observations over annual or multi-year time periods, given both the measurement uncertainty of 1ppm and the modelled variation in transport. In particular, we ask, what is the minimum amount of change in emissions that we can detect using this method, given a reference period of six years? We find that changes of 42aEuro-% or more could be detected in a new sample from one year at the same observation location or 22aEuro-% in the case of four years of new samples. This threshold is reduced and the method becomes more practical the more the size of the signal increases. For point sources 10 times larger than the Kapuni plant (a more typical size for power plants worldwide), it would be possible to detect sustained emissions changes on the order of 10aEuro-%, given suitable meteorology and observations.