  We compute the impact of dark energy at last scattering on measurements of
baryon acoustic oscillations (BAOs). We show that an early dark energy
component can contribute a systematic uncertainty to BAO measurements of up to
2.5%. Whilst this effect turns out to only slightly affect current BAO surveys,
the results of future BAO surveys might become biased. We find that BAO surveys
alone appear unable to resolve this systematic uncertainty, so supplementary
measurements are necessary.
