  Recently much attention has being attracted to the problem of limitations on
the attainable intensity of high power lasers [A.M. Fedotov {\it et al.} Phys.
Rev. Lett. \textbf{105}, 080402 (2010)]. The laser energy can be absorbed by
electron-positron pair plasma produced from a seed by strong laser field via
development of the electromagnetic cascades. The numerical model for
self-consistent study of electron-positron pair plasma dynamics is developed.
Strong absorption of the laser energy in self-generated overdense
electron-positron pair plasma is demonstrated. It is shown that the absorption
becomes important for not extremely high laser intensity $I \sim 10^{24}$
W/cm$^2$ achievable in the nearest future.
