  The ${\ell}^1$-convolution algebra of a semilattice is known to have trivial
cohom ology in degrees 1,2 and 3 whenever the coefficient bimodule is
symmetric. We ex tend this result to all cohomology groups of degree $\geq 1$
with symmetric coef ficients. Our techniques prove a stronger splitting result,
namely that the spli tting can be made natural with respect to the underlying
semilattice.
