  If lepton masses and mixings are explained by a flavour symmetry in seesaw
model which leads to U_{e3}=0 at leading order, we find that, under reasonable
assumptions, a future observation of low energy leptonic CP violation implies,
barring accidental cancellations, a lepton asymmetry both in flavoured
leptogenesis and in its one-flavour approximation. We explicitly implement this
approach with a predictive seesaw model for Tri-Bimaximal Mixing (TBM) and show
how cosmological baryon asymmetry can be directly trigged by low energy phases
appearing in $U_{e3}$. Thanks to this direct correlation we can derive a lower
bound on the reactor angle \theta_{13}: \sin^2 \theta_{13} \gtrsim 0.005.
