Geography Reference
In-Depth Information
Panel on Climate Change, he contributed to the reports of the panel, which
was awarded the Nobel Peace Prize in 2007. Dave and I taught together for
years at the University of Washington, so naturally I turned to him for advice
on how I might calculate the ef ect of subirdia's trees on carbon dioxide levels.
Dave sent me a mound of equations that I could use to calculate the amount of
living matter held within the trees of my yard and from that the l ux of carbon
in my part of subirdia.
It turns out that it is simple to determine the rough amount of carbon held
in a tree. About half of the total tree mass is carbon, and mass can be calcu-
lated for a particular tree species simply by measuring the tree's diameter.
Nearly all of my trees are Douglas-i r, so Dave sent me the equation that con-
verts a i r's diameter into its aboveground biomass, the weight of its stump,
trunk, and branches. Cool! I hit the yard with a measuring tape in hand and
wrapped it around a sample of my trees. The girth of my trees ranged from
four to thirty-two inches and averaged about twenty inches across. Using
Dave's equation I learned that if we had cut and dried one of my trees, it would
have weighed more than three thousand pounds and held about i fteen hun-
dred pounds of carbon. Amassing this carbon doesn't happen quickly; each
of my trees is about seventy years old. But the process is steady.
I got a feel for my trees' annual growth and carbon storage by measuring
their change in size over the past two decades. Each tree, of course, carries
this information within it, in its annual growth rings. So, all I had to do was
take a small core of wood from my trees, a standard forestry technique, and
examine the rings. As I counted and measured them, I discovered that for
each of the past twenty years my trees increased their waistlines by a bit
more than a tenth of an inch. As they fattened up on solar energy, they
gained just over i fty pounds in dry mass per year and sucked nearly thirty
pounds of carbon out of the atmosphere. To get that amount of carbon, each
tree would have photosynthesized roughly one hundred pounds of carbon
dioxide.
