We investigate future constraints on early dark energy ( EDE ) achievable by the Planck and CMBPol experiments , including cosmic microwave background ( CMB ) lensing .
For the dark energy , we include the possibility of clustering through a sound speed c _ { s } ^ { 2 } < 1 ( cold dark energy ) and anisotropic stresses parameterized with a viscosity parameter c _ { vis } ^ { 2 } .
We discuss the degeneracies between cosmological parameters and EDE parameters .
In particular we show that the presence of anisotropic stresses in EDE models can substantially undermine the determination of the EDE sound speed parameter c _ { s } ^ { 2 } .
The constraints on EDE primordial energy density are however unaffected .
We also calculate the future CMB constraints on neutrino masses and find that they are weakened by a factor of 2 when allowing for the presence of EDE , and highly biased if it is incorrectly ignored .