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Growth of a bias point thermal shift induced by dc bias in z-cut LiNbO3 optical intensity modulators is quantitatively discussed from the standpoint of its impact on field service. During 20 years of device operation under a worst-case dc bias condition, the thermal shift slope grows almost symmetrically with respect to a bias polarity. An initial small thermal shift of around ±7 mV/°C (3σ distribution bounds) increases continuously over time with biased operation at 55°C and approaches ±70 mV/°C after 20 years. This increased temperature sensitivity would generate approximately a 2-V bias point shift toward the bias rail when modulator is exposed to temperature variation from 55°C to 25°C.