  The so-called Vegetation Red-Edge (VRE), a sharp increase in the reflectance
around $700 nm$, is a characteristic of vegetation spectra, and can therefore
be used as a biomarker if it can be detected in an unresolved extrasolar
Earth-like planet integrated reflectance spectrum. Here we investigate the
potential for detection of vegetation spectra during the last Quaternary
climatic extrema, the Last Glacial Maximum (LGM) and the Holocene optimum, for
which past climatic simulations have been made. By testing the VRE
detectability during these extrema when Earth's climate and biomes maps were
different from today, we are able to test the vegetation detectability on a
terrestrial planet different from our modern Earth. Data from the Biome3.5
model have been associated to visible GOME spectra for each biome and cloud
cover to derive Earth's integrated spectra for given Earth phases and observer
positions. The VRE is then measured. Results show that the vegetation remains
detectable during the last climatic extrema. Compared to current Earth, the
Holocene optimum with a greener Sahara slightly increases the mean VRE on one
hand, while on the other hand, the large ice cap over the northern Hemisphere
during the LGM decreases vegetation detectability. We finally discuss the
detectability of the VRE in the context of recently proposed space missions.
