  Rigorous upper limits on the vertical heat transport in two dimensional
Rayleigh-Benard convection between stress-free isothermal boundaries are
derived from the Boussinesq approximation of the Navier-Stokes equations. The
Nusselt number Nu is bounded in terms of the Rayleigh number Ra according to
$Nu \leq 0.2295 Ra^{5/12}$ uniformly in the Prandtl number Pr. This Nusselt
number scaling challenges some theoretical arguments regarding the asymptotic
high Rayleigh number heat transport by turbulent convection.
