  Spin ice, a peculiar thermal state of a frustrated ferromagnet on the
pyrochlore lattice, has a finite entropy density and excitations carrying
magnetic charge. By combining analytical arguments and Monte Carlo simulations,
we show that spin ice on the two-dimensional kagome lattice orders in two
stages. The intermediate phase has ordered magnetic charges and is separated
from the paramagnetic phase by an Ising transition. The transition to the
low-temperature phase is of the three-state Potts or Kosterlitz-Thouless type,
depending on the presence of defects in charge order.
