  We investigate constraints on early dark energy (EDE) from the Cosmic
Microwave Background (CMB) anisotropy, taking into account data from WMAP9
combined with latest small scale measurements from the South Pole Telescope
(SPT). For a constant EDE fraction we propose a new parametrization with one
less parameter but still enough to provide similar results to the ones
previously studied in literature. The main emphasis of our analysis, however,
compares a new set of different EDE parametrizations that reveal how CMB
constraints depend on the redshift epoch at which Dark Energy was non
negligible. We find that bounds on EDE get substantially weaker if dark energy
starts to be non-negligible later, with early dark energy fraction Omega_e free
to go up to about 5% at 2 sigma if the onset of EDE happens at z < 100. Tight
bounds around 1-2% are obtained whenever dark energy is present at last
scattering, even if its effects switch off afterwards. We show that the CMB
mainly constrains the presence of Dark Energy at the time of its emission,
while EDE-modifications of the subsequent growth of structure are less
important.
