Of tree equivalents and methane

I am not the only one using tree equivalents to account for carbon offsetting. However, the term clearly means different things to different people. If you enter it as a Google search term, you’ll raise all sorts of esoteric stuff, mainly to do with analytical methods in economics, but here’s an interesting analysis from TransCanada Pipelines (http://www.methanetomarkets.org/events/2005/all/docs/cormack.pdf)

TransCanada Pipelines
Emissions saved in no of tree equivalents
Year	2002	2003
tonnes methane	191,000	223,000
tree equivalents	232,000	270,630
tonnes per tree (CO2E)	1.2	1.2

TransCanada is trying to show what impacts the prevention of emissions (basically leaks) from its pipelines will have. It assigns a factor of 1.2 t of carbon dioxide equivalent (CO₂E, different from the straight carbon, C_2, that I have been using – the factor would be 0.32 t C₂) to a tree, and claims that it has prevented emissions equivalent to hundreds of thousands of trees. I am assuming that the analysis attempts to quantify the offset tree planting program it would have to launch if it were to address emissions by this means.

If I have tried to do anything so far, it is to indicate that emissions offsetting must be viewed from the point of view of annual increments in sequestration per tree. A tree does not go from zero to 1 t of sequestration of CO₂E in a single year. Perhaps TransCanada wouldn’t actually plant, and has identified a stand of actively growing larger trees somewhere in the tropics of Costa Rica which have been assessed as capable of sequestering this much annually, and it is these which it has selected for its sequestration factor. This analysis is no better than that of my fuel-dump story yesterday. What is essential is to understand that the process of carbon sequestration in trees builds upon that which has already occurred (there is something called a relative growth rate), and that conditions remain such that the trees can grow actively for the period during which sequestration is being sought. In the early years, sequestration is in the order of kg, not t, per tree.

Methane has a much higher (21 times) global warming potential (GWP) than CO_2, so it is not entirely clear to me that the factor of 1.2 t is actually in CO₂E. This is actually 1.2 t of methane, equivalent to 25.2 t of CO₂ in global warming terms, so if I’ve done my calculations correctly, TransCanada’s tree equivalents should more honestly be numbered in the millions. These data were shown in a Powerpoint presentation, so I’m assuming that if no-one questioned the concepts espoused, they all went home feeling warm and fuzzy.