They point out that chlorophyll will only have as strong a signal as oxygen if the cloud cover is zero or if the planet has a higher proportion of its landmass covered in vegetation.
Although we may be waiting for some time until we can overcome the technological challenges to detect chlorophyll on an alien planet's surface, it's fascinating to think that the first hint of alien life could be through the detection of the signature of something that resembles terrestrial flora.
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But just because this hypothetical form of extraterrestrial life may extract energy from their host star using a form of photosynthesis, this doesn't mean we'd necessarily be detecting vegetation as we know it. There could be an entirely different kind of life we won't fully comprehend until we can view it up-close.
And who knows? Should we detect a nearby exoplanet rich in biomarkers, that could be the motivation we need to mount a future interstellar mission.
Publication: Prospects for Detecting Oxygen, Water, and Chlorophyll in an Exo-Earth, Brandt and Spiegel, 2014. arXiv:1404.5337 [astro-ph.EP]