  We study the prospects for constraining dark energy at very high redshift
with the Sandage-Loeb (SL) test - a measurement of the evolution of cosmic
redshift obtained by taking quasar spectra at sufficiently separated epochs.
This test is unique in its coverage of the ``redshift desert'', corresponding
roughly to redshifts between 2 and 5, where other dark energy probes are unable
to provide useful information about the cosmic expansion history. Extremely
large telescopes planned for construction in the near future, with ultra high
resolution spectrographs (such as the proposed CODEX), will indeed be able to
measure cosmic redshift variations of quasar Lyman-alpha absorption lines over
a period as short as ten years. We find that these measurements can constrain
non-standard and dynamical dark energy models with high significance and in a
redshift range not accessible with future dark energy surveys. As the cosmic
signal increases linearly with time, measurements made over several decades by
a generation of patient cosmologists may provide definitive constraints on the
expansion history in the era that follows the dark ages but precedes the time
when standard candles and rulers come into existence.
