  The trace anomaly term in the low energy chiral Lagrangian is established by
deriving a low energy theorem for $\gamma \gamma \rightarrow \pi^0\pi^0$
scattering. It contributes to $\sigma(\gamma \gamma \rightarrow \pi^0\pi^0)$ in
the same order as the previously computed pion loop contribution. The coherent
sum of the two contributions explains the shape of the measured cross section,
while the magnitude is determined by the anomalous dimension of the quark mass
operator.
