  The biosignatures of life on Earth do not remain static, but change
considerably over the planet's habitable lifetime. Earth's future biosphere,
much like that of the early Earth, will consist of predominantly unicellular
microorganisms due to the increased hostility of environmental conditions
caused by the Sun as it enters the late stage of its main sequence evolution.
Building on previous work, the productivity of the biosphere is evaluated
during different stages of biosphere decline between 1 Gyr and 2.8 Gyr from
present. A simple atmosphere-biosphere interaction model is used to estimate
the atmospheric biomarker gas abundances at each stage and to assess the
likelihood of remotely detecting the presence of life in low-productivity,
microbial biospheres, putting an upper limit on the lifetime of Earth's
remotely detectable biosignatures. Other potential biosignatures such as leaf
reflectance and cloud cover are discussed.
