  Employing a combination of \emph{ab initio} band structure theory and
dynamical mean-field theory we explain the experimentally observed linear
temperature increase of the magnetic susceptibility of the iron pnictide
material BaFe$_{2}$As$_{2}$. The microscopic origin of this anomalous behaviour
is traced to a sharp peak in the spectral function located approximately 100
meV below the Fermi level. This peak is due to the weak dispersion of
two-dimensional bands associated with the layered crystal structure of
pnictides.
