  This paper establishes the global in time existence of classical solutions to
the 2D anisotropic Boussinesq equations with vertical dissipation. When only
the vertical dissipation is present, there is no direct control on the
horizontal derivatives and the global regularity problem is very challenging.
To solve this problem, we bound the derivatives in terms of the $L^\infty$-norm
of the vertical velocity $v$ and prove that $\|v\|_{L^{r}}$ with $2\le
r<\infty$ at any time does not grow faster than $\sqrt{r \log r}$ as $r$
increases. A delicate interpolation inequality connecting $\|v\|_{L^\infty}$
and $\|v\|_{L^r}$ then yields the desired global regularity.
