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.
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