  Current investigations of exoplanet biosignatures have focused on static
evidence of life, such as the presence of biogenic gases like O2 or CH4.
However, the expected diversity of terrestrial planet atmospheres and the
likelihood of both false positives and false negatives for conventional
biosignatures motivate exploration of additional life detection strategies,
including time-varying signals. Seasonal variation in atmospheric composition
is a biologically modulated phenomenon on Earth that may occur elsewhere
because it arises naturally from the interplay between the biosphere and
time-variable insolation. The search for seasonality as a biosignature would
avoid many assumptions about specific metabolisms and provide an opportunity to
directly quantify biological fluxes--allowing us to characterize, rather than
simply recognize, biospheres on exoplanets. Despite this potential, there have
been no comprehensive studies of seasonality as an exoplanet biosignature.
Here, we provide a foundation for further studies by reviewing both biological
and abiological controls on the magnitude and detectability of seasonality of
atmospheric CO2, CH4, O2, and O3 on Earth. We also consider an example of an
inhabited world for which atmospheric seasonality may be the most notable
expression of its biosphere. We show that life on a low O2 planet like the
weakly oxygenated mid-Proterozoic Earth could be fingerprinted by seasonal
variation in O3 as revealed in its UV Hartley-Huggins bands. This example
highlights the need for UV capabilities in future direct-imaging telescope
missions (e.g., LUVOIR/HabEx) and illustrates the diagnostic importance of
studying temporal biosignatures for exoplanet life detection/characterization.
