Geyser eruptions are produced by a complex and poorly understood set of subsurface processes and conditions. They typically have an abundant supply of water, relatively permeable and competent subsurface material, a conduit to the surface, a driving mechanism (commonly believed to be the initiation of gas lift pumping by steam formation in the conduit), and a trigger. Here we present time series of dissolved CO2 concentrations in near-surface discharge waters of a thermal geyser in Yellowstone National Park (northwestern United States) that vary systematically over several eruption cycles. Chemical geothermometry, combined with a temperature profile in a nearby well, suggests that the geyser water ascends from non-boiling conditions (∼153–171 °C at a depth of 57–65 m). When the time series of near-surface measured CO2 concentrations are extrapolated to these subsurface conditions assuming dominantly adiabatic cooling, the additional gas pressure from dissolved CO2 is large enough to cause the total dissolved gas pressure to exceed bubbling pressure, inducing bubble formation. We postulate that CO2 is a necessary component to triggering eruptions in the geyser studied. Furthermore, unlike steam, CO2 bubbles do not completely re-condense during cooling in the geyser conduit, hence providing better sustenance for gas lift pumping than pure H2O boiling.