It is reported that the Schottky barrier between a semiconducting carbon nanotube (NT) and a metallic electrode is sometimes modified in the gaseous environment. There are three different cases: (1) there is no charge transfer between the gas and the NT/electrode (gas not charged) and the gas does not have a dipole moment (not polarized), (2) the gas is not charged, but is polarized, and (3) there is a charge transfer between the gas and the NT/electrode and the gas is charged. Case 1 will cause no Schottky barrier modulation. Case 2 will result in the Schottky barrier modulation through the modified work function in either the NT or the electrode due to the dipole moment in the gas. This is understood within the usual Schottky theory. Case 3 will also result in the Schottky barrier modulation, but depending on whether the NT and electrode are connected (closed) or not (open), the modulation is significantly different. The charged gas will attract the opposite charges in the NT and electrode. In the open circuit condition, the gas-NT and the gas-electrode interactions determine how much opposite charges are induced in the NT and electrode, respectively. However, in the closed-circuit condition, which is the usual condition in electronics applications, the induced opposite charges will move around the system and keep the Fermi level constant everywhere. This means that the induced opposite charges are redistributed in the NT and electrode. We have solved this redistribution problem and shown that the Schottky barrier modulation is large when the NT is in the depletion mode, while the modulation is negligible when the NT is consistently in the accumulation mode.