  Shear viscosity of a two-dimensional Fermi liquid is found to be a
nonanalytic function of temperature. In contrast to the quasiparticle lifetime
that is determined by the forward-scattering processes, the main contribution
to the viscosity arises from the quasiparticle scattering in the Cooper
channel. The viscosity is enhanced by the logarithmic singularity of the vertex
part. This singular behavior can manifest itself in the two-dimensional
electron transport, and in the momentum relaxation of fermions in atomic traps.
