Nodes of different colours represent the following:
Solid arrows point from a submodule to the (sub)module which it is
descended from. Dashed arrows point from a module or program unit to
modules which it uses.
Where possible, edges connecting nodes are
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large graphs.
read CO2 yearly values
open output file
no co2 file - default to basin parm
entire co2 period is before start of simulation
co2 record hasn't started so set to basin parm co2
write to co2.out