  We show that the non-adiabatic perturbations between Ricci dark energy and
matter can grow both on superhorizon and subhorizon scales, and these
non-adiabatic perturbations on subhorizon scales can lead to instability in
this dark energy model. The rapidly growing non-adiabatic modes on subhorizon
scales always occur when the equation of state parameter of dark energy starts
to drop towards -1 near the end of matter era, except that the parameter
\alpha\ of Ricci dark energy equals to 1/2. In the case where \alpha\ = 1/2,
the rapidly growing non-adiabatic modes disappear when the perturbations in
dark energy and matter are adiabatic initially. However, an adiabaticity
between dark energy and matter perturbations at early time implies a
non-adiabaticity between matter and radiation, this can influence the ordinary
Sachs-Wolfe (OSW) effect. Since the amount of Ricci dark energy is not small
during matter domination, the integrated Sachs-Wolfe (ISW) effect is greatly
modified by density perturbations of dark energy, leading to a wrong shape of
CMB power spectrum. The instability in Ricci dark energy is difficult to be
alleviated if the effects of coupling between baryon and photon on dark energy
perturbations are included.
