  A planet's surface conditions can significantly impact its climate and
habitability. In this study, we use the 3D general circulation model ExoPlaSim
to systematically vary dayside land cover on a synchronously rotating,
temperate rocky planet under two extreme and opposite continent configurations,
in which either all of the land or all of the ocean is centred at the
substellar point. We identify water vapour and sea ice as competing drivers of
climate, and we isolate land-dependent regimes under which one or the other
dominates. We find that the amount and configuration of land can change the
planet's globally averaged surface temperature by up to 20K, and its
atmospheric water vapour content by several orders of magnitude. The most
discrepant models have partial dayside land cover with opposite continent
configurations. Since transit spectroscopy may permit observations of M-dwarf
planets' atmospheres, but not their surfaces, these land-related climate
differences likely represent a limiting uncertainty in a given planet's
climate, even if its atmospheric composition is known. Our results are robust
to variations in atmospheric CO2 concentration, stellar temperature, and
instellation.
