  We introduce a new method for analyzing the carbon budget using box models
and a mass balance approach. The method describes the net flow of carbon
between the atmosphere and other reservoirs. The method assumes that the data
can be explained by having the carbon move in and out of the atmosphere through
two reservoirs, one consisting of isotopically light carbon (biotic) and the
other consisting of isotopically heavy carbon (abiotic). The systems are
underdetermined from the data, so the Occam's razor approach is to assume that
the least amount of carbon moves between the atmosphere and the reservoirs. As
data from known sources and sinks are added to the model, one can determine the
constraints on the unknown sources and sinks. To illustrate the method, we
analyze data from the Mauna Loa Observatory and data from Antarctic ice cores.
