  We combine IRAM Plateau de Bure Interferometer and Herschel PACS and SPIRE
measurements to study the dust and gas contents of high-redshift star forming
galaxies. We present new observations for a sample of 17 lensed galaxies at
z=1.4-3.1, which allow us to directly probe the cold ISM of normal star-forming
galaxies with stellar masses of ~10^10Msun, a regime otherwise not (yet)
accessible by individual detections in Herschel and molecular gas studies. The
lensed galaxies are combined with reference samples of sub-millimeter and
normal z~1-2 star-forming galaxies with similar far-infrared photometry to
study the gas and dust properties of galaxies in the SFR-M*-redshift parameter
space. The mean gas depletion timescale of main sequence galaxies at z>2 is
measured to be only ~450Myr, a factor of ~1.5 (~5) shorter than at z=1 (z=0),
in agreement with a (1+z)^-1 scaling. The mean gas mass fraction at z=2.8 is
40+/-15% (44% after incompleteness correction), suggesting a flattening or even
a reversal of the trend of increasing gas fractions with redshift recently
observed up to z~2. The depletion timescale and gas fractions of the z>2 normal
star-forming galaxies can be explained under the "equilibrium model" for galaxy
evolution, in which the gas reservoir of galaxies is the primary driver of the
redshift evolution of specific star formation rates. Due to their high star
formation efficiencies and low metallicities, the z>2 lensed galaxies have warm
dust despite being located on the star formation main sequence. At fixed
metallicity, they also have a gas-to-dust ratio 1.7 times larger than observed
locally when using the same standard techniques, suggesting that applying the
local calibration of the relation between gas-to-dust ratio and metallicity to
infer the molecular gas mass of high redshift galaxies may lead to systematic
differences with CO-based estimates.
