  Realistic 3-dimensional (3D), radiative hydrodynamical surface convection
simulations of the metal-poor halo stars HD 140283 and HD 84937 have been
performed. Due to the dominance of adiabatic cooling over radiative heating
very low atmospheric temperatures are encountered. The lack of spectral lines
in these metal-poor stars thus causes much steeper temperature gradients than
in classical 1D hydrostatic model atmospheres where the temperature of the
optically thin layers is determined by radiative equilibrium. The modified
atmospheric structures cause changes in the emergent stellar spectra. In
particular, the primordial Li abundances may have been overestimated by
0.2-0.35 dex with 1D model atmospheres. However, we caution that our result
assumes local thermodynamic equilibrium (LTE), while the steep temperature
gradients may be prone to e.g. over-ionization.
