  We study the properties of massive, galactic-scale outflows of molecular gas
and investigate their impact on galaxy evolution. We present new IRAM PdBI
CO(1-0) observations of local ULIRGs and QSO hosts: clear signature of massive
and energetic molecular outflows, extending on kpc scales, is found in the
CO(1-0) kinematics of four out of seven sources, with measured outflow rates of
several 100 Msun/yr. We combine these new observations with data from the
literature, and explore the nature and origin of massive molecular outflows
within an extended sample of 19 local galaxies. We find that
starburst-dominated galaxies have an outflow rate comparable to their SFR, or
even higher by a factor of ~ 2-4, implying that starbursts can indeed be
effective in removing cold gas from galaxies. Nevertheless, our results suggest
that the presence of an AGN can boost the outflow rate by a large factor, which
is found to increase with the L_AGN/L_bol ratio. The gas depletion time-scales
due to molecular outflows are anti-correlated with the presence and luminosity
of an AGN in these galaxies, and range from a few hundred million years in
starburst galaxies, down to just a few million years in galaxies hosting
powerful AGNs. In quasar hosts the depletion time-scales due to the outflow are
much shorter than the depletion time-scales due to star formation. We estimate
the outflow kinetic power and find that, for galaxies hosting powerful AGNs, it
corresponds to about 5% of the AGN luminosity, as expected by models of AGN
feedback. Moreover, we find that momentum rates of about 20 L_AGN/c are common
among the AGN-dominated sources in our sample. For "pure" starburst galaxies
our data tentatively support models in which outflows are mostly
momentum-driven by the radiation pressure from young stars onto dusty clouds.
