  We study the dissipative effects of baryon physics on cosmic statistics at
small scales using a cosmological simulation of a (50 Mpc/h)^3 volume of the
universe. The MareNostrum simulation was performed using the AMR code RAMSES,
and includes most of the physical ingredients which are part of the current
theory of galaxy formation, such as metal-dependent cooling and UV heating,
subgrid modelling of the ISM, star formation and supernova feedback. We re-ran
the same initial conditions for a dark matter only universe, as a reference
point for baryon-free cosmic statistics. In this paper, we present the measured
small-scale amplification of sigma^2 and S_3 due to baryonic physics and their
interpretation in the framework of the halo model. As shown in recent studies,
the effect of baryons on the matter power spectrum can be accounted for at
scales k <~ 10 h/Mpc by modifying the halo concentration parameter. We propose
to extend this result by using a halo profile which is a linear combination of
a NFW profile for the dark matter and an exponential disk profile mimicking the
baryonic component at the heart of the halo. This halo profile form is
physically motivated, and depends on two parameters, the mass fraction f_d of
baryons in the disk, and the ratio lambda_d of the disk's characteristic scale
to the halo's virial radius. We find this composite profile to reproduce both
the small-scale variance and skewness boosts measured in the simulation up to k
~ 10^2 h/Mpc for physically meaningful values of the parameters f_d and
lambda_d. Although simulations like the one presented here usually suffer from
various problems when compared to observations, our modified halo model could
be used as a fitting model to improve the determination of cosmological
parameters from weak lensing convergence spectra and skewness measurements.
