Canonical variational transition state theory rate calculations have been performed to assess the fate of methanediol in the troposphere. The calculations suggest that proton tunneling plays a very important role in the gas-phase decomposition of methanediol as it enhances the rate of the reaction by 1-9 orders of magnitude in the tropospherically relevant temperature range of 200-300 K The effect of proton tunneling is greatest at 200 K; the rate constant is enhanced up to 9 orders of magnitude. This is in stark contrast to previous calculations suggesting that tunneling would not play any role in the alkanediol decomposition under typical laboratory and interstellar conditions. Furthermore, the results imply that though water is the most dominant trace component of the troposphere, formic acid and the hydroperoxyl radical, which are relatively less abundant, outcompete water in catalyzing the decomposition. Methanediol may also catalyze its own decomposition below 280 K However, this autocatalytic pathway turns out to be less effective than the water-catalyzed one. These results may play a crucial role in improving our understanding of alkanediol chemistry, which has a broad appeal beyond the troposphere.