These files are from the CASA model as described by the following reference: 

Randerson, J. T., M. V. Thompson, T. J. Conway, I. Y. Fung, and C. B. Field.
 1997. The contribution of terrestrial sources and sinks to trends in the
 seasonal cycle of atmospheric carbon dioxide.  Global Biogeochemical Cycles.
 11: 535-560.

Reading the Data:

The data have a 1 degree lat by 1 degree lon gridded resolution (180x360)
The first pixel is centered at 89.5N, 179.5W, the last pixel at 89.5S, 179.5E.

One directory contains NEP, another Net Primary Production (NPP) and the 
third Heterotrophic Respiration (RESP). NEP is positive into the surface 
(opposite of the atmospheric convention).

There is one file for each month, 1-Jan, 12-Dec.
Each file is IEEE floating point (4 bytes) 
so that the size is 180x360x4 = 259200 bytes.

The units are in gC/m2/month.
The sum of the 12 NEP files should be close to zero, but not exactly. 
That is, the terrestrial biosphere, in this run, is close to annual 
steady state.