Signs of life on a global scale: Earth as a laboratory for exoplanet biosignatures

Thousands of planets orbiting stars other than the Sun (exoplanets) have been discovered, and the pace of discoveries will only accelerate as new observing missions are deployed. As the state of the field moves from exoplanet detection to characterization we are inching ever closer to indepth determination of the properties of rocky planets within the so-called ‘habitable zones’ of their host stars. The question now becomes---how would we recognize the signs of habitability and life on a distant exoplanet? We must begin with the only known example of a habitable and inhabited world---our own. But Earth affords more than one glimpse into conditions on a lifebearing world. Throughout geologic time the prevailing atmospheric and chemical state of our planet has undergone titanic shifts including from a hazy, orange, oxygen-free, methane-choked global habitat to the oxygen-rich pale blue dot we now take for granted. Here, we discuss ongoing efforts by astrobiologists and astronomers to catalogue the potential signatures of habitability and life we may find elsewhere in the universe by using Earth---both present and past---as a laboratory for the possible signatures of inhabited exoplanets.