  We prove short time existence for the Ricci flow on open manifolds of
nonnegative complex sectional curvature. We do not require upper curvature
bounds. By considering the doubling of convex sets contained in a
Cheeger-Gromoll convex exhaustion and solving the singular initial value
problem for the Ricci flow on these closed manifolds, we obtain a sequence of
closed solutions of the Ricci flow with nonnegative complex sectional curvature
which subconverge to a solution of the Ricci flow on the open manifold.
Furthermore, we find an optimal volume growth condition which guarantees long
time existence, and we give an analysis of the long time behaviour of the Ricci
flow. Finally, we construct an explicit example of an immortal nonnegatively
curved solution of the Ricci flow with unbounded curvature for all time.
