  We investigate how obliquity affects stratospheric humidity using a 3D
general circulation model and find the stratosphere under high obliquity could
be over 3 orders of magnitude moister than under the low obliquity equivalent,
even with the same global annual mean surface temperature. Three complexities
that only exist under high obliquity are found to be causally relevant. 1)
Seasonal variation under high obliquity causes extremely high surface
temperatures to occur during polar days, moistening the polar air that may
eventually enter the stratosphere. 2) Unlike the low obliquity scenario where
the cold trap efficiently freezes out water vapor, the high obliquity
stratosphere gets most of its moisture input from high latitudes, and thus
largely bypasses the cold trap. 3) A high obliquity climate tends to be warmer
than its low obliquity equivalent, thus moistening the atmosphere as a whole.
We found each of the above factors could significantly increase stratospheric
humidity. These results indicate that, for an earth-like exoplanet, it is more
likely to detect water from surface evaporation if the planet is under high
obliquity. The water escape could cause a high obliquity planet to loss
habitability before the runaway greenhouse takes place.
