  We argue that a few per cent of "Early Dark Energy" can be detected by the
statistics of nonlinear structures. The presence of Dark Energy during linear
structure formation is natural in models where the present tiny Dark-Energy
density is related to the age of the Universe rather than a new fundamental
small parameter. Generalisation of the spherical collapse model shows that the
linear collapse parameter delta_c is lowered. The corresponding relative
enhancement of weak gravitational lensing on arc-minute scales lowers the value
of sigma_8 inferred from a given lensing amplitude as compared to Lambda-CDM.
In presence of Early Dark Energy, structures grow slower, such that for given
sigma_8 the number of galaxies and galaxy clusters is substantially increased
at moderate and high redshift. For realistic models, the number of clusters
detectable through their thermal Sunyaev-Zel'dovich effect at redshift unity
and above, e.g. with the Planck satellite, can be an order of magnitude larger
than for Lambda-CDM.
