  The morphology of massive star-forming galaxies at high redshift is often
dominated by giant clumps of mass ~10^8-10^9 Msun and size ~100-1000 pc.
Previous studies have proposed that giant clumps might have an important role
in the evolution of their host galaxy, particularly in building the central
bulge. However, this depends on whether clumps live long enough to migrate from
their original location in the disc or whether they get disrupted by their own
stellar feedback before reaching the centre of the galaxy. We use cosmological
hydrodynamical simulations from the FIRE (Feedback in Realistic Environments)
project that implement explicit treatments of stellar feedback and ISM physics
to study the properties of these clumps. We follow the evolution of giant
clumps in a massive (stellar mass ~10^10.8 Msun at z=1), discy, gas-rich galaxy
from redshift z>2 to z=1. Even though the clumpy phase of this galaxy lasts
over a gigayear, individual gas clumps are short-lived, with mean lifetime of
massive clumps of ~20 Myr. During that time, they turn between 0.1% and 20% of
their gas into stars before being disrupted, similar to local GMCs. Clumps with
M>10^7 Msun account for ~20% of the total star formation in the galaxy during
the clumpy phase, producing ~10^10 Msun of stars. We do not find evidence for
net inward migration of clumps within the galaxy. The number of giant clumps
and their mass decrease at lower redshifts, following the decrease in the
overall gas fraction and star-formation rate.
